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

MATERIALS TRANSACTIONS Vol. 53 (2012), No. 6

  • Fabrication of TiN–Ni and TiN–Co Cermets by Mechanical Milling and Subsequent Pulsed Current Sintering and Their Fracture Strength

    pp. 1029-1033

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    DOI:10.2320/matertrans.MBW201109

    TiN–Ni and TiN–Co cermets were fabricated by mechanical milling of TiN and Ni or Co powders, followed by pulsed current sintering. The microstructure of the TiN–Ni and TiN–Co cermet samples prepared from the non-milled powder contained a large number of pores. However, these pores were eliminated from both the sintered samples through mechanical milling of the powders. Relative densities of over 98% were achieved. Mechanical milling also improved the three-point bending strength of the cermet samples. In this study, the samples sintered from the powders milled for 28.8 ks exhibited the maximum strength, i.e., the strength of TiN–Ni and TiN–Co ceremets was 1.4 and 1.2 GPa, respectively. The fracture surface of the sample sintered from the non-milled powders was intergranular whereas that of the sample sintered from the milled powder was both intergranular and transgranular in nature.
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    Readers Who Read This Article Also Read

    1. Aluminum Coating on Magnesium-Based Alloy by Hot Extrusion and Its Characteristics MATERIALS TRANSACTIONS Vol.53(2012), No.6
    2. PREFACE MATERIALS TRANSACTIONS Vol.53(2012), No.1
    3. Fabrication and Property Evaluation of Mo Compacts for Sputtering Target Application by Spark Plasma Sintering Process MATERIALS TRANSACTIONS Vol.53(2012), No.6
  • Aluminum Coating on Magnesium-Based Alloy by Hot Extrusion and Its Characteristics

    pp. 1034-1041

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    DOI:10.2320/matertrans.M2012026

    Although magnesium-based alloys have excellent mechanical properties, their very poor corrosion resistance limits their application. It has been considered that aluminum coating would solve this problem because aluminum has an excellent corrosion resistance. This study proposes a superior coating method based on the hot extrusion process. An aluminum plate set between the magnesium-based alloy billet and an extrusion die having an inversely angled face was successfully extruded together with the magnesium-based alloy billet and, as a result, the aluminum coated the extruded magnesium-based alloy with a uniform thickness. Corrosion tests based on dipping in HCl aqueous solution and potential measurement in NaCl aqueous solution showed that the extruded sample exhibited the same corrosion resistance as the aluminum. A three point bending test at room temperature resulted in fracture after plastic deformation, while at high temperatures between 100 and 300°C plastic deformation without fracture occurred. Notably, the intermetallic compound layer formed on the boundary between the magnesium-based alloy substrate and the aluminum coating layer plastically deformed without cracking at 300°C. A tensile test at room temperature resulted in a UTS of about 320 MPa and a plastic elongation of 18%.
  • Notched Tensile Fracture of Ti–6Al–4V Laser Welds at Elevated Temperatures

    pp. 1042-1047

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    DOI:10.2320/matertrans.M2011340

    Notched tensile tests of Ti–6Al–4V laser welds that were subjected to post-weld heat treatments (PWHTs) at 482 and 704°C were carried out in air at temperatures from 25 to 450°C. The experimental results were also compared with those of the mill-annealed base metal (MB) specimens tested at similar temperatures. Generally, the notched tensile strength (NTS) of the specimens was sensitive to the test temperature. The NTS of laser welds was lower than that of the MB specimen at room temperature, but the trend reversed at 450°C. The presence of thick grain boundary α layers, which promoted intergranular fracture at elevated temperatures, could account for the lowered NTS of the weld after PWHT at 704°C.
  • Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid

    pp. 1048-1055

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    DOI:10.2320/matertrans.M2012008

