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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 44 (2003), No. 4

  • Deformation Behavior and Controlling Mechanisms for Plastic Flow of Magnesium and Magnesium Alloy

    pp. 426-435

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    DOI:10.2320/matertrans.44.426

    Deformation behavior of pure Mg and Mg alloy were studied in the temperature range of 423 to 773 K and at strain rates of 10−4–10−2 s−1. Three temperature regions can be categorized both in Mg and Mg alloy. The deformation behavior in Mg can be described by an exponantional law at temperatures below 523 K. At the higher temperatures a power law of deformation is valid with the stress exponent close to n=7 in the intermediate (523–623 K) and 2.2 in the high (673–773 K) temperature ranges. The alloying of Mg with elements such as Zn changes the phenomenology of plastic deformation. An exponantional law is operative at temperatures below 473 K. At above 473 K deformation obeys a power law. The stress exponent is close to n=7 at intermediate temperatures (473–523 K) and 5 in the high temperature range. An analysis of experimental results shows that alloying changes the controlling mechanisms of plastic deformation and so leads to different deformation behavior in pure Mg and Mg alloy that can be associated with decreasing stacking fault energy (SFE) in Mg alloy. The effect of SFE on the mechanisms of plastic deformation when alloying Mg is discussed.
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  • Tensile Properties of Directionally Solidified AZ91 Mg Alloy

    pp. 436-439

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    DOI:10.2320/matertrans.44.436

    AZ91 Mg alloy consisting of elongated grains was processed by a directional solidification method and mechanical properties of the directionally solidified Mg alloy were compared with those of the non-directionally solidified Mg alloy by tensile tests at room temperature and at 473 K. The directionally solidified Mg alloy exhibited higher strength at 473 K than the non-directionally solidified Mg alloy because grain boundary sliding was suppressed. Also, the directionally solidified alloy exhibited higher strength and larger elongation at room temperature than the non-directionally solidified alloy. Suppression of intergranular fracture was responsible for high ductility for the directionally solidified alloy.
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  • Production of High Strength Mg97Zn1Y2 Alloy by Using Mechanically Alloyed MgH2 Powder

    pp. 440-444

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    DOI:10.2320/matertrans.44.440

    The Mg97Zn1Y2 bulk alloy has been produced by MA-HDH P/M process using MgH2 powders as the starting material. As-extruded Mg97Zn1Y2 bulk alloy of MAed powders showed a high compressive yield strength of 616 MPa and a plastic strain of 4.2%. These values were much higher than those of commercial ingot metallurgy (I/M) Mg alloys, such as AZ91 and ZK60. This is originated from the refinement of hcp-Mg grains, the solid solution hardening of Zn and Y, and the dispersion hardening by MgO and Y2O3 fine particles. No degradation was observed in mechanical properties of the alloy even after annealing at 673 K for prolonging time. This is considered to be due to the inhibition of the hcp-Mg grain growth by the densely dispersed MgO and Y2O3 fine particles. Therefore, MA-HDH P/M Mg97Zn1Y2 alloy is promising as the structural material for high temperature applications.
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  • Grain-Boundary Sliding in AZ31 Magnesium Alloys at Room Temperature to 523 K

    pp. 445-451

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    DOI:10.2320/matertrans.44.445

    Rolled sheets of AZ31 Mg alloys were subjected to tensile testing at temperatures ranging from room temperature to 523 K. The occurrence of grain-boundary sliding (GBS) at room temperature was demonstrated by the displacement of scribed lines across grain boundaries of deformed samples. Surface relief of deformed samples was measured by use of a scanning laser microscope. GBS strain was calculated from the measured surface step height, and its temperature dependence was analyzed by a Dorn-type constitutive equation. GBS above 423 K was found to be pure GBS that was activated by resolved applied shear stress acting on grain boundaries. The activation energy for GBS was found to be 80 kJ/mol, which is in agreement with the activation energy for grain boundary diffusion. Meanwhile, GBS below 373 K was found to be slip-induced GBS, and its extent was found to be significantly greater than that expected from extrapolation of high-temperature values. The slip-induced GBS is considered to occur by plastic compatibility conditions in the presence of plastic strain anisotropy and by absorption and dissociation of lattice dislocations at grain boundaries.
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  • Anisotropy and Non-Uniformity in Plastic Behavior of AZ31 Magnesium Alloy Plates

    pp. 452-457

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    DOI:10.2320/matertrans.44.452

    On the AZ31-O magnesium alloy plates of 20 mm in thickness, basal plane texture was studied for the samples taken from different layers. Compression tests were carried out in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. The formability in deep drawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties. The severity of the basal plane texture is higher at the surface layer than the inner layers. In tensile tests at room temperature, proof stress is higher for the surface layer than the inner layers, whereas elongation is lower and r-value is higher at the surface layer. In compression tests at room temperature, yield stress in the rolling and width directions is appreciably lower than in the thickness direction. At 573 K, anisotropic and non-uniform deformation behavior disappeared. Thin sheet specimens taken from inner layers of the plates showed higher formability than those from the surface layer in deep drawing and stretch forming. It is concluded that the formability of magnesium alloy sheets can be improved by decreasing the severity of the basal plane texture.
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  • Mechanical Properties and Microstructure of Mg-Al-Zn-Si-base Alloy

    pp. 458-462

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    DOI:10.2320/matertrans.44.458

    Magnesium alloys containing Mg2Si particles, as a promising low cost heat-resistant magnesium alloy for automobile engine applications, are attracting more and more attention of both material scientists and design engineers. Refinement of the Chinese script Mg2Si particle is a key for using this kind of alloy in sand casting or permanent mould casting. In the present work, a new type of heat-resistant magnesium alloy with low cost based on Mg–Al–Zn–Si was developed. The effect of Sb addition and mischmetal (MM, a mixture of rare-earth, RE elements) microaddition to a Mg–Al–Zn–Si alloy was investigated systematically from the viewpoints of microstructure, solidification process, mechanical properties and corrosion resistance. Microstructure observation revealed that Sb promoted the formation of fine polygonal type Mg2Si particles by providing the nucleation sites. Meanwhile, the grain sizes of modified Mg–Al–Zn–Si–Sb alloy are much finer than those of base alloy. Such improved microstructure brings about the significant improvement in tensile properties, toughness and creep resistance at elevated temperatures up to 200°C, which is superior to or comparable to AE42 alloy. Moreover, the corrosion resistance of the new alloy is improved significantly by the MM microaddition.
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  • Mechanical Properties of Mg-Y-Zn Alloy Processed by Equal-Channel-Angular Extrusion

    pp. 463-467

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    DOI:10.2320/matertrans.44.463

    Fine-grained WZ73 magnesium alloy with the grain size of 1.6 μm was produced by Equal-Channel-Angular Extrusion. The material exhibited tensile yield strength of 293 MPa, tensile strength of 350 MPa and relatively large elongation of 18% at room temperature. The yield strength was almost unchanged up to 473 K. In addition, large superplastic elongation of over 300% was obtained at a temperature of 673 K.
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  • Effect of Microstructural Factors on Tensile Properties of an ECAE-Processed AZ31 Magnesium Alloy

    pp. 468-475

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    DOI:10.2320/matertrans.44.468

    Mg–3%Al–1%Zn (AZ31) alloy was subjected to ECAE (Equal Channel Angular Extrusion) processing under various processing conditions. Then tensile tests were carried out at room temperature to investigate the relationship between tensile properties and microstructural parameters that include grain size and the texture generated by ECAE processing. In 4-pass ECAE specimens processed at 523 K, tensile ductility is improved as a result of easy basal slip during tensile test along the extrusion direction, because such specimens have textures in which the basal plane is inclined at 45° to the extrusion direction. On the other hand, in the specimens processed at 573 K, 0.2% proof stress is higher than those of specimens processed at lower temperatures, but elongation is smaller. This is because of difficult basal slip caused by the textures in which the basal plane is oriented parallel to the extrusion direction. However, 8-pass specimens processed at 473 K and subsequently annealed, which have similar textures but different grain sizes (d), exhibit clear grain size dependencies of 0.2% proof stress (σ0.2) according to Hall-Petch relationship; σ0.2=30+0.17d−1⁄2. Therefore, crystallographic orientation has a profound effect on the tensile properties of AZ31 alloy, and grain size has a little effect.
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  • Microstructures and Tensile Properties of ECAE-Processed and Forged AZ31 Magnesium Alloy

    pp. 476-483

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    DOI:10.2320/matertrans.44.476

    In order to achieve same level of high strength and high ductility as 6061 aluminum forging alloy that is currently used for automobile applications, AZ31 magnesium alloy rod with a large diameter of 40 mm was subjected to ECAE-processing, and the microstructures and mechanical properties of the ECAE-processed specimens were investigated. Furthermore, automobile knuckle arm was produced by forging using the ECAE-processed material, and the mechanical properties of the forged product and their strain rate dependencies were investigated under impact tensile load conditions. 4pass-ECAE-processed specimen has fine and uniform microstructure and a texture whose basal planes are mainly parallel to the extrusion direction with some inclined at angles up to 45° to the extrusion direction. Therefore, they show high ductility even if the tensile direction is parallel to the extrusion direction. The knuckle arm forged using the ECAE-processed material exhibits high elongation even in the high strain rate region. Furthermore, the tensile strength, fracture elongation and absorption energy of the forged product increase with increasing strain rate and their values are higher than those of T6-treated 6061 aluminum forging alloy specified by JIS.
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  • Mechanical Properties and Blow Forming of Rolled AZ31 Mg Alloy Sheet

    pp. 484-489

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    DOI:10.2320/matertrans.44.484

