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

MATERIALS TRANSACTIONS Vol. 45 (2004), No. 9

  • Ni-Al Functionally Graded Materials by Laser Self-Propagating High-Temperature Synthesis

    pp. 2791-2793

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    DOI:10.2320/matertrans.45.2791

    In recent years, functionally gradient materials have been developed. But we seldom see the paper about that FGMs are produced by making use of laser. In this paper, we make FGMs with laser in self-propagating high-temperature synthesis. We select nickel and aluminum powders, which can react and release much heat. The ignition time and reaction time are shortened. And the highest temperature which reaction can get is improved. Utilizing the laser of high power density, we can get more complete product. And the composite process of phase in this method is different from that in tradition method.
  • Enhancement of Magnetoimpedance in Fe74.5−xCuxNb3Si13.5B9 As Cast Ribbons

    pp. 2794-2796

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    DOI:10.2320/matertrans.45.2794

    With increasing Cu content x in Fe74.5−xCuxNb3Si13.5B9 as cast ribbons, an enhancement of magnetoimpedance can be observed. The magnetoimpedance (Z(H) − Z(0))/Z(0) for Fe72Cu2.5Nb3Si13.5B9 as cast ribbon reaches −35% under a dc field H = 7162 A/m at a frequency ƒ = 1 MHz. The longitudinal magnetoimpedance effect in Fe74.5−xCuxNb3Si13.5B9 as cast ribbons is connected with the variation of transverse permeability. The grain size of α-Fe(Si) in Fe72Cu2.5Nb3Si13.5B9 as cast ribbon was estimated as about 7—10 nm. This revealed that soft magnetic nanocrystalline materials with giant magnetoimpedance effect can be obtained directly from the melt - spinning technique without additional annealing processes.
  • Austenite Grain Coarsening Under the Influence of Niobium Carbonitrides

    pp. 2797-2804

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    DOI:10.2320/matertrans.45.2797

    This work investigates grain growth behaviour under the influence of pinning carbonitrides in a niobium microalloyed steel. The effect of temperature and heating rate on the grain size is studied. The grain coarsening temperature is determined as a function of the heating rate. It is found that unpinning by precipitates occurs around 40—70 K below the temperature of complete dissolution of carbonitrides. It has been found that experimental results are best described assuming a random distribution of carbonitrides in the matrix and niobium volume diffusion as the rate controlling process for the coarsening of precipitates. Austenite grain growth is explained theoretically taking as a basis the model proposed by Zener, which has been adapted for non-equilibrium kinetics, taking into account the experimental evidence that during a continuous heating, the amount of microalloyed element in solid solution is altered and different from that predicted by the solubility product.
  • Effect of Microstructure on Mechanical Properties of Friction-Welded Joints between Ti and AISI 321 Stainless Steel

    pp. 2805-2811

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    DOI:10.2320/matertrans.45.2805

    The microstructures and mechanical properties of the friction welded pure Ti/AISI 321 stainless steel have been investigated. From the Ti side from the interface to the Ti base metal, the sequence of recrystallized grain, elongated grain and many twin embedded grain structures were observed. The reaction layers were formed within 0.2 μm thickness at the interface under the conditions of relatively longer friction time (t1) and lower upset pressure (P2). These reaction layers formed at the central interface were identified as (Fe, Cr)2Ti, FeTi2 or Fe2Ti4O and β-Ti, while those of the peripheral interface were identified as FeCr (σ phase), (Fe, Cr)2Ti, FeTi and β-Ti. The σ phase was restrictedly formed at the peripheral interface. Higher mechanical properties were acquired under higher upset pressure condition due to higher compressive force between bonded materials, smaller grain size and narrower thickness of reaction layer. Therefore, maximum ultimate tensile strength of these joints was approximately 420 MPa with the conditions of 400 MPa of P2 and 0.5 s of t1.
  • Microstructure Evolution in Ferritic Stainless Steels during Large Strain Deformation

    pp. 2812-2821

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    DOI:10.2320/matertrans.45.2812

