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

MATERIALS TRANSACTIONS Vol. 43 (2002), No. 8

  • Role of Electrode Potential Difference between Lead-Free Solder and Copper Base Metal in Wetting

    pp. 1784-1790

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    DOI:10.2320/matertrans.43.1784

    The role of the electrochemical reaction between solder and copper base metal in soldering flux was investigated in relation to wetting. The research includes the contact polarization between solder and base metal in soldering flux of RA type. The difference of electrode potential between solder and base metal in soldering flux plays an important role in removing the oxide film by contact polarization. It is concluded that the electrode potential of Sn-based lead-free solders should be significantly lower than copper to obtain good wettability, because the accelerated anodic dissolution of tin by contacting with the copper base metal enhances the wettability by the removal of tin oxide which is one of the stable oxide on base metal and solder. Sn–Pb has the adequate electrode potential to be effectively dissolved when contacted by copper. On the other hand, the electrode potential of Sn–3.5Ag is very close to copper: the condition gives extremely small contact current. The addition of less noble elements that can lower the electrode potential is effective to enhance the wettability of Sn–3.5Ag. Sn–Zn solder has extremely low electrode potential than a copper base metal; the situation excessively accelerates the preferential anodic dissolution of zinc resulting in the no dissolution of tin. The addition of lead to Sn–3.5Ag solder lowered the electrode potential, which increased the potential difference between solder and copper base metal, as a result the contact current between them was increased. The improved wettability is confirmed by adding lead to Sn–3.5Ag; all lead added solders showed a larger spread area, i.e., a smaller contact angle than Sn–3.5Ag after the spreading test. This work proposed the role of electrochemistry in wetting based on the potential difference between base metal and solder including the degree of contact corrosion current between them.
  • Shear Strength and Interfacial Compound of Sn-Ag-Bi-In Alloy

    pp. 1791-1796

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    DOI:10.2320/matertrans.43.1791

    Sn–3Ag–8Bi–5In solder (melting point, 461–477 K) was investigated as a lead(Pb)-free solder. In order to evaluate the solderability of the Sn–Ag–Bi–In alloy, the shear strength of soldered joint and inter-metallic compound (IMC) were investigated. As an experimental procedure, a 0.5 mm diameter solder was set on the Ni/Cu/Cr-pad of a Si-substrate. The solder ball on the pad was reflowed with Rosin Mildly Activated (RMA)-flux in air, and the reflow temperature was controlled between 483 K and 533 K. The shear strength and microstructure of the solder ball were evaluated with and without aging. The results show that the shear strength of the Sn–3Ag–8Bi–5In solder ball had the highest value of 1.69N by reflowing at 513 K for 10 s. The shear strength decreased from 1.69N to 0.95N as the IMC thickness was increased from 1.75 \\micron to 1.9 \\micron. Needle-shaped Ni3Sn4 and plate-like (Ni, Cu)3Sn4 formed on the interface between the solder and the Under Bump Metallization (UBM) that was bonded for 10 s at 513 K.
  • Interface Reaction and Mechanical Properties of Lead-free Sn-Zn Alloy/Cu Joints

    pp. 1797-1801

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    DOI:10.2320/matertrans.43.1797

    The interfacial reaction and mechanical properties of Sn–Zn lead-free alloys/Cu joints were investigated under thermal exposure conditions. In the solder layer, Zn phases reacted with Cu and were transformed to Cu–Zn compounds with increasing exposure time. The microstructure change caused decreasing Vickers hardness of the solder layer. At the joint interface, although Cu–Zn compounds formed first, the formation of Cu–Sn compounds occurred with increasing exposure time. Simultaneously, the disappearance of Cu–Zn compounds and void formation occurred. The activation energy of the growth of Cu–Zn compounds at the joint interface was determined to be approximately 70 kJ/mol. That value is close to the activation energy of the diffusion of Zn in Sn crystal.
  • Mechanical Strength and Microstructure of BGA Joints Using Lead-Free Solders

    pp. 1802-1807

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    DOI:10.2320/matertrans.43.1802

    Au–Ni plated Cu pads were reflow soldered by using lead free solder balls. The microstructure and strength of the as-reflowed solder joints were investigated. For solder joints using Cu-free Sn–Pb and Sn–Ag solder balls, a Ni3Sn4 reaction layer was formed on the boundary between solder and pads. On the other hand, a Cu–Sn based (Cu, Ni)6Sn5 reaction layer (η) was formed in solder joints using Cu-containing solder balls. The growth rate for an η reaction layer during heat exposure at 423 K was much slower than that for a Ni3Sn4 reaction layer. This suppression of an η reaction layer growth can be attributed to the fact that the Cu in solder balls was mostly removed during the formation of the η layer. By ball shear test, cold bump pull and hot bump pull tests, mechanical properties of the obtained BGA joints were investigated. Fracture loads and crack propagation path changed by changing the mechanical tests, the BGA joints using Cu containing Sn–Ag–Cu solder or low P type Ni plating revealed better mechanical properties. We established the mismatch of the boundaries between reaction layers and the P-enriched Ni–P layer, which was caused by the chained voids formed due to the Kirkendall effect, led to low joint strength.
  • Mechanical Properties and Microstructure of Tin-Silver-Bismuth Lead-Free Solder

    pp. 1808-1815

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    DOI:10.2320/matertrans.43.1808