    Rolling was conducted at 373–673 K for AZ31 Mg alloy; mechanical properties of the rolled Mg alloy were investigated by tensile and blow forming tests. The grain sizes of all the rolled specimens were smaller than that of the specimen prior to rolling. At tensile temperatures under 373 K, the rolled specimens showed much higher 0.2% proof stresses than the non-rolled specimens due to their fine-grained microstructure. However, the strength of the rolled specimens decreased significantly at 473 K. Superplastic behavior was obtained at 573–723 K for the specimens rolled at 498 K. Blow forming tests demonstrated that specimens rolled at 498 K exhibited a high degree of formability at 723 K.
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  • Influence of Grain Size on Elongation at Elevated Temperatures in AZ31 Mg Alloy

    pp. 490-495

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    DOI:10.2320/matertrans.44.490

    Mechanical properties of an AZ31 Mg alloy with the grain sizes of 4, 12, 60 and 450 μm were investigated by tensile tests at 473–673 K with 1.7×10−5–1.7×10−1 s−1. The Mg alloy exhibited unique behaviors of low elongation of 17% at 473 K with 1.7×10−1 s−1 for the specimen with the grain size of 450 μm and large elongation of 234% at 673 K with 1.7×10−5 s−1 for the specimen with the grain size of 60 μm. These behaviors could not be explained from the viewpoint of the plastic stability. Microstructural observation revealed significant twin formation at 473 K with 1.7×10−1 s−1 for the specimen with the grain size of 450 μm and active grain boundary sliding at 673 K with 1.7×10−5 s−1 for the specimen with the grain size of 60 μm. Therefore, it is likely that enhancement of twining and grain boundary sliding gave rise to the unique behaviors of the Mg alloy that could not be explained from the plastic stability.
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  • The Grain Size Dependence on Diffusion Bonding Behavior in Superplastic Mg Alloys

    pp. 496-503

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    DOI:10.2320/matertrans.44.496

    Applicability of the diffusion bonding was examined in a superplastic magnesium alloy, AZ31, on two different grain sizes of 28 and 11 μm. In order to investigate the superplastic behavior, the tensile test was carried out at the strain rates from 10−4 to 10−2 s−1 at elevated temperatures. These materials showed a superplastic behavior at 673 K. The diffusion bonding tests were carried out in the superplastic region, which is the pressure range from 2 to 10 MPa and for the times up to 10 h at 673 K in air. The post-bonded mechanical properties were estimated by the compression lap shear test in order to determine the optimal diffusion bonding conditions. The diffusion bonded specimens exhibited more than 0.8 of parent material strength at several conditions for both materials. The bonding time on fine grained AZ31 could achieve much faster than that on coarse grained AZ31. Using this result, the comparison was carried out experimental results and previous theoretical diffusion bonding models. Many researchers constructed the theoretical models based on the void growth mechanism, diffusional controlled process, to predict its optimal bonding time and pressure. However, the previous modes were not agreement with experimental result. It was resulted from the previous models include only diffusional controlled process. Therefore, in this study, we developed new theoretical diffusion bonding model both diffusional and plastic controlled processes. From the comparison, this model was good agreement with experimental. Using the theoretical diffusion bonding model and experimental results, the prediction map for high quality diffusion bonding of the superplastic magnesium alloys was suggested.
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  • High-purity Magnesium Coating on Magnesium Alloys by Vapor Deposition Technique for Improving Corrosion Resistance

    pp. 504-510

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    DOI:10.2320/matertrans.44.504

    Microstructures in coated magnesium alloy with high purity magnesium fabricated by applying a vacuum deposition technique were investigated. Moreover, relationships between microstructures in coated and un-coated magnesium alloys and corrosion behaviors were interpreted by in-situ laser microscopic observations during salt immersion tests. Magnesium with 3N-grade and AZ31 magnesium alloy were used for an evaporation source and a substrate for deposition. Temperature of the substrates was changed resulting in change in temperature profile in a furnace in order to optimize deposition coating conditions for obtaining homogeneous microstructures and thickness in deposited layer. The coated specimen revealed superior corrosion resistance to those on 3N–Mg, AZ31 and AZ91E alloys, and comparable to that on 6N–Mg in salt immersion tests using 3% NaCl solution at 300 K for 587 ks. In-situ observations showed that inhomogeneity in microstructures, such as second phases and grain boundary segregations, deteriorate corrosion resistance in magnesium alloys. Therefore, pure magnesium coated layer without inhomogeneity in metallographic and electrochemical meanings can improve the corrosion resistance on magnesium alloys.
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  • Surface Treatment of Magnesium Alloys by Artificial Corrosion-oxidization Method

    pp. 511-517

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    DOI:10.2320/matertrans.44.511

    A new technique has been proposed for improving the poor corrosion resistance in magnesium and its alloys. The specimens were immersed into solutions with high pH values, such as 10% NaOH, (1% NaCl + 10% NaOH) and (10% NaCl + 10% NaOH) solutions, at R. T. for 3.6 ks, and then heat treated in air at 673–773 K for 3.6 ks. Corrosion resistance of the specimens were evaluated by the time for occurring filiform corrosion, tf, in salt immersion test using 1% NaCl solution. Hydrogen bubble evolution at the early stage of the salt immersion test was suppressed by the corrosion-oxidization treatment, and also tf was prolonged by this treatment. On the non-treated specimen of AZ31 alloy, tf was about 1.7 ks, and on the specimen only heat-treated in air at 673 K without the primary immersion, tf was about 1.9 ks. On the other hand, when the (10% NaCl + 10% NaOH) solution was used for the primary immersion and then heated in air at 673 K for 3.6 ks, tf was prolonged up to about 35 ks, about 20 times longer than that in the non-treated specimen. It is considered that magnesium hydroxide formed on the surface of the specimen by the primary immersion treatment changed into magnesium oxide which protected the specimen from corrosion. Formation of magnesium oxide on both the surfaces of the specimens treated by the corrosion-oxidization method and heated without the primary immersion was confirmed by TEM observations. Microstructures in both the oxide layers were different one another, cohesion of the oxide formed directly from metallic magnesium seemed to be weak compared with that formed from magnesium hydroxide.
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  • Improvement of Corrosion Resistance and Adhesion of Coating Layer for Magnesium Alloy Coated with High Purity Magnesium

    pp. 518-523

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    DOI:10.2320/matertrans.44.518

    AZ31 magnesium alloy was coated with high purity magnesium by vapor deposition coating process at high temperature in order to improve the corrosion resistance. Interdiffusion of each element was observed at the interface during the deposition coating process. The high purity magnesium coating showed superior corrosion resistance in salt solution. Although corrosion resistance was improved dramatically by the vapor deposition coating process, there were a lot of voids and pin-holes at the interface and in the coating layer, respectively. Hot press and HIP processes were carried out to decrease the voids and pin-holes. They were disappeared completely by hot press and HIP processes, resulting in improvement of adhesion property. Moreover, corrosion resistance was also improved due to disappearance of voids and pin-holes. Besides, the high purity magnesium coating worked as sacrificial anode in the cross cut test, resulting in protecting the substrate.
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  • Tribological Properties of Magnesium Composite Alloy with In-situ Synthesized Mg2Si Dispersoids

    pp. 524-530

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    DOI:10.2320/matertrans.44.524

    The tribological properties of the magnesium composite alloys reinforced with solid-state synthesized Mg2Si or Mg2Si/MgO dispersoids are evaluated under wet conditions (in engine oil lubricants) by using pin on disc type wear test equipment. Every composite shows a dependence of the friction coefficient on the applied load, which corresponds to Sribeck diagram based on elasto-hydrodynamic lubrication (EHL). The increase of Mg2Si content causes the increase of both the friction coefficient and the total wear by plowing, because Mg2Si dispersoids protruded from the surface are much harder than S35C mild steel counter material. When including MgO dispersoids formed in deoxidizing SiO2 particles by magnesium, the friction coefficient reduces to 0.01–0.02 in oil lubricant. This is due to the “mild offensive effect” by MgO dispersoids which ease the attacking or plowing phenomenon on the counter material, because they are not so hard.
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  • Improvement of Wear Resistance of Magnesium by Laser-Alloying with Silicon

    pp. 531-538

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    DOI:10.2320/matertrans.44.531