    The effect of copper (Cu) addition on the active corrosion behavior of hyper duplex stainless steels in sulfuric acid was investigated. The addition of Cu in the base alloy enhanced the resistance to general corrosion by decreasing the critical and corrosion current densities, and increasing the polarization resistance. There are two primary reasons for the considerable enhancement of the corrosion resistance of the experimental alloys containing Cu. First, the protective surface film was enriched with the noble metallic copper (Cu) due to the selective dissolution of the active metallic Cr, Fe, and Ni, and the electrochemical dissolution of the corrosion products such as iron-sulfide (FeS2), iron sulfate (FeSO4), ferrous oxide (FeO) and hydrous iron sulfate (FeSO4·7H2O). Second, chromium oxide (Cr2O3), chromium trioxide (CrO3), nickel oxide (NiO), molybdenum dioxide (MoO2), molybdenum trioxide (MoO3), and tungsten trioxide (WO3) in an oxide state, molybdenum oxy-hydroxide (MoO[OH]2) and chromium hydroxide (Cr[OH]3) in a hydro-oxide state, molybdate (MoO42−) and tungstate (WO42−) as corrosion inhibitors in an ion state, and ammonium (NH4+) elevating the pH in an ion state were increased and assisted in improving the corrosion resistance.
  • Fabrication and Property Evaluation of Mo Compacts for Sputtering Target Application by Spark Plasma Sintering Process

    pp. 1056-1061

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    DOI:10.2320/matertrans.M2012039

    Pure molybdenum compacts having a high density, purity and a fine-grained microstructure were fabricated by a spark plasma sintering process. Also, an optimized sintering condition was obtained by controlling process parameters such as temperature, pressure, and heating rate. Molybdenum compacts were prepared with diameters of Φ150 × 6.35 mm, and the characteristics of the compacts were analyzed by induction couple plasma (ICP), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). In addition, molybdenum thin films that were fabricated on a glass substrate by using sputtering equipment were analyzed by XRD, transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS). A relative density of up to 99% and the grain size of below 8 µm were reached at the sintering temperature of 1473.15 K and the uniaxial pressure of 60 MPa. Also, molybdenum compacts having a uniform relative density and grain size were fabricated through an optimully redesigned mold and process control method. The resistivity (~0.15 μΩm) of the optimized spark plasma sintering (SPS) molybdenum thin film showed properties that are similar to those of the film prepared with conventional sintering.
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    Readers Who Read This Article Also Read

    1. Fabrication of TiN–Ni and TiN–Co Cermets by Mechanical Milling and Subsequent Pulsed Current Sintering and Their Fracture Strength MATERIALS TRANSACTIONS Vol.53(2012), No.6
    2. PREFACE MATERIALS TRANSACTIONS Vol.53(2012), No.1
    3. Aluminum Coating on Magnesium-Based Alloy by Hot Extrusion and Its Characteristics MATERIALS TRANSACTIONS Vol.53(2012), No.6
  • Development and Microstructure of Cu–Zr Alloy Wire with Ultimate Tensile Strength of 2.2 GPa

    pp. 1062-1068

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    DOI:10.2320/matertrans.M2012064

    Round, bar-shaped ingots of hypoeutectic Cu–4- and Cu–5 at%Zr alloys were cast using the copper-mold casting method. The ingots were drawn into wires with a drawing ratio (η) of 5.9 or more. The relationship between the mechanical and electrical properties of these wires as well as their microstructure was investigated. It was found that the Cu–5 at%Zr alloy wire drawn down to 40 µm in diameter with η = 8.6 exhibited an ultimate tensile strength (UTS) of 2234 MPa, 0.2% proof stress of 1873 MPa, total strain to fracture of 4.2%, Young’s modulus of 126 GPa, and electrical conductivity of 16%IACS. As for the Cu–4 at%Zr alloy, it could be wire-drawn down to 27 µm in diameter with η = 9.4.
    Both UTS and Young’s modulus increase linearly with η. A nanosized lamellar structure was noticed in the α-Cu and Cu9Zr2 intermetallic compound phases. Furthermore, it was observed that a nanosized amorphous phase was formed within the layers of the Cu9Zr2 intermetallic compound phase. The increase in the strength of the wire-drawn Cu–4- and Cu–5 at%Zr alloy is due to the synergistic effects resulting from the development of deformation twins in the α-Cu phase and the formation of the nanosized lamellar structure in the α-Cu and Cu9Zr2 intermetallic compound phases.
    High electrical conductivity of 16%IACS was obtained for the wire-drawn Cu–5 at%Zr alloy. This high value can be attributed to the low density of dislocations in the α-Cu phase.
  • In-Situ Observation and Acoustic Emission Analysis for Corrosion Pitting of MgCl2 Droplet in SUS304 Stainless Steel

    pp. 1069-1074

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    DOI:10.2320/matertrans.M2011399