    Deformation microstructures were studied in ferritic stainless steels during cold bar rolling and swaging to total true strains about 7. Two steels, i.e. Fe-22Cr-3Ni and Fe-18Cr-7Ni with coarse-grained ferritic and fine-grained martensitic initial microstructures, respectively, were selected as starting materials. Microstructure evolution in the both steels was characterized by the development of highly elongated (sub)grains aligned along the rolling/swaging axis. The transverse size of these (sub)grains in the Fe-22Cr-3Ni steel gradually decreased to about 0.1 μm with increasing the strain. On the other hand, the transverse (sub)grain size in the Fe-18Cr-7Ni steel decreased to its minimal value of 0.07 μm with straining to about 3 followed by a little coarsening under further working. The strengthening of worked steels that revealed by hardness tests correlated with the microstructure evolution. The hardness of the Fe-22Cr-3Ni steel increased with cold working within the studied strain range, while that of the Fe-18Cr-7Ni approached a saturation after fast work hardening at strains below 3, leading to an apparent steady-state behaviour. Development of strain-induced (sub)grain boundaries and internal stresses in the steels with different initial microstructures during severe deformation is discussed in some detail.
  • Ordered Structures and Partial Phase Diagram of Pd-rich Palladium-Manganese Alloys Studied by Electron Diffraction and Microscopy

    pp. 2822-2825

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    DOI:10.2320/matertrans.45.2822

    In order to present the phase relation in Pd-rich Pd-Mn alloys, we investigated the ordered structures on the basis of transmission electron diffraction and microscopy studies. Among various ordered phases reported up to now, four of Pd3Mn (D023), Pd2Mn, Pd5Mn3 and PdMn (L10) phases were confirmed to be stable, but no indications of Pd3Mn I (L12) and Pd21Mn11 phases were obtained. In the region of 1d-APS from Pd3 Mn to about 35 at% Mn, c0/a0 gradually decreased from unity with increasing Mn content, indicating the deviation of structure from D023. The value of M of the 1d-APS was 2.00 up to about 30 at% Mn irrespective of annealing temperature, and then gradually increased to 2.87 with increasing Mn and elevating temperature. Controversial results on M reported by different investigators are ascribed to the annealing temperature as well as composition dependence of M. The structural models reported for Pd2Mn and Pd5Mn3 phases were supported by the present study. We finally propose a partial atomic phase diagram in Pd-rich region.
  • Microstructural and Textural Evolution by Continuous Cyclic Bending and Annealing in a High Purity Titanium

    pp. 2826-2831

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    DOI:10.2320/matertrans.45.2826

    Microstructural and textural evolution by continuous cyclic bending (CCB) and annealing in a high purity titanium sheet has been investigated. The hardness distribution through thickness in the CCBent sheet exhibits V-shape with marked difference between the surface and the inside. More deformation twins are observed in the surface of the sheet subjected by more CCB passes. After annealing, the texture in the sheet which was CCBent in rolling direction is randomized, while the as-received sheet has typical textural components in rolling and recrystallization of the rolled titanium sheet. On the other hand, in the CCBent sheet in transverse direction, development of the rolling texture component is found after annealing. Further, the textural evolution is compared with that in a commercial purity titanium.
  • Analysis of Interfacial Damage and Debonding Life Estimation of Cold Forging Tool Coated with Hard Film

    pp. 2832-2837

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    DOI:10.2320/matertrans.45.2832

    The behavior of the interface of a cold forging tool coated with a hard film is analyzed by the finite element method (FEM). The mechanical properties of the interface between a hard film and a tool material are modeled under the framework of the irreversible thermodynamic constitutive equation theory with continuum damage mechanics. The interface damage variable and its evolution law are introduced, which express the degradation of the interface. The hemispherical tool of SUJ2 in JIS coated with an electroplated coating of chromium indents into a cylindrical workpiece of S25C in JIS, which is performed to examine the validity of the proposed model. In FE analysis, the proposed constitutive equations are implemented by the nonlinear springs between opposite nodes of the interface. Calculated results show that the maximum interfacial normal displacement is observed close to the front of the lip of indentation, whereas maximum interfacial tangential displacement is observed close to the back of the lip of indentation. The region where debonding will first occur as indicated by the calculation results, is similar to the region of the behavior during an actual forging operation. Finally, a method of evaluating the interfacial debonding life of coated tools is proposed.
  • Development of Warm Forging Method for Magnesium Alloy

    pp. 2838-2844

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    DOI:10.2320/matertrans.45.2838