    The surface modification of magnesium by laser alloying using a powder injection method was carried out for the purpose of improving its wear resistance. Silicon powder was used as the feeding powder. The applied CO2 laser power conditions were 2 kW-1pass and 2 kW-2pass, while the moving speed of the substrate was 8.3 mm·s−1 in all the cases. The silicon powder reacts easily with molten magnesium to form fine Mg2Si compound in the modified layer. Under both laser power conditions of 2 kW-1pass and 2 kW-2pass, the modified layer becomes thick as the powder feeding rate increases, and the modified layer obtained using 2 kW-2pass is thicker than that of 2 kW-1pass at all powder feeding rates. Furthermore, fine and homogeneously distributed Mg2Si compound crystallizes on the whole modified layer in the 2 kW-2pass. At the laser power condition of 2 kW-1pass, the area fraction of Mg2Si compound increases with an increase in the powder feeding rate, while at 2 kW-2pass, the area fraction of Mg2Si compound is almost the same regardless of the powder feeding rate. Wear resistance of the modified layer improves with increasing area fraction of Mg2Si compound held in the soft matrix of magnesium solid solution, while there is almost no wear of the pin. Coefficient of friction rapidly decreases within a few minutes and then remains virtually constant during sliding wear test. Microstructure of the wear-tested surface reveals that the soft substrate of magnesium is worn by pin at the initial stage and pits which subsequently serve as oil pockets are developed, resulting in the remarkable improvement of wear resistance.
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  • Microstructure and Voltage-Current Characteristics of Anodic Films Formed on Magnesium in Electrolytes Containing Fluoride

    pp. 539-545

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    DOI:10.2320/matertrans.44.539

    Formation behavior of anodic oxide films on magnesium in various electrolytes including fluoride was investigated with attention to the effects of anodizing voltage, pH and aluminum content. In the range of formation voltage between 2 V and 100 V, porous film was formed in alkaline fluoride solution associated with high current density at around 5 V and at breakdown voltage. The critical voltage of breakdown to allow maximum current flow was approximately 60 V and relatively independent on substrate purity. Barrier type films or semi-barrier type films, which were composed of hydrated outer layer and inner layer, were formed at the other voltages. A peculiar phenomenon of high current density at around 5 V, which may be caused by trans-passive state, was not observed for anodizing in acidic fluoride solutions such as Dow17 and ammonium fluoride. In the case of AZ91D, the critical voltage increased to 70 V and peculiar phenomenon at 5 V was not observed, so that only barrier films were formed at less than the critical breakdown voltage. When AlO2 ion was added in the electrolytes, the critical voltage remarkably increased and current density effectively decreased with increasing AlO2 content. The passivation effect of aluminum addition in the electrolytes is more remarkable than the addition in magnesium substrates. The depth profiles of constituent elements showed that aluminum migrated into oxide film to reach near oxide/substrate interface. Atomic ratio of aluminum to magnesium increased with increasing voltage to attain 0.42 at 80 V and crystalline MgAl2O4 and MgO were found in the film.
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  • Electrorefining of Magnesium in Molten Salt and Its Application for Recycling

    pp. 546-551

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    DOI:10.2320/matertrans.44.546

    Electrorefining of Mg has been investigated in a molten salt system, and the electrolysis conditions for the effective purification have been discussed. A purified mixture of MgCl2–NaCl–CaCl2 was used as an electrolytic bath. Magnesium metal was dissolved anodically by potentiostatic electrolysis, and purified Mg was electrodeposited at the cathode. A certain degree of cathodic overpotential was required for the effective electrodeposition of Mg metal, while large anodic overpotential directly caused the deterioration in the purity of Mg electrodeposit; it was necessary for the anodic overpotential to be less than 1.0 V for good purification. In addition to the electrode potentials, some factors affected the electrorefining of Mg metal. Under the suitable electrolysis condition, the Fe content in the Mg electrodeposit was less than 10 ppm. A couple of subjects on recycling Mg metal and its alloys have been also studied: purification of Mg alloy by an electrorefining technique and distinction of Mg alloys. It was shown that pure Mg metal was electrodeposited at the cathode by the electrorefining of Mg alloy. X-ray fluorescence analysis was applied to distinction of Mg alloys, and the measuring conditions were discussed. It was concluded that the electrorefining process and X-ray fluorescence analysis were usable for the recycling of Mg metal and alloys.
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  • Observation of Manganese-Bearing Particles in Molten AZ91 Magnesium Alloy by Rapid Solidification

    pp. 552-557

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    DOI:10.2320/matertrans.44.552

    Magnesium alloy AZ91 contains 0.1 to 0.3 mass% manganese as an alloying element. Such manganese always reacts with aluminum and produces various compounds. Actually, numerous Mn-bearing particles are visible in polished surfaces under an optical microscope. Some Mn-bearing particles are also expected to be present in molten alloy. We believe that some Mn–Al compounds are closely related to the superheating effect. However, it is not clear specifically what compounds exist in molten alloy and how contribute to grain refinement by superheating. In order to study this subject, we utilized a diffusionless process based on rapid solidification. Two grams of AZ91 alloy was melted in a stainless steel tube and then injected onto a copper wheel rotating at high-speed under various conditions to obtain cast ribbons. Cast ribbon was analyzed by an X-ray diffractometer, an electron probe micro-analyzer and a transmission electron microscope. These analyses indicated that the cast ribbons consist of single phase and that the structure is quite homogeneous, i.e., diffusionless solidification occurs due to rapid cooling. Manganese-bearing particles completely disappear in the melt around 963 K and superheat temperatures, which is inconsistent with presently accepted superheat mechanisms, the temperature-solubility nucleation theory and the temperature-phase relationship theory. In the meantime, cross-shaped Mn-bearing particles are often observed in the ribbons when the melt is cooled from superheat temperatures to the injecting temperature (973 K) before injecting. This treatment may facilitate the formation of these particles.
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  • Semi-Solid Processing of Magnesium Alloys

    pp. 558-561

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    DOI:10.2320/matertrans.44.558

    Semi-solid processing is an emerging technology for near net-shape production of engineering components. Although there has been significant progress in semi-solid processing of Al alloys, very limited information is available on semi-solid processing of Mg alloys, except for the thixomolding process. In semi-solid casting process, it is necessary to properly control the flow and solidification behavior of semi-solid slurries for high quality products. There are a number of parameters that affect the flow and solidification behavior of semi-solid slurries such as viscosity, casting pressure, shapes of gate and mold cavity, gate velocity, mold temperature, etc. In the present study, the effects of various thermo-mechanical treatments were investigated on the change in viscosity of the semi-solid AZ91D magnesium alloys by using a concentric cylinder type viscometer. The effects of gate velocity and thickness on mold filling behavior of the semi-solid AZ91D alloys were also investigated by using a high-speed camera and the results were compared with those obtained from computer simulations. From these results and microstructure examination, a processing map for high pressure die casting of the semi-solid AZ91D alloy was constructed in order to produce sound castings.
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  • Development of New Die-castable Mg-Zn-Al-Ca-RE Alloys for High Temperature Applications

    pp. 562-570

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    DOI:10.2320/matertrans.44.562

    New die-castable Mg–Zn–Al–Ca–RE alloys are developed and evaluated in order to determine their suitability for high temperature applications. The crystallization of an Al–Ca compound along the grain boundaries and Al–RE compounds both within the grains and along the grain boundaries helps to improve creep properties of the alloys. The creep resistance of diecast specimens of some of the investigated alloys is comparable to that of ADC12 aluminum alloy that is currently used for diecasting of automobile powertrain parts.
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  • Quenched and Annealed Microstructures of Hot Worked Magnesium AZ31

    pp. 571-577

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    DOI:10.2320/matertrans.44.571

    The microstructures of magnesium AZ31 are examined following hot compression testing and annealing. The grain size, fraction dynamically recrystallized and, in a couple of cases, the crystallographic texture are reported. The progress of dynamic recrystallization and the recrystallized grain size were sensitive to processing conditions, as expected. This effect was more marked in the former than in the latter, compared to other metals. It was also found that, for structures containing between 80 and 95% dynamic recrystallization, abnormal grain growth occurred during annealing. Irrespective of the whether or not abnormal grain growth occurred, the annealing step weakened the crystallographic texture.
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  • Solid Recycling of an AZ31 Mg Alloy with a Vapor Deposition Coating Layer of High Purity Mg

    pp. 578-582

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    DOI:10.2320/matertrans.44.578

    Solid recycling of AZ31 Mg alloy with vapor deposition coating layer of high purity Mg was evaluated. In the open die forging experiments, two AZ31 Mg alloy specimens with the pure Mg deposition coating layer of the 30 μm in thickness were forged. The specimens were sufficiently bonded by forging at 673 K. Furthermore, the elements (Al and Zn) of the AZ31 substrate diffused up to the center of the pure Mg deposition coating layer. The theoretical analysis in which only the lattice diffusion was considered showed that the elements in the AZ31 substrate cannot diffuse to the center of the pure Mg deposition coating layer. The grain boundary diffusion coefficient of Mg at 673 K is about 25 times larger than the lattice diffusion coefficient. Therefore, it is suggested that the grain boundary diffusion enhanced by grain refinement due to hot forging largely contributes to the solid state bonding of the forged specimens. Also, the solid recycled specimen was fabricated from the AZ31 Mg substrate with pure Mg deposition coating layer by hot extrusion at 673 K. The solid recycled specimen showed almost the same tensile properties as the virgin extruded specimen. This is probably related not only to the grain boundary diffusion enhanced due to grain refinement, but also severe plastic deformation by hot extrusion.
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  • The Electronic Structure of MgY2H6+δ High-Pressure Hydride

    pp. 583-588

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    DOI:10.2320/matertrans.44.583