    Acoustic emission with near-field detection and optical video microscope monitoring was proposed to investigate the pitting corrosion caused by a small magnesium-chloride droplet. Four types of the detected waveforms were characterized by the wavelet analysis to investigate the corrosion of work-hardened SUS304 stainless steel. After high-activity AE signals observed, the corrosion product sheet grew with or without low-activity AE signals. With the cross-section observations, the detected AE signals were mainly attributed to longitudinal and transverse cracking around the covered pitting, cracking of the oxidation products, and the early stage of corrosion potential fluctuation.
  • Deoxidation of Ti Powder and Preparation of Ti Ingot with Low Oxygen Concentration

    pp. 1075-1077

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    DOI:10.2320/matertrans.M2012004

    In this experiment, we carried out a two-step deoxidation process to obtain Ti ingots with low oxygen concentration from commercial Ti powder. The first step was to primarily reduce oxygen in Ti powder using Ca and the second was to melt the deoxidized Ti powder in Ar atmosphere with added hydrogen using a vacuum arc melting system. We used two types of raw Ti powder, one with a high oxygen concentration of 5,600 ppm and average powder size of 35 µm, and the other with 2,200 ppm and 115 µm. The high oxygen of 5,600 ppm in the Ti powder was reduced to approximately 1,460 ppm by the two-step process of melting after deoxidizing the Ti powder with Ca at 973 K. On the other hand, in the case of the Ti powder with low oxygen of 2,200 ppm, the Ti ingot with low oxygen of 600 ppm could be prepared by the two-step process of melting after deoxidizing at 1,103 K. The Vickers hardness and c/a value of Ti ingots decreased as the oxygen concentration decreased.
  • Tie-Line Compositions of the γ and δ Phases in the Binary Re–Ni System

    pp. 1078-1083

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    DOI:10.2320/matertrans.M2012028

    Compositions with tie lines between the γ and δ phases in a binary Re–Cr–Ni system were investigated at 1423, 1573, and 1773 K by heat treating of Ar-arc-melted Re–40 at%Ni alloy. The microstructures of the Re–40 at%Ni alloy which had been water quenched after various heat treatment were observed and their concentration profiles for Re and Ni were measured using an electron probe microanalyzer. The Re–40 at%Ni alloy consisted of the γ and δ phases. The tie lined compositions of the γ and δ phases are summarized as follows (in at%); γ: 13.4 Re–86.6 Ni, δ: 71.7 Re–28.3 Ni at 1423 K, γ: 16.4 Re–83.6 Ni, δ: 69.3 Re–30.7 Ni at 1573 K, γ: 20.2 Re–79.8 Ni, δ: 65.3 Re–34.7 Ni at 1773 K. The Re–Ni alloy powder sintered using the Spark Plasma Sintering (SPS) method were investigated. Solubility limit of Ni in the δ phase in the binary alloy system at 1423 K was found to be 28.3 at%Ni.
  • Purification of 6N Aluminum by Ultrahigh-Vacuum Melting

    pp. 1084-1089

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    DOI:10.2320/matertrans.M2012041

    To improve the purity of 6N-grade aluminum, it was melted in a vacuum of 3–6 × 10−6 Pa for 30 min and solidified gradually. The melted samples were cut into several pieces and the residual impurities of each portion were analyzed by glow discharge mass spectrometry (GDMS). The residual resistivity ratio (RRR) of each portion was also measured and the relationship between RRR and residual impurities was investigated. The purity of the melted sample was improved in comparison with that of the raw material, and the measured RRR was almost doubled to 40000. GDMS analysis revealed that the concentration of the elements with partition coefficient k < 1 was markedly reduced, whereas that of the elements with k > 1 was not reduced. While each impurity concentration ratio in this study is about one-tenth of that in our previous report, the purification efficiency of each element in this study and that in the previous report were in good agreement. Although the values of RRR estimated from composition analysis were larger than the measured RRR values, both exhibited similar tendencies with regard to the specimen position and also corresponded well with impurity distribution.
  • Characterization of Hot-Steam Oxidation Tested Chromosiliconized Heat-Resistant Austenitic Stainless Steel

    pp. 1090-1093

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    DOI:10.2320/matertrans.M2011396