    A new warm forging method for magnesium alloys is proposed by taking their material properties into consideration. Since the ductility of Mg alloys is low at room temperature, the forging operation is conducted in the temperature range of 200—400°C. In the warm forging of Mg alloys, the billet is easily cooled down in the die cavity before the onset of forging due to high thermal conductivity. This problem is solved by heating the billet with high-temperature tools. Since the flow stress of Mg alloys exhibits a significant work softening phenomenon, a very high load tends to appear at the beginning of the forging process. In order to reduce the peak forging load, a new concept of billet shape is proposed; the shape is so chosen that pre-straining is caused without restraining the flow in the early stage of the process and then die filling is attained with a low flow stress. In this study, the proposed warm forging method is confirmed to be valid through finite element simulation and experiment using a servo-controlled press.
  • Computer Simulation of Rolling Textures Evolution of Pure Aluminum with Initial Texture

    pp. 2845-2850

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    DOI:10.2320/matertrans.45.2845

    The experimental of rolling texture development in pure aluminum with initial texture was carried out. The rolling texture mainly consists of components B and S while the C component is also observed. The rolling textures evolution was simulated by the Taylor-type models. For the full constraints (FC) Taylor model, the mean method over all the possible solutions was used to solve the ambiguity in the selection of the active slip systems. For the relaxed constraints (RC) lath and pancake models, the ambiguity can be solved effectively by some modified principle, in which the strain compatibility can be fulfilled as much as possible. It has been shown that using the principles proposed in this paper, the rolling texture formation and development of pure aluminum with initial texture could be explained by applying different models under different deformation degree.
  • Phase Relations and Distribution of Some Minor Elements in Cu-Fe-As System Saturated with Carbon at 1473 K

    pp. 2851-2856

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    DOI:10.2320/matertrans.45.2851

    As a fundamental study to develop a new process for eliminating detrimental elements and for recovering valuable ones from secondary Cu-Fe base alloys with a considerably high content of arsenic, both the phase relations in a miscibility gap of the Cu-Fe-As system saturated with carbon and the distribution of some minor elements of silver, platinum, cobalt, nickel and sulfur between two phases in the miscibility gap were investigated at 1473 K by using a quenching method. The phase separation into copper-rich and iron-rich phases occurred when the Cu-Fe-As system was saturated with carbon. The arsenic content in the copper-rich phase was larger than that in the iron-rich phase, and carbon mostly distributed in the iron-rich phase. Cobalt and nickel distributed preferentially in the iron-rich phase, and platinum and sulfur distributed almost evenly in both phases, while silver mostly in the copper-rich phase. The experimental results for the phase separation and the distribution of the minor elements were discussed on the basis of activity coefficients in the copper-rich and iron-rich phases. By utilizing this phase separation, recovery of valuable silver and copper into the copper-rich phase and elimination of less valuable iron into the iron-rich phase are feasible for treating the secondary Cu-Fe-As base alloys.
  • Wetting, Adhesion and Adsorption in Al-Si/(0112) α-Alumina System at 1723 K

    pp. 2857-2863

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    DOI:10.2320/matertrans.45.2857

    The wetting of (0112) α-Al2O3 single crystals by Al-Si alloys in the full composition range was studied by an improved sessile drop method in a reducing Ar-3%H2 atmosphere at 1723 K. The wettability and the adhesion are significantly enhanced when a small amount of Al is added to Si. In turn, the effect is less pronounced when Si is added to Al. Based on the Gibbs adsorption isotherm equation and a statistic thermodynamic model, the adsorptions of Si and Al at the liquid surface and the solid-liquid interface were determined, and the experimental results were explained by the fact that Si is a surface tensioactive element while Al is an interface tensioactive element in the Al-Si/(0112) α-Al2O3 system.
  • Surface Tension and its Temperature Coefficient of Liquid Sn-X (X=Ag, Cu) Alloys

    pp. 2864-2870

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    DOI:10.2320/matertrans.45.2864