    The electronic structure of the MgY2H6+δ high-pressure hydride has been investigated by means of the full potential linearized augmented plane wave method, and the stability of the octahedral and tetrahedral sites for H atoms has been discussed. The calculation models used were MgY3H8–12, corresponding to H/M = 2–3. From the calculation of the density of states for MgY3H11, it was found that the states originating from H-s and Y-d mainly formed valence and conduction bands, respectively. In the valence band, bonding states originating from H-s of tetrahedral sites, Y-d, Y-p, and Mg-p can be observed in a wide energy range, especially between −3∼−5 eV below the Fermi level. In addition, the contour plots of the valence electron charge density revealed that the bonding between Y and H atoms on the tetrahedral sites was predominantly covalent, while the H atom on the octahedral site showed a weak interaction with Y and H atoms on the tetrahedral sites. The cell volume optimization indicated that the calculated equilibrium volume linearly increased with increasing the number of vacancies on the octahedral sites, while it decreased in the case of introducing the vacancy on the tetrahedral site. From these observations, it can be concluded that the H atoms on the tetrahedral sites seem to be responsible for holding its crystal structure, while ones on the octahedral sites had a certain level of contribution to enhancing the bond strength. These observations support the experimental results that the crystal structure of MgY2H6+δ is stable even after the partial desorption of hydrogen, which presumably occupy the octahedral sites, at around 600 K.
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  • Improvement of Protium Absorption/Desorption Characteristics of Mg-x mass%LaNi5 (x=50, 70) Composites by Interface-control

    pp. 589-594

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    DOI:10.2320/matertrans.44.589

    An attempt is made to form composites of pure magnesium that has high hydrogen capacity of about 7.6 mass% and LaNi5 alloy that is capable of protium absorption/desorption at relatively low temperatures by mechanical alloying (MA) for the utilization of synergy effect to acquire both high capacity and ability to absorb/desorb protium at low temperatures. The Mg–LaNi5 composites have a well-bonded and uniform structure. The maximum protium content of a specimen of mechanically-alloyed Mg–50 mass%LaNi5 composite is 2.77 mass% at 40°C. Furthermore, the maximum protium content of a specimen of mechanically alloyed Mg–70 mass%LaNi5 composite is 1.81 mass% at 40°C, and the protium absorption kinetics of the composite become fast at the initial region of the hydriding curve. Consequently, the results suggest the occurrence of synergy effect during the absorption process since the hydrogen capacity of LaNi5 phase of the composites is lower than 2.77 mass% and 1.81 mass% at 40°C and Mg cannot absorb protium at a relatively low temperature of 40°C.
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  • Fabrication of Porous Magnesium Alloys by Pulse Electric Current Sintering Process Using Machined Chips

    pp. 595-600

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    DOI:10.2320/matertrans.44.595

    Magnesium based porous materials are made by Pulse-Electric-Current-Sintering (PECS) method using cut chips. Commercial ingots of AZ91D and AM60B alloys and extruded AZ31 alloy were selected as raw materials in order to change the amount of eutectic compounds and the range of semi-solid temperatures. Porous materials of various plateau stresses could be made by PECS process depending on pore ratio, alloy composition and sintering temperature. In well-joined parts of porous samples of AZ91D and AM60B alloy, Mg–Al system compounds crystallize, while the aluminum content at the joined part of AZ31 porous sample is concentrated. This means that the non-equilibrium solidified Mg–Al compound is preferentially remelted by rapid heating during PECS. Thus the difference in the microstructures of the joined parts is caused by the different aluminum contents of the alloys. The condition for making the well-joined porous materials using PECS process is to use alloys that have large amount of low melting compounds and large semi-solid temperature range. The plateau stresses of the porous materials investigated in this study are in the range from 1.3 to 37.3 MPa and pore ratio is the most important factor affecting plateau stress.
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  • Solid State Synthesis of Non-Equilibrium Phase in Mg-Co and Mg-Fe Systems via Bulk Mechanical Alloying

    pp. 601-610

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    DOI:10.2320/matertrans.44.601

    Mg–Co and Mg–Fe systems were employed as a candidate hydrogen storage alloy. Different from Mg–Ni system, there exist no line compounds of Mg2Co and Mg2Fe. Non-equilibration of these compounds is indispensable to make solid state synthesis. Bulk mechanical alloying was applied to this non-equilibration of Mg2Co with success. Planetary ball milling was also utilized to discuss the process efficiency of bulk mechanical alloying. In particular, the on-line monitored energy density was used to describe the homogeneous refining and solid-state reaction with increasing the number of cycles. Through SEM observation of intermediate phase change, the solid-state reaction commences when the total energy density exceeds the critical limit. SEM/EDX and XRD analyses assured that the synthesized non-equilibrium phase should be Mg2Co. The Goldschmidt-factor analysis was used to determine that the synthesized Mg2Co has mainly fcc-structure. No significant change of XRD profiles was observed even when increasing the holding temperature. This Mg2Co is quasi-stable, non-equilibrium phase even at the elevated temperature. In case of Mg–Fe system, the initial elemental particle mixture was homogeneously refined. Under the similar condition to the solid-state synthesis of Mg2Co, however, Mg2Fe was not synthesized even via bulk mechanical alloying. Through precise analysis, non-equilibrium phase with high iron content was recognized, so that non-equilibration via the bulk mechanical alloying might well be effective to investigate the solid state synthesis of binary compounds even in Mg–Fe system.
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  • Magnesium Matrix Composite with Solid-state Synthesized Mg2Si Dispersoids

    pp. 611-618

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    DOI:10.2320/matertrans.44.611

    For improvement of the mechanical properties of magnesium alloys, the material and process design for MMC was established to disperse solid-state synthesized Mg2Si in the magnesium alloys, in employing the elemental magnesium and silicon powder mixture. In the repeated plastic working on the raw mixture for the refinement of silicon particles and their uniform distribution in the AZ31 primary powder, Mg2Si/Mg composite alloys show fine microstructures and superior mechanical properties: UTS is about 350 MPa and YS is over 300 MPa. This is because of fine Mg2Si dispersoids with a mean particle size less than 5 μm as well as refined matrix grain of 2–5 μm. The specific tensile strength is much higher than that of the conventional magnesium and aluminum alloys such as AZ31, AZ91 and 2014(T6) alloys. This material and process design is possible to apply to the mass-production process in using a large scale manufacturing equipments.
  • Precipitation in Mg-(4-13)%Li-(4-5)%Zn Ternary Alloys

    pp. 619-624

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    DOI:10.2320/matertrans.44.619

    Precipitations in Mg–Li–Zn ternary alloys containing 4 to 13%Li and 4 to 5%Zn (in mass%) with α or β single phase, or with (α+β) dual phases were investigated using a micro-Vickers hardness measurement and transmission electron microscopy. Age hardening in the α phase alloy was found to occur, which was attributed to the precipitation of the stable θ (MgLiZn) phase with the following orientation relationships: [10\\bar10]α||[110]θ, (0001)α||(1\\bar1\\bar1)θ. In the (α+β) phases alloy, the precipitation of the α phase together with the metastable θ′ (MgLi2Zn) phase occurred at grain boundaries between the α and β, and also β and β grains. The orientation relationships between the α and θ′ were as follows; (0001)α||(01\\bar1)θ′, [0\\bar110]α||[111]θ′. Age hardening in the β alloy was caused by the precipitation of the θ′ phase and over-aging was attributed to the precipitation of the α and θ phases.
  • Evaluation of Silicon Twinning in Hypo-Eutectic Al-Si Alloys

    pp. 625-628

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    DOI:10.2320/matertrans.44.625

    It is generally accepted that growth of eutectic silicon in aluminium–silicon alloys occurs by a twin plane re-entrant edge (TPRE) mechanism. It has been proposed that modification of eutectic silicon by trace additions occurs due to a massive increase in the twin density caused by atomic effects at the growth interface. In this study, eutectic microstructures and silicon twin densities in samples modified by elemental additions of barium (Ba), calcium (Ca), yttrium (Y) and ytterbium (Yb) (elements chosen due to a near-ideal atomic radii for twinning) in an A356.0 alloy have been determined by optical microscopy, thermal analysis, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Addition of barium or calcium caused the silicon structure to transform to a fine fibrous morphology, while the addition of yttrium or ytterbium resulted in a refined plate-like eutectic structure. Twin densities in all modified samples are higher than in unmodified alloys, and there are no significant differences between fine fibrous modification (by Ba and Ca) and refined plate-like modification (by Y and Yb). The twin density in all modified samples is less than expected based on the predictions by the impurity induced twining model. Based on these results it is difficult to explain the modification with Ba, Ca, Y and Yb by altered twin densities alone.
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  • Development of Thermal Conductivity Prediction System for Composites

    pp. 629-632

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    DOI:10.2320/matertrans.44.629