    In this study, a duplex chromizing-siliconizing coating was produced on heat-resistant austenitic stainless steel by pack cementation. The chromizing-siliconizing of the duplex chromosiliconized coating was evaluated in 873 K hot steam for 3.6 × 103 ks, and the oxidized as well as the coated samples were characterized. A duplex chromosiliconizing coating has been successfully applied to heat-resistant austenitic stainless steel by pack cementation. This coating containing Cr and Si can effectively improve the oxidation resistance of heat-resistant austenitic stainless steel in 873 K hot steam for 3.6 × 103 ks.
  • Effect of Injection Speed on Microstructure of AZ91D Magnesium Alloy in Semi-Solid Injection Process

    pp. 1094-1099

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    DOI:10.2320/matertrans.F-M2012811

    We have developed a new type of semi-solid injection process that can obtain a high material yield of about 90% for magnesium alloys. The morphology of the solid particles in the semi-solid slurry has significant effects on the rheological behavior. In this study, semi-solid injection moldings of AZ91D magnesium alloy were conducted with injection speeds of 220, 300, and 400 mm/s and for fraction solids of fs = 0.3, 0.4, and 0.5 in order to investigate the effects of shear stress on the microstructure. With a high injection speed, the α-phase solid particles became spherical and the average particle size decreased. The average roundness of the α-phase solid particles was correlated to the shear stress calculated on the assumption that the slurry characteristics correspond to that of the steady state at the nozzle. The reduction of the average particle size suggests that the solid particles were broken up and spheroidized by the shear stress. Under all experimental conditions, the α-phase particles in the plane perpendicular to the flow direction were concentrated at the center of specimen rather than on the surface. On the other hand, distribution of the α-phase particles in the plane parallel to the flow direction depended on the injection speed. A high fraction solid zone was generated at the end of the specimen for a high injection speed.
  • Injection Molding of Mechanical Alloyed Ti–Fe–Zr Powder

    pp. 1100-1105

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    DOI:10.2320/matertrans.M2012031

    This study focuses on the injection molding of mechanical alloyed Ti–Fe–Zr alloys. Injection molded samples were produced using mechanical alloying based on hydride-dehydride (HDH) titanium and pure iron and zirconium powders. Mechanical alloyed powders were mixed with a polymeric binder and hot injection molded to form standard tensile bars. The critical powder loading for injection molding was 50 vol% for feedstock. Molded bars were debound by solvent and then thermal steps, under ultra pure argon. Debound samples were sintered at 1300°C for 60 min in a high level vacuum (10−5 mbar). After sintering, the performances of the sintered materials was characterized using tensile and hardness testing, optical microcopy (OM) and scanning electron microscopy (SEM). The strengths and weaknesses of the test conditions have been analyzed from the microstructure and mechanical properties. Theoretical density, ultimate tensile strength, and hardness of injection molded Ti powders increased with the additions of 5%Fe and 5%Zr.
  • Interface Microstructure and Mechanical Properties of Dissimilar Friction Stir Welded Joints between Zr55Cu30Ni5Al10 Bulk Metallic Glass and Pure Al

    pp. 1106-1112

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    DOI:10.2320/matertrans.M2011376

    Dissimilar friction stir welding of Zr55Cu30Ni5Al10 bulk metallic glass (BMG) to pure Al was carried out at different welding conditions. Sound BMG/Al joints can be obtained at a rotation speed of 350 and 400 rpm. However, defects or crack may form in the stir zone when the rotation speed was either too low or too high. In the sound joints, a clear and straight BMG/Al interface was formed and no intermetallic compounds caused either by chemical reaction or crystallization from the amorphous phase can be detected within the entire stir zone. The hardness of Al side in the stir zone decreases with increasing of the rotation speed and is lower than that in the base material due to frictional heat input. The sound BMG/Al joints fractured in the heat affect zone of Al side and the tensile strength can reach about 91% of that of the base material.
  • Critically-Percolated, Cluster-Packed Structure in Cu–Zr Binary Bulk Metallic Glass Demonstrated by Molecular Dynamics Simulations Based on Plastic Crystal Model

    pp. 1113-1118

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    DOI:10.2320/matertrans.M2012025