    This paper presents the mechanical properties and microstructure of Sn–Ag–Bi Pb-free solder. We evaluated the effects of Bi content on the mechanical properties of Sn–Ag–Bi solder such as tensile strength, elongation and deformation behavior at cross-head speeds of 0.1 mm/min and 500 mm/min. The experimental results show that at low cross-head speeds, the addition of Bi to Sn–Ag solder initially increases the tensile strength and decreases elongation due to solid-solution hardening of Sn-phase. As the Bi content is increased to 10 mass% and more, however, elongation increases to a maximum at Sn–Ag–Bi solder containing 57 mass%Bi. Deformation of Sn–Ag solder is governed by slip within the Sn phase, and for high-Bi solders (about 57 mass%Bi) deformation occurs due to slip at Sn–Bi grain boundaries. Intermediate-Bi solders, on the other hand, do not slip in either the Sn phase or at Sn–Bi grain boundaries. At high cross-head speeds, the elongation of both intermediate-Bi solders and high-Bi solders was low and almost constant, indicating slip at Sn–Bi grain boundaries becomes difficult. The impact resistance of these solders was investigated through charpy impact tests, and it is found that Bi has a marked effect on impact resistance. The impact absorption energy of Sn–Ag solder decreases rapidly with the addition of Bi.
  • Numerical Simulation of Dynamic Wetting Behavior in the Wetting Balance Method

    pp. 1816-1820

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    DOI:10.2320/matertrans.43.1816

    Wetting balance test is known to be the most versatile method to evaluate wettability during soldering process, because it provides quantitative information and time dependent wetting behavior. There are many other studies exist related with the wetting force, however, as to wetting time, there are few theoretical backgrounds and mathematical analysis. In this study, the wetting time is focused on a wettability index. The wetting curve, which shows time dependant wetting behavior, is predicted by using computer simulation and compared with oil experiment also, the mechanism of meniscus rise is analyzed. As a result, relationship of calculated and measured wetting curve can be obtained. It is found that the viscosity of a liquid is the major variable that determine the wetting time and wetting rate.
  • Interfacial Reactions Between Sn-58 mass%Bi Eutectic Solder and (Cu, Electroless Ni-P/Cu) Substrate

    pp. 1821-1826

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    DOI:10.2320/matertrans.43.1821

    The growth kinetics of intermetallic compound layers formed between eutectic Sn–58Bi solder and (Cu, electroless Ni–P/Cu) substrate were investigated at temperature between 70 and 120°C for 1 to 60 days. The layer growth of intermetallic compound in the couple of the Sn–58Bi/Cu and Sn–58Bi/electroless Ni–P system satisfied the parabolic law at given temperature range. As a whole, because the values of time exponent (n) have approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by diffusion mechanism over the temperature range studied. The apparent activation energies of Cu6Sn5 and Ni3Sn4 intermetallic compound in the couple of the Sn–58Bi/Cu and Sn–58Bi/electroless Ni–P were 127.9 and 81.6 kJ/mol, respectively.
  • Effects of Alloying in Near-Eutectic Tin-Silver-Copper Solder Joints

    pp. 1827-1832

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    DOI:10.2320/matertrans.43.1827

    This study included a comparison of the baseline Sn–3.5Ag eutectic to one near-eutectic ternary alloy, Sn–3.6Ag–1.0Cu and two quaternary alloys, Sn–3.6Ag–1.0Cu–0.15Co and Sn–3.6Ag–1.0Cu–0.45Co, to increase understanding of the effects of Co on Sn–Ag–Cu solder joints cooled at 1–3°C/s., typical of reflow assembly practice. The results revealed joint microstructure refinement due to Co-enhanced nucleation of the Cu6Sn5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 h at 150°C. Some recent additional results with Co and Fe additions are consistent with this catalysis effect, where a reduced total solute level was tested. The baseline Sn–3.5Ag joints exhibited significantly reduced strength retention and coarser microstructures.
  • Melting and Joining Behavior of Sn/Ag and Sn-Ag/Sn-Bi Plating on Cu Core Ball

    pp. 1833-1839

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    DOI:10.2320/matertrans.43.1833