    For composites, it is possible to control their effective thermal conductivity through selecting the component materials and designing the structure. For this purpose, a computer system that can help a designer to find the optimum solution of materials and structure is necessary. In this work, we have developed a high-speed and light-weight thermal conductivity evaluation engine, basing on analytical solutions of effective thermal conductivity of composites. A package of calculation modules corresponding to the analytical solutions for different structure models is developed. As input data, the constitution of a composite is described by a file in XML data format. The engine analyzes the file, recognizes the structure model, and automatically selects a proper calculation module for the composite. The evaluation engine is connected to a materials database, which is used to stores and provides information of component materials. The calculation result of a designed composite can also be stored in the database for sharing and reusing. This system is expected to be used as an effective decision support tool for composites design.
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  • An Experimental Investigation of Effects of Specimen Size Parameters on Compressive and Tensile Properties in a Closed Cell Al Foam

    pp. 633-636

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    DOI:10.2320/matertrans.44.633

    Effects of specimen size parameters, i.e. the specimen volume and the ratio of cross sectional dimension to cell size, on compressive and tensile properties were investigated in a closed cell Al foam. For compressive tests, the stress in a plateau region decreased with decreasing specimen volume. This is likely because constraint of cell walls at the free surface is reduced with decreasing specimen volume. The scatter of the stress was large for the small volume specimens. The critical ratio of cross sectional dimension to cell size was 4 for negligible scatter of the stress. For tensile tests, the ultimate tensile strength decreased, the elongation to failure increased and the work hardening coefficient decreased with decreasing specimen volume. It is suggested that the reduced constraint of cell walls at the free surface by decreasing specimen volume affects the tensile strength as well as the compressive strength.
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  • The Extraction of Sm from Sm-Co alloys by the Glass Slag Method

    pp. 637-640

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    DOI:10.2320/matertrans.44.637

    The use of the glass slag method in the extraction of Sm from Sm–Co alloys was studied. The magnetic SmCo5 phase decomposed into Sm oxide phase and Co phases by the glass slag method. The Sm oxide phase was extracted by the surrounding molten glass slag materials in the glass slag method. The resultant alloys consisted of neither SmCo5 phase nor Sm oxide phase. In the glass slag method, the Sm–Co alloys were separated into Sm-containing glass slag material and a Co–B alloy. The glass slag method was suitable for the extraction of samarium from the Sm–Co alloys as was the case for the extraction of neodymium from the Nd–Fe–B alloys.
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  • Hydrogen Isotope Effects on Absorption Properties of Ti-Cr-V Alloys

    pp. 641-644

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    DOI:10.2320/matertrans.44.641

    Ti–Cr–V alloys are known to absorb protium (hydrogen atom) up to H/M = 2, while Cr-rich alloys absorb up to H/M = 1 because of the formation of mono-protides (mono-hydrides). However, few hydrogen isotope effects of the Ti–Cr–V alloys have been reported. This paper aims to clarify the hydrogen isotope effects on the absorption properties of the Ti–Cr–V alloys in a composition range near the boundary between the appearance regions of mono- and di-protides. It was found that the appearance region of the mono-dueteride was Cr-richer composition than that of the mono-protides. The absorption plateau pressure for deuterium was lower than that for protium in the appearance region of the di-protide and the di-deuteride, but higher in the appearance region of the mono-protides and the mono-deuteride. The hydrogen isotope effects on the plateau pressure were found to cause the difference in the appearance regions of the protides and the deuterides.
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  • Greenness Assessment of Products in PLCA by DEA Approach

    pp. 645-648

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    DOI:10.2320/matertrans.44.645

    How to assess the greenness of products from the viewpoint of LCA (Life Cycle Assessment) is one of the important issues in PLCA (Products Life Cycle Analysis). In this report, we present a non-uniform assessment method for greenness assessment of products based on DEA (Data Envelopment Analysis) model after analyzing the shortcomings of the existing comprehensive assessment methods for green products. This method not only assesses the greenness of given products with regard to technical reference product constructed by means of the information of products, but also illustrates how to improve the greenness of products quantitatively by utilizing the project theory on DEA efficient frontier of DMU (Decision Making Unit). Finally we take a numerical example concerning greenness assessment of refrigerators to show how to use the established method and prove its availability.
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  • Preparation and Characterization of Electrodeposited Fe-Pd Binary Alloy Film

    pp. 649-652

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    DOI:10.2320/matertrans.44.649

    Fe–Pd binary alloy films with various alloy compositions (from 7.8 at% Pd to 77.1 at% Pd) were electrodeposited in a novel bath containing the ammonia solution and the ammonium tartrate as complexing agents by changing the bath composition and the current density. The chemical states of the deposited films were analyzed by using ESCA, and the crystallographic structures of deposited films with wide alloy compositions were determined by using XRD and HRTEM. The result indicated that the crystallographic structures of deposited films are the α-Fe solid solution when the Pd content is below 14.3 at% Pd, the mixture of α-Fe solid solution and (γ-Fe, Pd) solid solution with face-centered cubic structure when the Pd content is from 28.5 to 61.6 at% Pd, and the (γ-Fe, Pd) solid solution when the Pd content is from 69.5 at% to 77.1 at%.
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  • Preparation and Characterization of Electrodeposited Co-Pt Binary Alloy Film

    pp. 653-656

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    DOI:10.2320/matertrans.44.653

    Co–Pt alloy films were electrodeposited from a novel bath which was designed in the present study containing the ammonia solution and ammonium tartrate as complexing agents. The Co–Pt deposited films were prepared with wide alloy compositions ranging from 16.5 at% Pt to 56.8 at% Pt by changing the bath concentration and current density. The structures of these deposited films were analyzed by means of XRD and HRTEM. The results indicated that the crystallographic structure of Co–Pt deposited film are the ε-Co solid solution when the platinum content in deposited film is below 31.1 at%, the mixture of ε-Co solid solution and (α-Co, Pt) face centered-cubic solid solution when the platinum in the deposited film is ranging from 35.7 at% to 45.7 at%, and the (α-Co, Pt) face centered-cubic solid solution when the platinum in the deposited film is ranging from 51.1 at% to 56.8 at%. Moreover, the ordered fct phase (L10) was detected with the help of XRD from Co–Pt deposited film with 51.1 at% Pt which was annealed in vacuum with pressure no lower 10−5 Pa for 60 min at the temperature of 450°C.
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  • Microstructures and Mechanical Properties of Porosity-Graded Pure Titanium Compacts

    pp. 657-660

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    DOI:10.2320/matertrans.44.657

    Microstructures and mechanical properties of porosity-graded Ti compacts were investigated in this study. To fabricate the porosity-graded compacts, Ti powders with three different particle sizes, 65, 189 and 374 μm were prepared by the plasma rotating electrode process (PREP) and the gas atomization process, and these powders were sintered with and without applied stress. Porosity and pore size of porosity-graded Ti compacts decrease significantly with decreasing initial powder size and by applying stress. The most porous layer (374 μm) is severely deformed in compression tests compared to other layers. The compressive strength of porosity graded-compacts is found to be in fairly good agreement with the strength of most porous layer. Bend strength of compact sintered at 1223 K and 1 MPa (223.3 MPa) shows a higher value than that of human bone (156.9 MPa).
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  • Magnetocrystalline Anisotropy in a Single Crystal Fe-Ni-Ga Ferromagnetic Shape Memory Alloy

    pp. 661-664

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    DOI:10.2320/matertrans.44.661

    In order to discuss the advent of the magnetic-field-induced strains for a single crystal Fe22.0Ni51.5Ga26.5 shape memory alloy, the magnetic easy axis in the 14 M martensite phase and the magnetocrystalline anisotropy constant K have been investigated. From the data on the spontaneous thermal expansion and the magnetization curves, the magnetic easy axis of the 14 M martensite phase was determined to be the [010]14 M. The value of K in the martensite phase was estimated to be 1.3×105 J/m3, implying to exhibit a large value of the magnetic-field-induced strain.
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  • Large Magnetostriction of Fe-29.6 at%Pd Alloy Ribbon under Tensile Stress

    pp. 665-668

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    DOI:10.2320/matertrans.44.665

    Rapidly solidified Ferromagnetic shape memory alloy Fe–29.6 at%Pd ribbon has large magnetostriction of 1000 ppm (0.1%). The strain is caused by martensite twin’s movements induced by magnetic field. In order to determine whether the ribbon is useful for a sensor/actuator material, we investigated a behavior of magnetostriction of Fe–29.6 at%Pd ribbon under tensile stress. Three samples investigated are (1) melt-spinning single-roll ribbon (70 μm), (2) melt-spinning twin-roll ribbon (100 μm), (3) the layered structure of 4 ribbons (280 μm). The magnetostriction for three samples has a maximum, −250 ppm under tensile stress 15–20 MPa, which make to rearrange martensite twin boundary. The results show that rapidly solidified Fe–29.6 at%Pd alloy ribbon should be useful as sensor/actuator materials under low tensile stress.
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  • Magnetic Domain of Fe73.5Cu1Nb3Si13.5B9 Studied by Electron Holography Coupled with Thickness Mapping Method

    pp. 669-672

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    DOI:10.2320/matertrans.44.669