    Molecular dynamics (MD) simulations based on a plastic crystal model (PCM) were performed for a Cu0.618Zr0.382 binary bulk metallic glass (BMG). The local atomic arrangements of the Cu0.618Zr0.382 BMG were demonstrated with MD-PCM under an assumption that the BMG is composed of randomly-rotated as well as distorted hypothetical clusters. The Cu-rich Cu0.618Zr0.382 alloy was computationally created from a tentatively-created Zr-rich Zr0.73Cu0.27 alloy through two steps. The first step includes the Zr0.73Cu0.27 alloy quenched from a liquid through conventional MD simulation, whereas the second step has a replacement of the Zr and Cu atoms in the Zr0.73Cu0.27 alloy with randomly-rotated icosahedral and tetrahedral clusters, respectively, and subsequent structural relaxation. The Cu0.618Zr0.382 alloy, thus created with MD-PCM, was formed in a noncrystalline state as a critically-percolated cluster-packed structure. The analyses with total pair-distribution and interference functions revealed that the calculation results tend to reproduce the experimental data in an as-quenched state. The results explain that the glass-forming ability of the Cu0.618Zr0.382 BMG is due to (1) a critically-percolated and distorted tetrahedral clusters forming a network and (2) atomistic-level inhomogeneity for the local atomic arrangements with keeping macroscopic homogeneity in terms of the chemical composition and dense random atomic arrangements.
  • Eutectic Growth in Laser Cladding of Zr-Coating on AZ91D Magnesium Substrate

    pp. 1119-1123

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    DOI:10.2320/matertrans.M2012048

    Zr-bearing coatings were fabricated on an AZ91D magnesium substrate by means of a two-step method. First, pure zirconium powder was plasma sprayed on the substrate and, subsequently, laser cladding was performed on top of the spray deposited layer. The process led to the formation of a zirconium coating containing zirconia. The coating comprises three layers; on top of the substrate is the plasma sprayed layer, followed by the laser re-melt layer and finally an outermost laser-clad layer. The microstructure and phase of the coating were studied. The results show that rapid solidification occurred in the laser-clad layer and resulted in a directional eutectic cellular and eutectic dendrite growth of (Zr) + ZrO2. The maximum velocity for co-operative lamellar growth of the (Zr) + ZrO2 eutectic and its corresponding lamellae spacing were calculated based on the TMK model.
  • Fabrication of Al2O3–YAG Equilibrium Eutectic Composites via Transformation from Fine Al2O3 and YAP Powder Mixtures

    pp. 1124-1129

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    DOI:10.2320/matertrans.M2012047

    A process for the fabrication of Al2O3–YAG eutectic composites using an undercooled melt formed by the melting of an Al2O3–YAP eutectic specimen has been developed. An Al2O3–YAG eutectic structure with fine lamellar spacing was formed. However, the Al2O3–YAG casting has porosities with diameters of 4–9 µm. The composition is limited to the Al2O3–YAP eutectic composition. In this paper, a new method for the fabrication of Al2O3–YAG eutectic composites through the transformation from fine Al2O3 and YAP powder mixtures is proposed. The enhancement of YAG formation due to the fine Al2O3 and YAP powders and the 11% volume expansion in the transformation from the Al2O3–YAP system to the Al2O3–YAG system contributed to the formation of dense Al2O3–YAG eutectic compact. The Al2O3–YAG compact featured a uniform microstructure consisting of spherical-like Al2O3 and YAG grains. In addition, the forming temperature was reduced to 1873 K, which is approximately 100 K lower than that in the previous process using the undercooled melt. The proposed process provides a dense Al2O3–YAG eutectic compact with a fine and uniform microstructure over a wide composition range of Al2O3x mol%Y2O3 (10 < x < 30).
  • Microscopic Observation of the Interface between a Tool Surface and Deposits of Low-Carbon Free Cutting Steel

    pp. 1130-1137

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    DOI:10.2320/matertrans.M2012034

    Microstructures and element distribution at the interface between a tool surface and a residue of low-carbon free cutting steel, which was left behind and accumulated on the tool surface during machining, were observed and analyzed by transmission electron microscopy (TEM) and laser-assisted three dimensional atom-probe tomography (LA3DAP). The residue of the machined steel was composed of fine ferrite grains a few hundred nanometers in size, which were considered to be a result of a grain refinement process by severe plastic deformation. Tungsten carbide (WC) particles embedded in the tool were sharply worn at the interface, and the interface was found to be flat and sharp on a nanometer scale. The deposited steel on the worn tool surface was tightly bound and the interface was successfully analyzed by LA3DAP in transverse direction. It was shown that tungsten atoms from the WC particles had diffused into the deposited steel with a distance of approximately 10 nm from the interface. These findings indicate that the wear of tool upon machining low-carbon free cutting steels proceeds mainly by the diffusional wear mechanism.
  • Tensile and High Cycle Fatigue Properties of a Minor Boron-Modified Ti–22Al–11Nb–2Mo–1Fe Alloy

    pp. 1138-1147

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    DOI:10.2320/matertrans.M2012043