    We fabricated Cu core Sn–Ag solder balls by plating pure Sn and Ag on Cu balls and clarified that Sn/Ag plating began to melt at a rather low temperature, the eutectic temperature of Sn–Ag–Cu. This early melting at the eutectic temperature was ascribed to the diffusion of Cu and Ag into the Sn plating during the heating process. We investigated the solderability of the BGA joint with the Ni/Au coated Cu pad to compare it with that of the commercial Sn–Ag and Sn–Ag–Cu balls. After reflow soldering, we observed a eutectic microstructure composed of β-Sn, Ag3Sn, and Cu6Sn5 phases in the solder, and a η-(Au, Cu, Ni)6Sn5 reaction layer was formed at the interface between the solder and the Cu pad. The BGA joint using Cu core solder balls could prevent the degradation of joint strength during aging at 423 K because of the slower growth rate of the η-(Au, Cu, Ni)6Sn5 reaction layer formed at the solder-pad interface. Furthermore, we were able to fabricate Cu-cored, multicomponent Sn–Ag–Bi balls by sequentially coating binary Sn–Ag and Sn–Bi solders onto Cu balls. The coated balls also exhibited almost the same melting and soldering behaviors as those of the previously alloyed Sn–2Ag–0.75Cu–3Bi solders.
  • Influence of Phosphorus Concentration in Electroless Plated Ni-P Alloy Film on Interfacial Structures and Strength between Sn-Ag-(-Cu) Solder and Plated Ni-P Alloy Film

    pp. 1840-1846

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    DOI:10.2320/matertrans.43.1840

    One of the critical issues which needs to be solved in the packaging technology of high speed and high density semiconductor devices is the enhancement of micro-solder joint reliability and strength. The reliability and strength of the solder joints depend on the interfacial structures between metallization and lead free solder. Both the interfacial structures and the strengths of the solder joints between plated Ni–P alloy films with various P concentrations and various solder materials have been investigated. The places where intermetallic compounds crystallized were found to vary according to the P concentration in plated Ni–P alloy films and the composition of the solder. Pyramidal intermetallic compounds that formed on plated Ni–P alloy films had the following compositions: Sn–3.5 mass%Ag/Ni–2 mass%P, Sn–3.5Ag–0.7 mass%Cu/Ni–P(2, 8 mass%) and Sn–50 mass%Pb/Ni–P(2, 8 mass%). Whereas intermetallic compounds were crystallized in the solder of the Sn–3.5 mass%Ag/Ni–8 mass%P sample. A P-enriched layer was formed between the plated Ni–P alloy films and the intermetallic compounds. The thickness of the P-enriched layers of each sample increased with the reaction time. In experiments using the same solder material, the P-enriched layer of the solder/Ni–8 mass%P sample was much thicker than that of the solder/Ni–2 mass%P sample. In experiments with plated Ni–8 mass%P alloy films, the P-enriched layers became thicker in this order: Sn–50 mass%Pb/Ni–8 mass%P; Sn–3.5Ag–0.7Cu/Ni–8 mass%P; Sn–3.5 mass%Ag/Ni–8 mass%P . The strengths of the solder joints decreased with the P concentration in plated Ni–P alloy films for all solder materials. However, it was found that the strength degradation ratio varied with the solder materials and they increased in the following order: Sn–50 mass%Pb; Sn–3.5Ag–0.7 mass%Cu; Sn–3.5 mass%Ag. Therefore, it was found that the solder joint strength is very sensitive to the thickness of the P-enriched layer at the solder joint and the solder joint strength decreased with the thickness of the P-enriched layer independent of the solder materials.
  • Anomalous Creep in Sn-Rich Solder Joints

    pp. 1847-1853

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    DOI:10.2320/matertrans.43.1847

    This paper discusses the creep behavior of example Sn-rich solders that have become candidates for use in Pb-free solder joints. The specific solders discussed are Sn–3.5Ag, Sn–3Ag–0.5Cu, Sn–0.7Cu and Sn–10In–3.1Ag, used in thin joints between Cu and Ni/Au metallized pads. The creep behavior of these joints was measured over the range 60–130°C. The four solders show the same general behavior. At all temperatures their steady-state creep rates are separated into two regimes with different stress exponents (n). The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95 to 60°C. The anomalous behavior of the solders appears to be due to the dominant Sn constituent. Joints of pure Sn have stress exponents, n, that change with stress and temperature almost exactly like those of the Sn-rich solder joints. Surprisingly, however, very similar behavior is found in Sn–10In–3.1Ag, whose primary constituent is γ-InSn. Research on creep in bulk samples of pure Sn suggests that the anomalous temperature dependence of the stress exponent is due to a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating Sn-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.
  • Tensile Properties of Sn-3.5Ag and Sn-3.5Ag-0.75Cu Lead-free Solders

    pp. 1854-1857

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    DOI:10.2320/matertrans.43.1854