    The surface tension of liquid Sn-X (X=Ag, Cu) alloys was measured by the constrained drop method in the temperatures between 700 and 1500 K across whole composition range. Surface tension of the alloys increased with the content of Ag and Cu, and the temperature coefficient of the surface tension (dσ/dT) had both positive and negative values. Experimental results were compared with the calculated results based on Butler's model. The calculated results reasonably accorded with the measurements. The effect of thermo-physical parameters on the surface tension and the temperature coefficient were examined using the model. It was found that the temperature coefficient increases as the difference in the surface tension of component metals or the excess free energy increases in the high composition range of the component metal having higher surface tension, because of the surface enhancement of the other component metal.
  • New Activity Measurement Technique by Vacuum-Sealed Quartz Cell/Atomic Absorption Spectrophotometer Combination and Its Application to Liquid Bi-In Binary System

    pp. 2871-2877

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    DOI:10.2320/matertrans.45.2871

    The problems concerned with non-proportionality between the absorbance and the vapor density and its dependence on the vapor temperature, which are inherently encountered by vacuum-sealed quartz cell/atomic absorption spectrophotometry combination, were solved through model calculations in this study. Model calculations revealed that “colligated analytical-curve” is useful. Activities of Bi and In in the Bi-In liquid alloy were measured over the entire composition range at the temperature from 850 to 1050 K. An alloy was vacuum-sealed in a quartz cell and heated at the temperature of interest. The absorption for Bi 307 nm radiation from Bi lamp was measured for Bi atom vapor in the cell. By heating a pure metal as a standard and measuring the absorbance as a function of the temperature, a colligated analytical-curve for Bi atom vapor was constructed and used for conversion of the absorbance to the vapor density. Bi activity was determined as the ratio of the Bi atom vapor density over the alloy to that over a pure metal. The same procedure was applied to the In 304 nm radiation from In lamp and In activity was determined independently of Bi. Thermodynamic behavior of the Bi-In liquid alloy was optimized with a sub-regular solution model by taking into account activity data obtained in this study. The agreement between activities optimized in this study and those in the literature was fairly good. The model also well predicted the liquidus curve on the Bi side and the heat of mixing in Bi-In binary, both of which are comparable with the literature values. Finally it was concluded that by constructing colligated analytical-curves the vacuum-sealed quartz cell/atomic absorption spectrophotometer combination was established as a useful technique to measure the activities of elements in alloy systems.
  • Electrical Discharge Surface Alloying of Superalloy Haynes 230 with Aluminum and Molybdenum

    pp. 2878-2885

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    DOI:10.2320/matertrans.45.2878

    Surface alloying of superalloy Haynes 230 was performed using either Al-Mo mixture powders suspended in kerosene or Al-Mo composite electrode during the electrical discharge alloying (EDA). For suspending Al-Mo powders in kerosene, the Cr23C6, WC1−x, and graphite phases exist in the alloyed layer of the EDA specimen with negative electrode polarity, while the specimen with positive electrode polarity exhibits no alloying phase except the raw material phase. For employing the Al-Mo electrode with negative polarity, the specimen represents many discontinuous piled-layers, with Al3Mo8 and AlMo3 phases accumulating on the substrate surface. The specimen produced using Al-Mo electrode with positive polarity shows an alloyed layer constituted mainly with NiAl phase. This specimen also shows the smallest surface roughness and highest hardness among the four EDA specimens. The kinetics of isothermal oxidation at 1000°C in static air demonstrate that the specimen using Al-Mo electrode with positive polarity possesses the best oxidation resistance among the tested specimens.
  • Process Dependence of Ir-Based Bond Coatings

    pp. 2886-2890

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    DOI:10.2320/matertrans.45.2886