    The Ti3Al (α2 phase)-based Ti–22Al–11Nb–2Mo–1Fe (in at%) alloy is a derivative of Ti2AlNb (O phase)-based orthorhombic Ti–22Al–27Nb (in at%) and was designed to reduce Nb content and thereby material cost. In the present study, two microstructure modification methods, a minor addition of boron (B) and a thermo-mechanical treatment in the (B2 + α2) two-phase region, were applied to refine the microstructure of this alloy and thus to improve its ductility. The prior B2 grain size of the as-cast ingot was drastically reduced, from 600–1000 to 100–200 µm with the addition of 0.1 mass% B, and thereby a refined full lamellar microstructure was obtained. The duplex microstructure consisting of spherical α2 phases and lamellar microstructure area was formed by the thermo-mechanical treatment, which was much finer in the B-modified alloy than in the B-free alloy. The B-modified alloy with a refined full lamellar or duplex microstructure showed higher tensile ductility, as well as higher high cycle fatigue strength and superior creep properties compared to those of the B-free counterpart. The fatigue crack initiated neither from the TiB/matrix interface nor from the TiB itself, but rather from the matrix area of the alloy. Thus, it was confirmed that the addition of a small amount of B exerts a favorable effect as a whole on the mechanical properties of the present Ti–22Al–11Nb–2Mo–1Fe alloy.
  • A Comparison of Corrosion Behavior of a Super Duplex Stainless Steel and an Austenitic Stainless Steel in a Molten Sn–3.0Ag–0.5Cu Lead-Free Solder

    pp. 1148-1153

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    DOI:10.2320/matertrans.M2011389

    To determine the endurance of a super duplex stainless steel (SDSS) used for wave soldering bath materials, the corrosion behaviors of a SDSS, SAF2507, and a comparative austenitic stainless steel, SUS304L (conventional material for tin-lead soldering container) in a Sn–3.0Ag–0.5Cu molten lead-free solder were investigated. After testing, the samples were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), and semi-quantitative phase identification under an energy dispersive spectrometer (EDS) to evaluate the effects of the composition of test materials and immersion conditions on their microstructure evolution and corrosion behaviors.
    As results show, when compared to the SUS304L, SAF2507 has better corrosion resistance to lead-free solder after immersion at the assigned temperatures (350, 450, and 550°C) and times (from 250 up to 1500 h). When the test temperatures of 350 and 450°C were employed, no obvious dissolution occurred for SAF2507, whereas SUS304L exhibited severe dissolution. However, if the immersion temperature of 550°C was used, the dissolution rates of SAF2507 increased significantly. It was found that the failure type of both materials was related to atom diffusion, formation of the reaction layer (RL), and finally dissolution, which is a typical failure type of Liquid Metal Corrosion. Moreover, SEM and EDS results reveal that the major intermetallic phases in the RL for both stainless steels are Fe/Sn and Cr/Sn compounds.
  • Immersion-Induced Changes in Structure and Properties of a Calcium Sulfate Cement

    pp. 1154-1158

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    DOI:10.2320/matertrans.M2011362

    This research was devoted to the investigation of changes in structure and properties of a calcium sulfate hemihydrates (CSH) cement immersed in Hanks’ solution for different periods of time. XRD patterns indicated a quick phase change (a hydration process) of CSH into calcium sulfate dihydrate (CSD) after the cement powder was mixed with the setting solution. After the hardened cement was immersed in Hanks’ solution for 1 d, CSD became the dominant phase. The long-term (up to 30 d) pH value of the Hanks’ solution wherein the cement was immersed remained in the range of 6–7. The hardened cement gradually lost its weight and increased its porosity level with immersion time. When immersed for 1 d, the average compressive strength (CS) value of the cement reached its maximum value. After 30 d, the compressive strength (CS) value of the cement still remained >25 MPa. SEM showed that, after being immersed for 30 d, numerous large, faceted CSD crystals were observed throughout the sample. Cytotoxicity test indicated that the present cement is a biocompatible implant material.
  • Effects of Porosity and Temperature on Oxidation Behavior in Air of Selected Nuclear Graphites

    pp. 1159-1163

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    DOI:10.2320/matertrans.MBW201107