    The tensile properties of Sn–3.5Ag and Sn–3.5Ag–0.75Cu lead-free solders were investigated. The effect of annealing at 100°C for 1 h before the tensile test on mechanical properties was small in both solders. The tensile strength decreased with decreasing strain rate, and with increasing test temperature. However, the ductility of each solder was relatively constant in the strain rate ranging from 1.67×10−4 s−1 to 1.67×10−2 s−1 and in the test temperature ranging from −40 to 120°C. From the results of the strain-rate-change tests, the strain sensitivity for Sn–3.5Ag and Sn–3.5Ag–0.75Cu were found to be 0.077 and 0.078, respectively.
  • Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

    pp. 1858-1863

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    DOI:10.2320/matertrans.43.1858

    The ball shear strength of BGA solder joints during isothermal aging was studied with Sn–3.5Ag–0.75Cu solder on three different pads (Cu, electroless Ni–P/Cu, immersion Au/Ni–P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days. The reliability of solder ball attachment was characterized by mechanical ball shear tests. As a whole, the shear strength of BGA joints decreased with increasing temperature and time. The shear strength for both the immersion Au/Ni–P/Cu and electroless Ni–P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions. The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad. The P-rich Ni layer formed at the interface between (Cu, Ni)6Sn5 and Ni–P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni–P/Cu and electroless Ni–P/Cu pad.
  • Mechanical Properties and Shear Strength of Sn-3.5Ag-Bi Solder Alloys

    pp. 1864-1867

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    DOI:10.2320/matertrans.43.1864

    Mechanical properties and ball shear strength of Sn–3.5Ag–Bi were investigated with Bi addition of 0–9 mass%. Solder characteristics of Sn–3.5Ag–Bi were improved with Bi addition to decrease the melting temperature, to promote wetting to Cu and Ni substrates, and to increase the tensile strength and fracture energy. Shear strength of Sn–3.5Ag–Bi solder bumps increased with Bi addition up to 5 mass%, and was kept almost unchanged with further increase of Bi addition. Shear strength exhibited a parabolic relationship with the tensile fracture energy of the solder alloys.
  • Thermodynamic Properties of Liquid Al-Sn-Zn Alloys: A Possible New Lead-Free Solder Material

    pp. 1868-1872

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    DOI:10.2320/matertrans.43.1868

    Because of its potential to be used as a lead free solder material the thermodynamic properties of the liquid Al–Sn–Zn system were investigated. Using an appropriate galvanic cell, the partial free energies of Al were determined as a function of concentration and temperature. Thermodynamic properties were obtained for 30 alloys. Their composition was situated on three cross-sections with constant ratios of Sn:Zn=2:1, 1:1 and 1:2. The integral Gibbs energy and the integral enthalpy for the ternary system at 973 K were calculated by Gibbs-Duhem integration.
  • Thermodynamics-Aided Alloy Design and Evaluation of Pb-free Solders for High-Temperature Applications

    pp. 1873-1878

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    DOI:10.2320/matertrans.43.1873

    High temperature solders that will not be affected in the subsequent thermal treatment are required in the step soldering process of multi-chip module (MCM) packaging. High-Pb solder alloys such as 95Pb–5Sn (numbers are all in mass% unless specified otherwise) are currently being used for this purpose. However, the development of the Pb-free solder alloy for high temperature applications is needed due to environmental issues. The solder alloys of Bi–Ag, Sn–Sb and Au–Sb–Sn systems are considered as candidates in this study. Aided by thermodynamic calculations, several specific compositions have been chosen and they were investigated in terms of melting behavior, electrical resistivity, wetting angle and hardness. The Bi–Ag alloy exhibited poor electro-conductivity while the Sn–Sb system had low melting temperatures. The ternary Au–Sn–Sb solder alloy shows prospects for high temperature applications in spite of poor wetting properties.
  • Thermodynamic Calculation of Phase Diagram in the Bi-In-Sb Ternary System

    pp. 1879-1886

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    DOI:10.2320/matertrans.43.1879

    A thermodynamic description of the Bi–In–Sb ternary system of lead-free solder alloys using the CALPHAD (Calculation of Phase Diagram) method is presented. Phase equilibria information such as vertical sections, liquidus projection and thermochemical quantities were calculated and compared with the experimental data. The calculated and experimental data are in excellent agreement in most cases.
  • Influence of P Content in Electroless Plated Ni-P Alloy Film on Interfacial Structures and Strength between Sn-Zn Solder and Plated Au/Ni-P Alloy Film

    pp. 1887-1890

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    DOI:10.2320/matertrans.43.1887