    The process dependence of Ir-based-alloy-coated Ni-based single crystal superalloys was investigated. An Ir-4 at%Ta alloy, 10 μm thick, was coated on a Ni-based single crystal superalloy TMS-82+, by means of magnetron sputtering deposition (SD) and electron beam physical vapor deposition (EB-PVD). X-ray analysis revealed that the grain size of the as-SD-coated layer (∼20 nm) has five times smaller than that of the as-EB-PVD coated layer (∼100 nm), and the SD-coated specimens showed more rapid outer diffusion of Ni and Al from the substrate after annealing, than the EB-PVD-coated ones. On the other hand, the aluminizing treatment reduces the differences between the SD coated and EB-PVD coated specimens in terms of compositional distribution and oxidation resistance. This study also revealed that even without aluminizing, the simple annealing treatment enhances the oxidation resistance of the Ir-4%Ta-coated TMS-82+.
  • Measurement of Friction in Cold Upsetting with Mist Lubrication

    pp. 2891-2896

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    DOI:10.2320/matertrans.45.2891

    The coefficient of friction during upsetting under semi-dry conditions is measured using the ring compression test. A small amount of mist lubricant is sprayed onto the mirror surfaces of cemented tungsten carbide (WC) tools, and pure aluminium specimens are compressed by the tools. It is found that spraying a small amount of lubricant (0.5 g/m2) reduces the friction effectively. The roughness of the workpieces after compression increases as the amount of the lubricant increases to Ra = 0.20—1.0 μm which is in the same order as the lubricant film thickness. Mist lubrication results in very small dots of lubricant particles sticking to the tool surface, and the lubrication mechanism of mist lubricant is discussed.
  • Basic Characteristics of the Explosive Welding Technique Using Underwater Shock Wave and Its Possibilities

    pp. 2897-2901

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    DOI:10.2320/matertrans.45.2897

    A novel explosive welding technique which uses underwater shock waves to weld thin sheets and the technical advantages of this technique are reported. Using this technique, a thin metal plate is uniformly accelerated by underwater shock waves. The initial angle of inclination of the explosive pack is determinant in high-explosive welding systems, with respect to decreasing the horizontal collision velocity. Any change in the welding conditions along the cladding plates influences the weld strength and, therefore, the parameters should be judiciously selected so as to lie within the weldability boundary. The size of the waves generated at the welded interface is discussed based on the angle of collision. Future applications for multilayered explosive welding are also suggested.
  • Improved Response Time of a Thin Film Hydrogen Storage Alloy Unimorph Structure after Platinum Surface Treatment

    pp. 2902-2905

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    DOI:10.2320/matertrans.45.2902

    In this study, the effect of a platinum surface treatment on the mechanical response of a LaNi5 thin film actuator deposited on polyimide substrates was investigated. Since this actuator could be reversibly driven by hydrogen pressure control, it could function as a sensor and/or controller of the hydrogen gas flux in various hydrogen related devices. In the experiments, the incubation period of the actuation after hydrogen gas exposure was reduced from 100 to 10 s by the platinum surface treatment. This significantly modified mechanical response is attributed to a switching of the reaction rate determining steps. The platinum treatment enhanced the decomposition rate of hydrogen gas molecules at the surface. The platinum treatment should bring about a change of rate determining step to a permeation/diffusion of the hydrogen atoms into the film.
  • Effects of Nitrogen Irradiation on Photoluminescence around 1.54 μm from Er-containing ZnO

    pp. 2906-2908

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    DOI:10.2320/matertrans.45.2906

    The Er-containing ZnO specimens were treated through the N+ irradiation and the subsequent annealing, and the effect was investigated on the photoluminescence (PL) spectra around 1.54 μm. When the specimens were annealed at 1273 K in air after the N+ irradiation, the PL spectra changed remarkably. The PL intensity increased by about 35 times after the N+ irradiation and the subsequent annealing, and there was the attendant shift in the wavelength of the strongest peak from 1.534 to 1.539 μm. Thus, the N+ irradiation and the subsequent annealing could modify the local structure around Er3+ ions in ZnO, resulting in the surprisingly large enhancement of the PL intensity.
  • Direct Synthesis of L10 (Fe0.55Pt0.45)77—78Zr2—5B17—20 Nanocrystalline Alloys with High Coercivity by Melt-Spinning

    pp. 2909-2915

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    DOI:10.2320/matertrans.45.2909