    Nuclear graphite endures gas oxidation in High Temperature Gas-cooled Reactor (HTGR), which may threaten the safety of reactor. To study the oxidation behavior of nuclear graphite, weight loss curve is usually measured through Thermo Gravimetric Analysis (TGA) method. In this work, three brands of nuclear graphite for HTGR (i.e., HSM-SC, IG-11, and NBG-18) are oxidized under 873 and 1073 K in open air, and their weight loss curves are obtained. The acceleration of oxidizing rate is observed for both HSM-SC and IG-11, and is attributed to the large porosity increase during oxidation process. For HSM-SC, the porosity increase comes from preferential binder oxidation, and thus its binder quality shall be improved to obtain better oxidation resistance. Temperature effects on oxidation for HSM-SC are also studied, which shows that oxidizing gas tends to be exhausted at graphite surface at high temperature instead of penetrate into the interior of bulk.
  • Cation Distribution Dependence on Thermoelectric Properties of Doped Spinel M0.6Fe2.4O4

    pp. 1164-1168

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    DOI:10.2320/matertrans.M2012023

    The electrical conductivity, Seebeck coefficient, and thermal conductivity of polycrystalline M0.6Fe2.4O4 (M = Ni, Ni0.5Mg0.5, Ni0.5Zn0.5, Zn) were measured to elucidate cation distribution-dependent changes. Preferential occupation by the doped cation in the iron spinel has been noted: Zn2+ ions prefer to occupy the tetrahedral A-site, while Ni2+ and Mg2+ prefer to occupy the octahedral B-site. While the electrical conductivity and Seebeck coefficient are almost cation distribution-independent, the thermal conductivity at room temperature is sensitive to the cation distributions. The lowest thermal conductivity of 2.0 W m−1 K−1 at room temperature is observed for Zn0.6Fe2.4O4. The value is about one third of that of Ni0.6Fe2.4O4. The thermal transport of MxFe3−xO4 is mainly affected by cation distribution at the A-site, while the electrical transport is affected by the B-site, which is discussed in terms of the point defects at the A- and B-sites. Due to the disordering at the A- and B-sites, the thermal conductivity of MxFe3−xO4 could be reduced without decreasing the electrical conductivity. Doped spinel-ferrite MxFe3−xO4 would be a kind of “phonon-glass electron-crystal” material.
  • Growth of Cu(In,Ga)Se2 Films Selenized Using Dimethylselenium

    pp. 1169-1171

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    DOI:10.2320/matertrans.M2011355

    Cu(In,Ga)Se2 (CIGS) films were prepared by thermal treatment of In/CuGa metallic precursors using dimethylselenium, which is a less hazardous Se source than H2Se gas. CIGS films were fabricated using various heating times and dimethylselenium supply rates. We investigated the effect of the selenization temperature and the dimethylselenium supply rate on the crystal phase and surface morphology of the films. It was demonstrated that deimethylseleniumt is effective in preparation of the CIGS films as the alternative material of H2Se.
  • Clarification of the Fracture Location in Friction Stir Processed AZ31 Mg Alloy through Examination of Globe and Miniature Specimens

    pp. 1172-1177

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    DOI:10.2320/matertrans.M2012050

    Sections in a friction stir processed AZ31 Mg alloy were extracted primarily from specific positions corresponding to the stir zone to explore the tensile responses of mini-regions. According to the orientation distribution examined by the thin film X-ray method, basal planes in the stir zone tended to lie in a valley shape; meanwhile, a banded structure could also be recognized in the stir zone. The variation of basal plane alignment affects the deformation resistance and tensile ductility so that the tensile responses behaved as a function of locations in the stir zone. Combining sections with variant deformation resistance, the yield stress of a global specimen was similar to that of the mini-region with the minimum deformation resistance. With locations of low fracture resistance, the banded structures in the stir zone strongly affected the fracture feature and tensile ductility of each friction stir processed mini-region. The banded structures in the stir zone were also responsible for the reduction in the ductility of the globe specimen.
  • The Effect of TiB2 Particles on the Microstructure of Semi-Solid 7075 Alloy Slurry

    pp. 1178-1183

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    DOI:10.2320/matertrans.M2011328