    The interfacial structure and strength of solder joints between Sn–9 mass%Zn solder and plated Au/Ni–P alloy film on a Cu substrate have been investigated. Three reaction layers with 0.2 to 0.5 \\micron thickness were formed along the interface between the plated Ni–P alloy films and Sn–9 mass%Zn solder. The outermost layer contains a Ni–Sn intermetallic compound. The middle layer contains approximately 40 mass%Au, 35 mass%Zn, 20 mass%Ni and 5 mass%Sn. The thickness of the Au layer is 0.1 \\micron, so the Au layer does not dissolve. The innermost layer contains about 63 mass%Zn, 25 mass%Ni, 10 mass%Au and 2 mass%Sn. The strength of the Sn–9 mass%Zn solder joints take almost the same values independent of P concentration. The strength of Sn–Zn solder joints with Sn–Zn/Ni–2 mass%P, Sn–Zn/Ni–4 mass%P and Sn–Zn/Ni–8 mass%P joints were found to be almost constantly independent of reflow cycles. Therefore, Sn–9 mass%Zn solder is considered to be an excellent solder material for plated Ni–P alloy films.
  • Recent Progress in Bulk Glassy Alloys

    pp. 1892-1906

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    DOI:10.2320/matertrans.43.1892

    This paper deals with recent progress in stabilized supercooled liquid and the resulting bulk glassy alloys and focusing on the following factors; alloy composition, forming ability, formation mechanism, computed glass-forming ability, computed glass formation range, atomic configuration, production techniques, mechanical properties, corrosion resistance, soft magnetic properties, micro-forming ability, applications, significance to science and engineering, and future trends for bulk glassy alloys including supercooled liquid. As demonstrated in this review, the high stability of metallic supercooled liquid has already opened up new fields of investigation in basic science and yielded new engineering applications. There is every reason to expect that their importance will continue to increase.
  • Kinetics and Thermodynamics of Bulk Glass Formation in a Zr52.5Cu17.9Ni14.6Al10Ti5 Alloy

    pp. 1907-1912

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    DOI:10.2320/matertrans.43.1907

    Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glasses have been prepared by copper moulding. In order to obtain a fully amorphous sample, a careful control of the atmosphere conditions in the casting chamber was necessary. In particular, the presence of oxygen contamination promotes the nucleation of a fcc “big cube” phase, reducing glass formation. The crystallisation of bulk amorphous samples has been followed by DSC and values of about 4 kJ·mol−1 have been obtained for the heat release at about 740 K. By high temperature DSC, a metastable-to-stable phase transformation has been evidenced at 938 K, which gives a heat evolution of 1.12 kJ·mol−1. Melting and solidification of the alloy have been observed at 1070 K, giving an average value for the enthalpy change of about 8.2 kJ·mol−1. The temperature dependence of the enthalpy difference between undercooled liquid and equilibrium crystal phases has been described on the basis of different models for the specific heat difference (ΔCp) between the two phases. A best fitting of experimental data gives values of ΔCp of about 21 J·mol−1·K−1 at the glass transition temperature (690 K), in good agreement with experimental values obtained from DSC measurements.
  • Direct Comparison between Critical Cooling Rate and Some Quantitative Parameters for Evaluation of Glass-Forming Ability in Pd-Cu-Ni-P Alloys

    pp. 1913-1917

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    DOI:10.2320/matertrans.43.1913

    The quantitative parameters such as supercooled liquid region or several reduced glass transition temperatures were applied for evaluating the glass-forming ability of Pd-based metallic glasses. A distinct proportional tendency was recognized between measured critical cooling rates and reduced glass transition temperature by liquidus temperature rather than eutectic temperature. Significance and physical meanings of the quantitative parameters for evaluating of glass-forming ability were also discussed. Furthermore, a new concept of a modified reduced glass transition temperature was proposed to evaluate the glass-forming ability. The modified reduced glass transition temperature as a function of critical cooling rate was found to exhibit a much clear linearity.
  • Determination of Atomic Sites of Nb Dissolved in Metastable Fe23B6 Phase

    pp. 1918-1920

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    DOI:10.2320/matertrans.43.1918

    The primary metastable phase in the Fe70Nb10B20 amorphous alloy is the complex fcc Fe23B6 phase with a large lattice parameter. The atomic site of Nb atoms in this metastable phase was determined by the anomalous X-ray scattering (AXS) method. Nb atoms in the metastable phase located at the similar position expected in the local structural unit in the amorphous state. Thus, the precipitation of the metastable phase does not require long-distance diffusion of Nb atoms in the amorphous matrix.
  • Relation Between Time and Temperature Dependence of Diffusion and the Structural State in ZrTiCuNiBe Bulk Glasses

    pp. 1921-1930

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    DOI:10.2320/matertrans.43.1921