    The structure and the coercivity (Hc) of the rapidly quenched (Fe0.55Pt0.45)bal. Zr0—8B0—24 alloys prepared by the melt-spinning technique have been investigated. The ordered L10-FePt phase with a size of 20—200 nm was directly formed by rapidly quenching the melt in the compositional range of 2—5 at% Zr and 17—20 at% B, and the alloys exhibit Hc greater than 200 kA/m in an as-quenched state. In particular, the melt-spun (Fe0.55Pt0.45)78Zr2B20 and (Fe0.55Pt0.45)78Zr4B18 alloys exhibit high Hc of 341 kA/m and 649 kA/m, respectively. The melting temperature (Tm) remarkably decreases by the addition of Zr and B, e.g., from 1833 K for Fe55Pt45 to 1360 K for (Fe0.55Pt0.45)78Zr4B18. The L10 phase, which is directly formed by the rapidly quenching method, is considered to include Zr and B, shows high thermal stability and maintains up to Tm. On the other hand, the (Fe0.55Pt0.45)77—85Zr0—8B15—22 alloys, produced by the Cu-mold casting with a lower cooling rate than the melt-spinning technique are found to comprise of mixed structures of L10-FePt and some compound phases such as ZrB12, PtZr, Fe3B, and FeB, and the alloys have a Hc of 40—100 kA/m, which is lower than that of the melt-spun alloys. Apart from the L10 phase, the other phases such as ZrB12, PtZr, Fe3B, and FeB are suppressed and the L10 phase containing Zr and B elements is formed by rapidly quenching the alloy melt with low Tm. The simultaneous addition of Zr and B facilitates the direct formation of the ordered L10 phase with a grain size of 20—200 nm by rapidly quenching the melt through the effect of the decreasing Tm and the increasing the stability of the L10 phase by the solution of Zr and B into the phase.
  • Effects of Y2O3 and Y Addition on Thermoelectric Properties of β-FeSi2 Synthesized by Mechanical Alloying and Hot Pressing

    pp. 2916-2921

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    DOI:10.2320/matertrans.45.2916

    The n-type Fe0.98Co0.02Si2 compacts with Y2O3 (1—6 mass%) dispersion were synthesized by mechanical alloying with Y2O3 powder and subsequent hot pressing. The effects of Y2O3 addition on the thermoelectric properties of the β-FeSi2 were investigated. The microstructures of the hot pressed samples were observed by transmission electron microscopy (TEM). The TEM observation showed that the fine Y2O3 particles around 10 nm in size were dispersed in the β phase matrix by mechanical alloying, resulting in a significant reduction in the thermal conductivity due to enhancing phonon scattering. The Seebeck coefficient was also enhanced by Y2O3 addition especially below 800 K, corresponding to the extrinsic conductive region of the β-FeSi2. Consequently, the figure of merit was significantly improved by 2 mass% Y2O3 addition. The chemical composition of these samples with Y2O3 addition was examined by the energy dispersive X-ray spectroscopy (EDX). The EDX analysis revealed that the added Y2O3 was partially decomposed and a small amount of Y was dissolved in the β phase matrix. Based on this fact, the enhancement of the Seebeck coefficient caused by Y2O3 addition is considered to be due to reduction in carrier concentration resulted from this Y solution as a p-type dopant in the β phase matrix, and the behavior of the Seebeck coefficient was found to be well consistent with that of the Y-doped samples synthesized by Y powder addition.
  • Effect of Hydrothermal Treatment on Microstructural Feature and Bonding Strength of Plasma-Sprayed Hydroxyapatite on Ti-6Al-4V

    pp. 2922-2929

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    DOI:10.2320/matertrans.45.2922

    The effects of hydrothermal treating temperature on bonding strength pertaining to the microstructural evolution of plasma-sprayed hydroxyapatite coatings (HACs) on Ti-6Al-4V substrate were investigated. On the basis of the observed microstructure, the as-sprayed amorphous and impurity phases, such as α-Ca3(PO4)2, Ca4P2O9 and CaO transform into crystalline HA after performing the hydrothermal treatment. Additionally, ultrafine crystallized particles can be recognized that the nucleation and grain growth occur by hydrothermal treatment at elevated temperatures of 175°C and 200°C. Furthermore, the hydrothermal treatment between 100°C and 150°C shows a significant decrease in microcracks, corresponding to an increase in bonding strength. This study reveals that even a small variation of hydrothermal heating temperature under 200°C can cause significant changes of the microstructural morphologies. Combining the experimental results of spraying defects, microstructural evolution and bonding strength, the knowledge of the hydrothermal-treated HACs will be helpful for the understanding and prediction in the biological stability and mechanical stability of HACs in the long-term clinical use.
  • Fatigue Strength of Ferritic Ductile Cast Iron Hardened by Super Rapid Induction Heating and Quenching

    pp. 2930-2935

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    DOI:10.2320/matertrans.45.2930