    In situ reaction and low superheat degree pouring technologies have been successfully used to prepare semi-solid TiB2/7075 aluminum matrix composite slurry. The experimental results have shown that the TiB2 particulates become the stable nucleus of α-Al during the solidification of 7075 Al alloy. Moreover, high speed impact formed in the serpentine channel and stainless steel beaker is beneficial to get fine globular grains. The microstructure of 3 mass%TiB2/7075 composite is composed of uniform globular grains. The mean grain sizes and shape factors of the composite are 23 µm, 0.90 and 26 µm, 0.89 at 655°C pouring temperature when one or two curved channels were used respectively.
  • Microstructural Characteristics of Electric Discharge Alloyed Layers on Spheroidal Graphite Cast Iron with Different Electrode Materials

    pp. 1184-1190

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    DOI:10.2320/matertrans.M2011395

    In this study, electrical discharge alloying (EDA) is used to modify the surface of ferritic spheroidal graphite cast iron. Under the conditions of fixed EDA parameters and variable elements (i.e., electrode materials Fe–75 mass%Si, Al, Ni, Mg and Cr), these alloying elements are dissolved in substrate. Then, the effects of alloying with different alloying elements on the EDAed layers are investigated. Experimental results reveal that the Mg electrode has produced the thickest EDAed layer while the Cr electrode has produced a thinner layer. Additionally, EPMA/mapping is also used to analyze the distribution of different elements in their respective EDAed layers and X-ray diffraction is used to analyze the phase compositions of these layers. The results of hardness tests reveal that the EDAed layer with Cr has the highest hardness value. This may be related to the formation of carbides that contain Cr in the layer.
  • Effect of Grain Boundary Resistivity on Solid State Phase Transformation Induced by Electric Current Pulse in a Cold-Rolled Cu–Zn Alloy

    pp. 1191-1194

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    DOI:10.2320/matertrans.M2011368

    To clear the effect of grain boundary resistivity on solid state phase transformation, an electrical current pulse (ECP) was applied to a 40% cold-rolled Cu–Zn alloy, and the angle between the exerted current direction (CD) and rolling direction (RD) varied from 0 to 90°. Results showed that with the change of angle, the average grain sizes of ECP treated samples were varied. Through the thermodynamic analysis, it was known that the different diffusion activation energy induced by different grain boundary resistivity was attributed to the evolution of microstructure.
  • Thermal Relaxation Behavior of Residual Stress and Microstructure in Shot Peened S30432 Steel at Elevated Temperatures

    pp. 1195-1198

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    DOI:10.2320/matertrans.M2012058

    The thermal relaxation behavior of residual stress and microstructure at elevated temperatures in S30432 austenite steel after shot peening was investigated by X-ray residual stress analyzer. The effect of the exposure time and the applied temperature on the residual stress and microstructure relaxation was particularly analyzed and discussed. A significant decrease of the residual stress values and peak breadth was observed in the first period of exposure time, followed by slowing down and then stabilization. It was also observed that a higher applied temperature produced greater relaxation. The relaxation behavior could be described by Zener–Wert–Avrami function. The thermal stability of microstructure was higher than that of the residual stress. Both of the activation enthalpy of residual stress and microstructure relaxation were higher than that of iron self-diffusion in γ-Fe.
  • Control of Aluminum Concentration on Iron Surface by Powder Eutectic Coating Using Laser-Heating

    pp. 1199-1202

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    DOI:10.2320/matertrans.M2011383

    A method for coating iron with a mixture of aluminum, titanium and iron powder was successfully developed using laser-heating. The aluminizing process included control of the aluminum concentration on the iron surface. A double heating process is considered to be effective for homogenization of the coated layer.
  • Effect of Molybdenum-Rich Carbides on Wear Behavior of Powder Metallurgy Steel

    pp. 1203-1205

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    DOI:10.2320/matertrans.M2012063

    Effect of molybdenum-rich carbides on wear behavior of the powder metallurgy (P/M) steel was studied. Wear-resisting property of the couple part was also studied. The friction pair materials have low wear ratio. M2C carbides and M6C carbides exit in the P/M steels. Crisp M2C carbides decrease wear-resisting property of the materials obviously. While fine grainy M6C carbides are beneficial to good wear-resisting property of the materials.

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