    Magnetic flux in soft magnetic alloy Fe73.5Cu1Nb3Si13.5B9 was investigated by electron holography coupled with a thickness mapping method. First the reconstructed phase image of a soft magnetic material with a simple wedge shape was simulated, and the contribution of both magnetic flux and inner potential to the reconstructed phase image was estimated. In the experiment, the magnetic flux was evaluated by removing the effect of inner potential with thickness mapping. The magnetic flux density of Fe73.5Cu1Nb3Si13.5B9 obtained was 1.21 T which agreed well with that of a bulk specimen (1.28 T).
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  • Evaluation Technique of the Hardness and Elastic Modulus of Materials with Fine Microstructures

    pp. 673-676

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    DOI:10.2320/matertrans.44.673

    Quantitative data of mechanical properties such as hardness, H, and elastic modulus, E, are required for the constituent phases of an alloy in the process of alloy design. To meet the needs, the evaluation technique of H and E of phases in composites through nanoindentation tests is proposed. Moreover, H and E of Nb solid solution (NbSS) and niobium silicide (Nb5Si3) phases in Nb-base in-situ composites were characterized by the proposed method. To clarify the quantitative relationship between the nanohardness, Hn, and the micro-Vickers hardness, Hv, nanoindentation tests were carried out on the Hv Standard blocks with Hv100, 500, 700, 900 and 1600, under a wide range of applied loads from 0.1 to 40 mN. As a result, it was clarified that Hv and Hn are linearly related under each applied load. Therefore, Hv could be estimated from Hn by applying the linear relation. It was also confirmed that the elastic modulus is almost independent of the applied loads. Therefore, the elastic modulus, E, could also be directly estimated by nanoindentation tests with Poisson’s ratios of tested materials. Hv and E of NbSS and Nb5Si3 in the Nb-base composites determined by the method show good agreement with the reported values for both phases. Accordingly, it is possible to conclude that the proposed method is useful to quantitatively evaluate the hardness and elastic modulus of constituent phases in a composite.
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  • Crystal Structure of Fe-N Clusters Prepared by Plasma-Gas-Condensation

    pp. 677-680

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    DOI:10.2320/matertrans.44.677

    Fe–N clusters were prepared by a plasma-gas-condensation cluster deposition apparatus at various nitrogen gas flow rate RN2, and their crystal structures were investigated by transmission electron microscopy. For RN2>2.2×10−7 mol/s, fcc single-phase FeN clusters are obtained and their lattice parameter is a=0.428 nm, being close to that (a=0.433 nm) of ZnS-type FeN films. When RN2≥7.5×10−7 mol/s, almost all clusters are of a tetrahedron shape with cluster sizes of d=8–25 nm. This reveals that the tetrahedron shape of FeN compound clusters is stable in such small sizes, implying a low (111) surface energy and/or high elastic strain energy and twin boundary energy compared with pure metal clusters with fcc structure.
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  • Fracture Toughness Evaluated by Indentation Methods and Its Relation to Surface Energy in Silicon Single Crystals

    pp. 681-684

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    DOI:10.2320/matertrans.44.681

    Fracture toughness of silicon crystals has been investigated by indentation methods, and their surface energy has been calculated using molecular dynamics (MD). When a conical indenter was forced into a (001) silicon wafer at room temperature, {110} cracks were mainly introduced from the indent, indicating that fracture occurs most easily along the {110} plane among the crystallographic planes of the ⟨001⟩ zone. To confirm this orientation dependence, surface energies for those planes were computed using molecular dynamics. The surface energy calculated exhibits the minimum value of 1.50 J·m−2 at the {110} plane, and it increases up to 2.26 J·m−2 at the {100} plane. Fracture toughness was derived from these computed surface energies, and it was shown that KIC value for the {110} crack plane was the minimum among those for the planes of the ⟨001⟩ zone. This result is in good agreement with that obtained by indentation fracture (IF) methods, although the absolute KIC values evaluated by the IF method were larger than those obtained by the calculation.
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  • Partial Sulfurization of Laser-ablated Titanium Oxide Film for the Improvement in Photocatalytic Property

    pp. 685-687

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    DOI:10.2320/matertrans.44.685

    Photocatalytic titanium oxide (TiO2) thin films were prepared on quartz substrate by the pulsed laser deposition using Nd:YAG (λ=1.064 μm) pulse laser. Subsequently, the films were sulfurized in H2S or CS2 atmosphere under various conditions. After the sulfurization of the film in H2S or CS2 at 1273 K for 1 h, the surface morphology and structure of the deposited TiO2 film were changed. The photocatalytic property of the film might be improved by the sulfurization using CS2 at the temperatures lower than 1273 K.
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  • Temperature Dependence of Inward Diffusion of Underpotentially-Deposited Cd into a Au (100) Bulk Phase under Electrochemical Environment

    pp. 688-691

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    DOI:10.2320/matertrans.44.688

    We investigated the inward diffusion of underpotentially-deposited (UPD) Cd into a Au (100) substrate in 50 mol m−3 sulfuric acid solution under potential control by anodic voltammetry at the temperature range from 295 to 333 K, as well as by the electrochemical quartz crystal microbalance (EQCM) analysis and X-ray diffractmetry (XRD). The anodic voltammograms show that inward diffusion of the Cd adatoms into a Au (100) bulk phase is classified into two types of steps. One is a very fast step, which is the diffusion of UPD Cd into a Au (100) electrode at the surface layer; the activation energy of this step is ≅70 kJ mol−1. The other is a much slower step; the activation energy of this step (110 kJ mol−1) agrees well with that of Cd bulk diffusion into a Au–Cd compound. The amount of UPD Cd is calculated from EQCM analysis, and it nearly agrees with that obtained from anodic voltammetry. We found that the amount of UPD Cd increases with temperature. We confirmed the formation of β Au–Cd layer on the sample by XRD after maintained at the UPD region of −1100 mV vs. Hg/Hg2SO4 for 1 h at 333 K.
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  • Effects of Boron-strontium Interactions on Eutectic Modification in Al-10 mass%Si Alloys

    pp. 692-695

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    DOI:10.2320/matertrans.44.692

    The effects of boron and strontium interactions on the eutectic silicon in hypoeutectic Al–Si alloys have been studied. Samples were prepared from an Al–10 mass%Si base alloy with different Al–B additions, alone and in combination with strontium. In alloys containing no strontium, boron additions do not cause modification of the eutectic silicon, while in strontium containing alloys, boron additions reduce the level of modification of the eutectic silicon. Thermal analysis parameters and eutectic silicon microstructures were investigated with respect to the Sr to B ratio. In order to modify the eutectic silicon, a Sr/B ratio exceeding 0.4 is required.
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  • Hydrogen Absorption of Oxide Composite Materials Prepared from Melt-Spun Zr-Pd-Ni Alloys

    pp. 696-699

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    DOI:10.2320/matertrans.44.696

    Melt-spun Zr65Pd35−xNix (x=0–35) amorphous alloys were produced by the single-roller melt-spinning technique and then oxidized at 553 K (x=0) or 673 K (x=5–35) in air. Hydrogen absorption of those oxide specimens was measured at 323 K by the conventional volumetric technique (Sieverts method). There was a tendency that the plateau of Pd disappeared and the maximum hydrogen absorption content (at about 5 MPa) decreased with increasing Ni (decreasing Pd) content in the starting alloys in the composition range of precursors, x=0–25 as shown in the P-C-T curves. However, hydrogen absorption content of the oxidized specimen prepared from precursors of x=27.5–35 peculiarly increased and the oxidized specimen prepared from the precursors of x=32.5 showed the largest hydrogen absorption of about 1.8 mass%H among the series of specimens used in this study. Moreover, comparing XRD patterns of these oxidized specimens obtained before and after hydrogen absorption, it can be known that the mixed microstructure of ZrO2 and PdO appears before hydrogen absorption in the oxidized specimens prepared from Zr65Pd35−xNix (x=0–25) precursors. On the contrary, in the oxidized specimen prepared from Zr65Pd5Ni30 precursor, Zr3NiO and NiO oxides appear before hydrogen absorption and their peaks move to lower angle after hydrogen absorption due to the lattice expansion by hydrogenation while ZrO2 phase did not appear so much. The significant behavior of the oxidized specimen prepared from Zr65Pd35−xNix (x=27.5–35) alloy precursors is related to the outstandingly different microstructure produced in this specimen. The authors expect these specimens to be used not only as hydrogen storage materials but also as catalysts in future.
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  • An Analysis of Crystallization Process in Amorphous Alloys Using Time-Scaling Factor

    pp. 700-704

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    DOI:10.2320/matertrans.44.700

    We have investigated the time-scaling properties of the isothermal crystallization process for Cu66Ti34 and Zr65Cu35 amorphous alloys by differential scanning calorimetry. Local atomic structures have also been studied by extended X-ray absorption fine structure (EXAFS) measurements. The results are discussed in comparison with the previous results obtained for metal-metalloid amorphous alloys. The time-scaling factor is defined as the time when the crystallization has reached half completion. By rescaling the time axis for each annealing temperatures, the crystallization curves measured at various temperatures for each alloy can be superimposed on a single curve in each case. The Williams-Landel-Ferry formula based on a free-volume consideration gives a universal function for the temperature dependence of the time-scaling factor for all the alloys. This suggests that we have to take into account the relaxation process occurring in the amorphous phase during the crystallization. The WLF analysis reveals that there exists a significant difference in the temperature dependence of the scaling factor between the metal-metal and metal-metalloid amorphous alloys. From the EXAFS study, such difference is considered to be due to the local structural differences between those two types of amorphous alloys.
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  • Effect of Sodium Hypophosphite on the Structure and Properties of Electrodeposited Ni-W-P Alloys

    pp. 705-708

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    DOI:10.2320/matertrans.44.705