    To clarify the effects of Super Rapid Induction Heating and Quenching (SRIHQ) on fatigue properties of Ferrite Ductile Cast Iron (FDI), rotational bending fatigue tests were carried out on specimens treated with four types of heating cycle. Results showed that; (i) the SRIHQ process generated a thin dark gray area around the graphite. This dark area was composed of a martensite structure (ringed martensite). (ii) The ringed martensite generated a compressive residual stress field at the surface hardened layer. Two types of compressive residual stress generative mechanisms were observed. One was a microscopic residual stress generative process due to the formation of ringed martensite and the other was a macroscopic residual stress generative process due to the expansion of the surface hardened layer. (iii) The fatigue strength of SRIHQ treated FDI specimen was higher than that of the untreated one. This was because the compressive residual stress field generated by the ringed martensite suppressed initiation and propagation of fatigue cracks.
  • Excellent Mechanical Properties of Cu-Hf-Ti-Ta Bulk Glassy Alloys Containing In-Situ Dendrite Ta-based BCC Phase

    pp. 2936-2940

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    DOI:10.2320/matertrans.45.2936

    A glassy phase containing high volume fractions (Vf) of bcc Ta-based phase was formed in as-cast (Cu0.6Hf0.25Ti0.15)94Ta6 alloy rods with diameters up to 2 mm. Its Vf increased from 9.5 to 11% with increasing rod diameter from 1 to 2 mm. The in-situ bcc phase has a dendritic morphology with a size of about 10 μm and its size tends to increase with increasing Vf. The bcc phase has analytical compositions of 1.4 at% Cu, 7.6 at% Ti and 91.0 at% Ta. The lattice parameter of the bcc phase is 3.306 nm which approximates to that of Ta metal. The thermal stability of the remaining glassy matrix remains unchanged in coexistence with the bcc phase. The mixed structure is presumed to be formed by the primary precipitation of ductile dendritic bcc phase, followed by the solidification of the remaining liquid to glassy phase. The Young's modulus (E), yield strength (σy), true compressive fracture strength (σt,f) and true plastic strain (εp) were 104 GPa, 2125 MPa, 2100 MPa and 0.34, respectively, for the φ2 mm rod with 11% Vf. The σy and σt,f are higher than those (2010 MPa and 2005 MPa) for the Cu60Hf25Ti15 glassy alloy rod with a diameter of 2 mm. Especially, the εp for the (Cu0.6Hf0.25Ti0.15)94Ta6 mixed structure alloy is dramatically enhanced in comparison with that (εp = 0.016) for Cu60Hf25Ti15 single phase alloy. The enhancement of σy, σt,f and εp in the mixed structure of glassy and bcc phase for the new Cu-based system is promising for future uses as a new type of high-strength bulk glassy alloy.
  • Preparation of Iron Fine Particles Coated with Boron Nitride Layers

    pp. 2941-2944

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    DOI:10.2320/matertrans.45.2941

    Boron (B) powders mixed with hematite or iron (Fe) nanoparticles were annealed at 1573 K in nitrogen atmosphere. X-ray diffraction measurement shows that mixture of B and hematite changed to boron nitride (BN) and metallic Fe after the annealing without the formation of FeB compounds like FeB and Fe2B. FeB compound was generated from a mixture of B and Fe. High-resolution transmission electron microscopy shows that Fe particles were ∼300 nm in diameter and they were coated with BN layers. Mössbauer spectroscopy revealed a reduction of hematite to Fe. Synthesized Fe fine particles have an oxidation resistance up to 673 K.

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