    Time and temperature dependence of impurity diffusion is measured in detail for several ZrTiCuNiBe bulk glasses in a wide temperature range. We find that neither the quenched-in free volume nor a small volume fraction of crystals have a significant effect on the diffusivity. It is shown that the composition of these glasses has a much weaker effect on the diffusion coefficients than have structural changes of the glass. By long time relaxation the reversible metastable equilibrium of the glass structure could be reached at temperatures below the calorimetric glass transition. For this metastable equilibrium state a single set of diffusion parameters, activation enthalpy and pre-factor of the diffusion coefficient, is derived in the entire temperature range above and below the glass transition. This indicates that the usually observed non-linear Arrhenius behavior of diffusion coefficients in the ZrTiCuNiBe bulk glasses is caused by insufficient structural relaxation at temperatures below the calorimetric glass transition.
  • Electrical Resistivity and Mössbauer Studies for the Structural Relaxation Process in Pd-Cu-Ni-P Glasses

    pp. 1931-1936

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    DOI:10.2320/matertrans.43.1931

    The structural relaxation process in several Pd–Cu–Ni–P glasses was investigated by the electrical resistivity and the Mössbauer experiments. The change in the resistivity, ρ(300), and the corresponding slope, α(300)=(dρ⁄dT)300, at room temperature was measured after a given heat treatment. The change in ρ(300) with isochronal annealing below the glass transition temperature (≈572 K), starting from room temperature, demonstrated that the ρ(300) increased with temperature and decreased beyond 550 K. The isothermal annealing at 570 K exhibited that the ρ(300) showed an increase, about 1%, during an initial annealing stage for 3.0×102 s and remained almost constant up to 6.0×103 s, and hereafter monotonously decreased. The change in α(300) showed an opposite sign to that in ρ(300). The reversible change in ρ(300) due to isochronal annealing was observed in the heat treatment repeated between 350 and 550 K. The Mössbauer spectroscopy experiments were performed at room temperature to investigate the local structure change due to structural relaxation. The isomer shift and the quadrupole splitting for an annealed sample decreased in comparison with that of an as-quenched sample. Based on these results, the structural relaxation process and the structure change were discussed within the framework of the free volume model.
  • Stability of Supercooled Liquid and Transformation Behavior in Zr-Based Glassy Alloys

    pp. 1937-1946

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    DOI:10.2320/matertrans.43.1937

    We investigated the stability of the supercooled liquid in Zr-based glassy alloys by examination of their transformation behavior. The primary phase is an fcc Zr2Ni phase in a Zr65Al7.5Ni10Cu17.5 glassy alloy with high glass-forming ability (GFA). By substitution of Ag, Pd, Au or Pt for only 1 at%Cu, an icosahedral quasicrystalline phase (I-phase) precipitates with the fcc Zr2Ni phase. The primary phase changes to the single I-phase at higher noble metal contents. It is further found that the I-phase precipitates by a small amount of substitution for Cu with other elements as well as the noble metals, which have a weak or positive chemical affinity with one of the constitutional elements in the Zr–Al–Ni–Cu glassy alloy. Thus, the slight deviation from the three component rules for high GFA is effective for the I-phase formation. The I-phase is also formed as a primary phase in the Zr65+yAl7.5Ni10Cu17.5−y (y=1–7) glassy alloys. A slight change in the composition has the similar effect as the addition of element such as noble metals and so on. An icosahedron is contained as a structure unit in the fcc Zr2Ni and I-phases and hence the glassy structure is correlated with the local icosahedral atomic configuration. The high-resolution transmission electron microscopy image of the Zr70Al7.5Ni10Cu12.5 glassy alloy reveals a possibility of the existence of the icosahedral ordered regions. It is therefore, concluded that the icosahedral short- or medium-range order exists in the supercooled liquid as well as in the glassy phase and it stabilizes the supercooled liquid state in the Zr-based alloys. An I-phase also precipitates as a primary phase by substituting Pd, Au or Pt for only 1 at%Cu in the Zr70Cu30 glassy alloy. From these results, the appearance of the supercooled liquid region is attributed to the existence of the icosahedral atomic configuration consisting of Zr and Cu.
  • Structure of Zr52Ti5Cu18Ni15Al10 Bulk Metallic Glass at Elevated Temperatures

    pp. 1947-1951

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    DOI:10.2320/matertrans.43.1947