    Amorphous and nanocrystalline Ni–W–P alloy films have been prepared by electrodeposition. The effect of sodium hypophosphite in the bath is important in determining the W and P contents of the alloy. The alloys prepared in the bath containing more than about 0.15 mol/L of sodium hypophosphite are only Ni–P alloys containing high P contents (>21% (at)) and no W at all. However, as the sodium hypophosphite content is decreased, the W content in the alloy increases and reaches about 20% (at) with decreasing P content to about 1% (at) at 6×10−4 mol/L of sodium hypophosphite. The alloys containing high P contents are amorphous, while those containing high W contents have a nanocrystalline phase with apparent grain size of 2.3 nm at 20% (at) W content. These nanocrystalline alloys show high hardness values up to about 780 MVH.
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  • Study of the Crystallization Kinetics of Zr60Al15Ni25 Bulk Glassy alloy by Differential Scanning Calorimetry

    pp. 709-712

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    DOI:10.2320/matertrans.44.709

    This paper investigates the crystallization kinetics of Zr60Al15Ni25 bulk glassy alloy under isothermal annealing by differential scanning calorimetry (DSC). The results show that the incubation time is very small even at low annealing temperature (743 K) and increases slightly with the annealing temperature decreasing, which suggests a growth-controlled crystallization process. However, the exothermic peak width, reflecting the time for the actual crystallization process, increases greatly when annealing temperature decreases. The Avrami exponents indicate that the crystallization mechanism changes at different annealing temperatures. The change of the crystallization mechanism is attributed to the great difference of the mobility ability of atoms at different temperatures.
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  • Influence of Frequency on High-Temperature Fatigue Behavior of 17-4 PH Stainless Steels

    pp. 713-721

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    DOI:10.2320/matertrans.44.713

    The effect of frequency (2 and 20 Hz) on the high-cycle fatigue and fracture behavior was investigated at 573–773 K for 17-4 PH stainless steels in three conditions: Condition A (unaged), H900 (peak-aged) and H1150 (overaged). S–N results indicated that at 573 and 673 K, there was generally no difference in fatigue strength between 2 and 20 Hz, except for H900 at 673 K where the fatigue strength at 2 Hz was lower than that at 20 Hz. At 773 K, the fatigue strength of each condition at 2 Hz was lower than that at 20 Hz due to the occurrence of creep mechanism at this low frequency. At 773 K and 2 Hz, the fatigue fracture mode exhibited a mixed mode involving transgranular and intergranular cracking and the grain boundary cavities were also observed. At a given temperature and frequency, the fatigue strength for the three conditions generally took the following order: H900 > Condition A > H1150, except for Condition A at 773 K in the long life regime where the fatigue strength was close to that of H1150 due to a precipitate-coarsening effect. With the exception of Condition A tested at 673 K, the fatigue strength of each condition was decreased with increasing temperature as a result of a reduction in yield strength. At both frequencies, the fatigue strength of Condition A at 673 K was greater than that at 573 K as a result of an in-situ precipitation-hardening effect. Fractography observations indicated that the fatigue crack initiation site, crack propagation path and fracture surface morphology were functions of testing temperature, loading frequency and applied cyclic stress level.
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  • Carbothermic Reduction of MgO by Microwave Irradiation

    pp. 722-726

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    DOI:10.2320/matertrans.44.722

    A new method has been developed to produce magnesium vapor for deoxidation or desulfurization of molten iron. Microwave heating of magnesium oxide and graphite powder mixtures was carried in air or argon flowing atmosphere, utilizing a commercial microwave oven operated at 2.45 GHz. Progress of carbothermic reduction of MgO was observed, and the influences of morphology and carbon content of the samples on the heating behavior and the fractional reduction of MgO were investigated. Particularly, the initial graphite particle size was found as an important factor for heating and reduction behavior. Also, MgO–C brick was subjected to the microwave treatment and was found to be a candidate as Mg sources.
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  • In-situ Electrochemical Atomic Force Microscopy with Atomic Resolution of Ni(110) in Neutral and Alkaline Aqueous Solution

    pp. 727-730

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    DOI:10.2320/matertrans.44.727

    Bare and anodically oxidized Ni(110) surfaces in 0.05 kmol m−3 Na2SO4 (pH = 6.5) and 0.01 kmol m−3 NaOH (pH = 12) aqueous solution have been investigated by in-situ electrochemical atomic force microscopy (EC-AFM) with atomic resolution. We have succeeded in in-situ observation of unreconstructed Ni(110)-(1×1) structures in both 0.05 kmol m−3 Na2SO4 and 0.01 kmol m−3 NaOH solution. Under passive region, we have observed well-ordered structures differing from those of the bare surfaces and we found that surface structure of anodically oxidized Ni(110) in 0.05 kmol m−3 Na2SO4 solution agrees with NiO(110), whereas that in 0.01 kmol m−3 NaOH solution agrees with β-Ni(OH)2 (0001). These EC-AFM observations reveal the following orientation relationship of the surface structures on the anodic oxide layers and the substrates: NiO(110) [001] || Ni(110) [001] and NiO(110) [1\\bar10] || Ni(110) [1\\bar10] in neutral solution, and β-Ni(OH)2(0001) [11\\bar20] || Ni(110) [001] and β-Ni(OH)2(0001) [1\\bar100] || Ni(110) [1\\bar10] in alkaline solution.
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  • Coatings of Nb-based Alloy by Cr and/or Al Pack Cementations and Its Oxidation Behavior in Air at 1273-1473 K

    pp. 731-735

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    DOI:10.2320/matertrans.44.731

    An Nb–5Mo–15W alloy (mass%) was surface-treated in vacuum by Cr and/or Al pack cementations, and the oxidation behavior of the coated alloys was investigated in air at temperatures up to 1473 K for 32.4 ks. The Al packed alloy formed a coating layer with the outer NbAl2 and the inner Nb3Al, and showed poor oxidation resistance. The Cr packed alloy formed a coating with outer α-Cr and inner Cr2Nb layer, and showed good oxidation resistance up to 1373 K. The alloy packed with Cr and then Al formed a coating layer with outermost α-Cr(Al), outer α-Cr(Nb), and inner Cr2Nb layers, and this coated alloy showed good oxidation resistance up to 1473 K by forming an Al2O3 scale.
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  • High Temperature Oxidation of Y2O3 Partially-Stabilized ZrO2 Composites Dispersed with Ni Particles

    pp. 736-742

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    DOI:10.2320/matertrans.44.736

    High temperature oxidation of Y2O3-partially stabilized ZrO2 composites dispersed with Ni particles was investigated at temperatures ranging from 873 to 1073 K in air. Cracks between Ni particles forms parallel to the surface during high temperature oxidation. Thickness of the cracked zone increases proportionally with increasing oxidation time. A model for the growth of the cracked zone was established as a basis of the oxidation of Ni particles by permeation of oxide ions through the Y2O3-partially stabilized ZrO2 matrix, which is rate-controlled by hole conduction. Growth behavior calculated by the model is in good agreement with the experimental results.
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  • Microstructure of Plasma-Sprayed Cast Iron Splats with Different Particle Sizes

    pp. 743-748

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    DOI:10.2320/matertrans.44.743

    The superior wear-resistant property of cast irons is closely linked with their microstructure, in which graphite formation in plasma-sprayed cast iron coatings causes distinct characteristics owing to its self-lubricating property. Since the solidification rate generally affects graphite formation, the optimum in spray parameters such as substrate temperature, ambient pressure, particle size and spray distance is required to slow down the solidification rate, as well as to improve the adhesive strength of splats. In this study, cast iron splats were induced on an aluminum alloy substrate by plasma spraying using alloyed cast iron powder high in silicon and aluminum in a low pressure argon atmosphere. Then, the effects of particle size on the microstructure and adhesive strength of splats were investigated by introducing the correlation between the solidification rate and the microstructure. Spraying with large particles leads to an increase in the number fraction of disk splats and a slight decrease in their adhesive strength. Cross-sectional observations reveal fine graphite growing in splats nearly perpendicular to the substrate surface.
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  • Corrosion Behavior of Cu-Zr-Ti-Nb Bulk Glassy Alloys

    pp. 749-753

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    DOI:10.2320/matertrans.44.749