    The structural behavior of the Zr52Ti5Cu18Ni15Al10 bulk glass has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The crystallization starts with the formation of an extremely fine nanostructure followed by the transformation into tetragonal NiZr2-type crystals plus an unknown phase. Both phases are metastable and transform at about 1123 K into the stable equilibrium phases. The temperature dependence of the scattering curve I(q) of the glass can be well described within the framework of the Debye theory. At the glass transition the first derivative dI(q)⁄dT changes. A Debye temperature θ=412 K was estimated for the glassy, and θ=162 K for the liquid state of the Zr52Ti5Cu18Ni15Al10 alloy. The short-range order of the glass, of the supercooled liquid state, and of the equilibrium melt at T=1193 K is found to be quite similar. The results point to the formation of complex chemically ordered clusters already in the melt.
  • Thermal Stability and Mechanical Strength of Bulk Glassy Ni-Nb-Ti-Zr Alloys

    pp. 1952-1956

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    DOI:10.2320/matertrans.43.1952

    Bulk glassy Ni-based alloys with high fracture strength exceeding 2700 MPa were prepared in Ni60Nb40−xyTixZry system by copper mold casting. The glassy alloys with distinct glass transition were obtained in the wide composition range from 0 to 35%Ti and 0 to 30%Zr and the largest supercooled liquid region before crystallization was 76 K for Ni60Nb15Ti10Zr15. The maximum diameter was 2 mm for Ni60Nb20Ti15Zr5 and the glass transition temperature (Tg), crystallization temperature (Tx) and reduced glass transition temperature (TgTl) of the bulk glassy alloy were 841 K, 898 K and 0.61, respectively. The Young’s modulus (E), compressive fracture strength (σc,f) and compressive fracture elongation (εc,f) were 156 GPa, 2770 MPa and 2.4%, respectively, for the bulk alloy. There is a tendency for fracture strength to increase with increasing E, Tg and liquidus temperature (Tl). It is therefore interpreted that the high strength is due to strong bonding nature among the constituent elements.
  • Anelastic Behavior under Tensile and Shearing Stresses in Bulk Metallic Glasses

    pp. 1957-1960

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    DOI:10.2320/matertrans.43.1957

    It is known that an anelastic deformation occurs more remarkably for amorphous ribbon alloys than for crystalline metallic alloys by the tensile test. In addition, the result of the molecular dynamics simulation on tensile and shearing tests for Cu single component amorphous metal has indicated that the anelastic deformation occurs more remarkably under the shearing stress rather than under the tensile stress. In this report, tensile and torsional tests were actually performed for bulk metallic glasses to examine the difference in the anelastic behavior under shearing and tensile stresses. Single-phase bulk metallic glasses, La60Al20Ni10Cu5Co5, Pd40Cu30Ni10P20 and Zr55Cu30Ni5Al10 (at%), were chosen together with a steel, JIS SGD 400-D, as a representative of metallic crystals. The test specimens were a round bar shape and the diameters of a parallel gage section were 4 to 10 mm. No anelastic behavior was observed for the steel under tensile and shearing stresses. Although the metallic glasses did not exhibit distinct anelastic deformation under the tensile stress, the shearing stress mode leads to a significant anelastic deformation even at low stress level. The amount of the anelastic deformation increases with an increase in the shearing stress level.
  • Preparation of Fe65Co10Ga5P12C4B4 Bulk Glassy Alloy with Good Soft Magnetic Properties by Spark-Plasma Sintering of Glassy Powder

    pp. 1961-1965

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    DOI:10.2320/matertrans.43.1961

    With the aim of developing a large size bulk glassy Fe-based alloy with good soft magnetic properties by the powder metallurgy technique, we have applied the spark-plasma sintering technique to a Fe65Co10Ga5P12C4B4 glassy alloy powder with a large supercooled liquid region of 50 K before crystallization. The existence of the supercooled liquid region enabled us to form a large size bulk glassy alloy of 20 mm in diameter and 5 mm in thickness with a high relative density of 99%. The resulting bulk glassy alloy exhibits good soft magnetic properties, i.e., 1.20 T for saturation magnetization, 14 A/m for coercive force and 6000 for maximum permeability. The good soft magnetic properties for the multicomponent Fe-based bulk alloy are attributed to the combination of the high relative density and the maintenance of the single glassy structure. The success of forming the large size bulk glassy alloy with good soft magnetic properties by the powder metallurgy techniques is promising for future use as practical soft magnetic materials.
  • Magnetic Properties and Phase Transformations of Bulk Amorphous Fe-Based Alloys Obtained by Different Techniques

    pp. 1966-1973

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    DOI:10.2320/matertrans.43.1966