    Bulk glassy (Cu0.6Zr0.3Ti0.1)100−xNbx (x=0–5 at%) alloys with high thermal stability and good mechanical properties were synthesized and the effects of Nb on the glass formation and corrosion behavior were clarified. As the Nb content increases, the glass transition temperature (Tg) remains constant while the crystallization temperature (Tx) decreases slightly, resulting in a decrease in ΔTx (=TxTg) from 42 K at 0 at%Nb to 36 K at 5 at%Nb. The maximum diameter for glass formation was 4.0 and 2.0 mm for the 2 at% and 5 at%Nb alloys, respectively. The corrosion rates in various solutions significantly decrease with increasing Nb content. The addition of Nb to the glassy alloys shows higher corrosion potential and lower current density in 1 N HCl and 3% NaCl solutions. In 1 N H2SO4 and 1 N HNO3 solutions, they are spontaneously passivated in wide passive region and low passive current density range of 10−2–10−1 A·m−2. The XPS results revealed that the Zr was enriched in the surface film by addition of Nb to the Cu–Zr–Ti bulk glassy alloy. It was suggested that the role of Nb is to improve the protectiveness of Zr-rich surface film by decreasing Cu concentration in the surface film.
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  • Superplasticity-like Behavior of Rapidly Solidified Ti-rich TiAl Powders and its Application to In-situ Sintering Bonding

    pp. 754-758

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    DOI:10.2320/matertrans.44.754

    The application of rapidly solidified Ti-rich TiAl alloy powders to bonding technique has been proposed. The alloy powders are produced by plasma rotating electrode process (PREP). Among them, the powders (denoted as M powders) having surface relief of martensitic phase are used as an interlayer of the joint. The bonding of two Ti–50 mol%Al alloy disks was performed at 1273 K for 0.6 ks under 1.9 kN using the M powders of Ti–40 mol%Al alloy. Since the M powder shows superplasticity-like deformation, there are no defects such as void and oxide film in the bonding interface and the powders in the interlayer are indistinguishable. It is also confirmed that the bonding strength of the obtained joint increases notably by the use of the powder interlayer. Consequently, the bonding of the TiAl disks is achieved well by the excellent deformation behavior of the M powder even at relatively lower temperature and shorter holding time.
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  • The Characterization of HIP and RHIP Consolidated NiAl Intermetallic compounds Containing Chromium Particles

    pp. 759-765

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    DOI:10.2320/matertrans.44.759

    NiAl intermetallics with dispersive Cr particles consolidated by the Hot Isostaic Press (HIP) and Reactive HIP (RHIP) techniques, has been investigated. Mechanical properties of HIPed and RHIPed materials with various Cr concentrations ranging form 0 to 30 mass%Cr, were examined at various temperatures. The consolidation temperature of RHIP materials decreases with Cr addition; when RHIPing NiAl+xCr compounds with 25 mass% has attained remarkable high ductility over 17% elongation and over 400 MPa tensile strength at 873 K. A very high tensile strength of HIPed NiAl+20 mass%Cr over 600 MPa with 10% elongation at 873 K can be achieved at 873 K.
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  • Grain Refinement in As-Cast 7475 Aluminum Alloy under Hot Equal-Channel Angular Pressing

    pp. 766-774

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    DOI:10.2320/matertrans.44.766

    Microstructural evolution taking place during equal channel angular pressing (ECAP) was studied in an as-cast 0.16%Zr modified 7475 aluminum alloy at a temperature of 673 K. The structural changes are characterized by development of deformation bands due to large strain inhomogeneity occurring through the ECAP die. Repeated deformation leads to an increasing of number and misorientation angle of the boundaries of deformation bands, finally followed by formation of new fine grains at high strains. The misorientation angle distribution for newly developed boundaries shows a single peak type at relatively low misorientations in low strain and changes to a bimodal distribution with two peaks at low and high misorientations in moderate strain. Pressing to a strain of 12 leads to a full development of new grains surrounded by medium to high angle boundaries with an average size of about 1.7 μm. It is concluded that grain refinement occurs by a deformation-induced continuous reaction, that is essentially similar to continuous dynamic recrystallization.
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  • Materials Processing for Structural Stability in a ZK60 Magnesium Alloy

    pp. 775-781

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    DOI:10.2320/matertrans.44.775

    The materials processing, consisted of a four step process of solution treatment, pre-strain, overaging and hot working, was developed for attaining structural stability at high temperatures in a commercial Mg–Zn–Zr alloy, ZK60. The grain size of the material processed by this schedule was refined to 2.5 μm, and the material behaved in a superplastic manner at 548 K and at a strain rate of 1×10−2 s−1 owing to the grain size stability at this temperature. From the analysis of the chemical composition of precipitates, it was suggested that the phase Mg7(Zn,Zr)3, which was not observed in material without pre-strain, was effective in pinning the grain boundaries at high temperatures.
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  • Characterization of the Silica Conversion Film Formed on Zinc-Electroplated Steel

    pp. 782-786

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    DOI:10.2320/matertrans.44.782

    The chemical conversion of zinc-electroplated steel sheet using colloidal silica was investigated as an alternative to chromate conversion. The corrosion resistance of zinc-electroplated steel sheet with and without this treatment was estimated from electrochemical impedance spectroscopy, open circuit potential in 3 mass% NaCl solution and the salt spray test. The solution used for the colloidal silica coating consisted of colloidal silica, titanium sulfate, cobalt sulfate, and succinic acid. This solution produced an adhesive film and maintained good corrosion resistance. Adding cobalt ions to the colloidal silica solution enhanced the adhesion between the silica film and the zinc layer electroplated on the steel sheet. In the electrochemical impedance analysis, the corrosion protection conferred by the colloidal silica film was about five times greater than that of a normal zinc-electroplated steel sheet, but not as good as that of a chromate film. In the salt spray test, rust appeared more quickly on chromate-coated specimens than on colloidal silica film surfaces.
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  • Effect of Aluminum Content on Tribological Behavior of a Cu-Fe-C Based Friction Material Sliding against FC30 Cast Iron

    pp. 787-793

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    DOI:10.2320/matertrans.44.787

    An experimental study was made on the effect of aluminum content on the tribological behavior of a Cu–Fe–C based friction material sliding against FC30 cast iron. As a result, the densities of all Cu–Fe–C materials decreased after sintering. In general, when Al content increased, the density decreased. Sintered 65.5mass%Cu–11.4mass%Fe–6.6mass%C based friction material (A0) had much lower hardness than 57.7mass%Cu–12.6mass%Fe–7.3mass%C and 48.1mass%Cu–14.0mass%Fe–8.0mass%C based friction materials (A10 and A20). During sintering in air, large amounts of oxides formed. The presence of Al caused Cu–Sn to decompose, Al–Cu and Fe–Sn phases to form, Cu to be oxidized, and Fe2O3 to be reduced. A10 demonstrated the highest and most stable friction coefficient, as well as the smallest weight loss among three materials. Both film type and particulate type debris morphologies were observed on worn surfaces of Cu–Fe–C specimens. Local delamination of debris film was frequently observed on A0 and A20 surfaces, but seldom observed on A10 surface. Significant amounts of iron transfers from FC30 to A10 and A20 during sliding, but less such transfer occurred in A0.
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  • Compressive Deformation of Precursor-Derived Si-C-N Ceramics at Elevated Temperatures

    pp. 794-797

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    DOI:10.2320/matertrans.44.794

    The deformation behavior of Si–C–N ceramics derived from polyvinylsilazane polymer precursor has been investigated by compression tests at elevated temperatures. The dense Si–C–N ceramic monolith was fabricated by hot isostatic pressing of a compact prepared from pyrolyzed powders at 1500°C and at 950 MPa without any sintering additives. The microstructure of the ceramic monolith was a nano-composite structure consisting of α-Si3N4 crystalline grains and turbostratic graphite-like phases. Only slight plastic deformation was observed at 1500°C in spite of high compressive stress over 1000 MPa. On the other hand, the sample showed plastic flow at 1600 and 1700°C. A compressive strain of about 11% was obtained at 1700°C and 4×10−5 s−1. The feature of the microstructure was maintained after the compressive deformation, although the grains were coarsened during the deformation. The strain rate exponent of the compressive stress during the deformation at 1600 and 1700°C was about 0.1. The deformation mechanism of the ceramic material is thought to be different from those of creep deformation in amorphous Si–C–N ceramics and of superplastic deformation in silicon nitride ceramics containing glassy grain boundary phases.
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  • Direct Extraction and Recovery of Neodymium Metal from Magnet Scrap

    pp. 798-801

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    DOI:10.2320/matertrans.44.798

    An environmentally sound process for the extraction of neodymium (Nd) from magnet scrap was devised and its feasibility was demonstrated. The developed Nd extraction apparatus circulates the magnesium (Mg) as an extraction medium by maintaining a temperature difference within the reaction vessel, thus achieving continuous extraction of metal Nd from scrap, re-extraction of Mg from a Mg–Nd alloy, and finally, pure Nd metal with 97.7% purity was directly recovered from magnet scrap without oxidation. The concept of “scrap combination” for recycling variable materials, that is demonstrated in this study, is an important technology that can be instrumental in creating a highly developed self-sustainable society.
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  • Preparation of BaTi2O5 Single Crystal by a Floating Zone Method

    pp. 802-804

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    DOI:10.2320/matertrans.44.802

    Single crystalline BaTi2O5 was prepared by a floating zone method. The single crystal was transparent having a (001) cleavage plane. The space group was C2, and the lattice parameters were a=1.6909(9) nm, b=0.3937(1) nm, c=0.9419(4) nm and β=103.12(6)°. The permittivity perpendicular to a (010) plane showed the maximum value of 20500 at 748 K. The permittivity perpendicular to (100) and (001) planes were about 140 and 70, respectively, independent of temperature.
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