    Bulk amorphous Fe65.5Cr4Mo4Ga4P12C5B5.5 rods with diameters of 1.5–3 mm were prepared by copper mold casting. Besides casting, bulk amorphous Fe77Al2.14Ga0.86P8.4C5B4Si2.6 samples in the shape of discs (diameter of 10 mm and thickness of 3 mm) were prepared from melt-spun ribbons by high-energy ball milling and subsequent compaction of the resulting powders in the supercooled liquid region. The as-cast amorphous FeCrMoGaPCB samples exhibit a low coercivity, below 10 A·m−1. In the case of the FeAlGaPCBSi alloy, the milling-induced stress causes significant differences in coercivity between the ribbons and the powders. The relatively low coercivity of about 5–10 A·m−1 characteristic for the melt-spun ribbons increases after 1 hour of ball milling to a value of about 2200 A·m−1. Subsequent annealing of the ball-milled powders leads to a decrease of Hc by a factor of 10 to about 220–250 A·m−1. The bulk samples prepared by hot pressing of the crushed ribbons show a coercivity of about 120–140 A·m−1. For both alloys, thermal stability measurements show a distinct glass transition, followed by a supercooled liquid region of 60 K for Fe65.5Cr4Mo4Ga4P12C5B5.5 and of 30 K for Fe77Al2.14Ga0.86P8.4C5B4Si2.6. For the Fe65.5Cr4Mo4Ga4P12C5B5.5 alloy, crystallization of the amorphous phase as observed by in-situ X-ray diffraction measurements in transmission configuration occurs via the formation of a metastable intermediate phase. The phases observed in the crystalline state obtained by heating do not correspond to those occurring after slow cooling.
  • Thermal Stability and Soft Magnetic Properties of (Fe, Co)-(Nd, Dy)-B Glassy Alloys with High Boron Concentrations

    pp. 1974-1978

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    DOI:10.2320/matertrans.43.1974

    Fe-based glassy alloys with high boron concentrations in (Fe, Co)–(Nd, Dy)–B system were found to exhibit a large supercooled liquid region (ΔTx) and good soft magnetic properties. The Fe62Co9.5Nd3Dy0.5B25 glassy alloy with ΔTx of 56 K exhibits high saturation magnetization (Is) of 1.41 T, low coercive force (Hc) of 2.6 A/m and large permeability (μe) at 1 kHz of 12000. The Fe62Co9.5Nd3Dy0.5B25 glassy alloy rods were produced in the diameter range up to 0.75 mm by copper mold casting. The substitution of 2 at% elements TM (TM=Nb, Ta, Mo and W) for Fe and Co, remarkably increases the ΔTx and TgTl, leading to an increase in the glass-formation ability (GFA) for Fe60.3Co9.2Nd3Dy0.5B25TM2. The ΔTx and TgTl increase from 56 to 87 K and 0.56 to 0.57, respectively, and the glassy alloy rods were produced in the diameter range up to 1.2 mm. The saturation magnetization Is decreases slightly, while the coercive force Hc remains almost unchanged by the addition of 2 at% TM elements.
  • High Strength Magnesium-based Glass Matrix Composites

    pp. 1979-1984

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    DOI:10.2320/matertrans.43.1979

    Magnesium-based glass matrix composites containing oxide particles were produced by mechanical alloying of Mg55Cu30Y15 elemental powder mixtures with the addition of MgO, CeO2, Cr2O3 or Y2O3 oxide particles. The formation of the glassy phase was characterized by X-ray-diffraction and transmission electron microscopy methods and was found to proceed almost unaffected by the presence of the oxides. Differential-scanning-calorimetry-analysis revealed, that the amorphous matrix features a wide supercooled liquid region with an extension of about 40–50 K. Differences in the thermal stability of the composites depending on the oxide addition are discussed. Viscosity measurements proved the existence of a characteristic minimum of viscosity in this temperature range which was used to consolidate the powders into bulk samples by uniaxial hot pressing. The deformation behaviour under compression at room temperature as well as at elevated temperature of 423 K yielded excellent properties compared to conventionally produced magnesium-based alloys.
  • Bulk Amorphous and Partially Crystallized Alloys in Nd-Fe-(Al, B) System with Hard Magnetic Properties Prepared by Arc Melting

    pp. 1985-1991

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    DOI:10.2320/matertrans.43.1985

    Bulk Nd60Fe30Al10−xBx (x=0, 2.5, 5, 7.5 and 10 at%) alloys with hard magnetic properties were prepared by an arc melting method. The bulk alloys have weights of 10 and 100 mg with button shapes of 1 and 6 mm in thickness, respectively. The X-ray diffraction and TEM observation data reveal that the Nd60Fe30Al10 bulk alloy exhibits an amorphous phase while the other alloys consist of partial crystalline and amorphous phases. The bulk Nd60Fe30Al10 amorphous alloy of 1 mm in thickness exhibits crystallization temperature (Tx) of 802 K, eutectic temperature (Te) of 925 K, and reduced crystallization temperature (TxTe) of 0.87. The alloy exhibits hard magnetic properties at 298 K, i.e., saturation magnetization of 0.13 T, remanance of 0.09 T, and intrinsic coercive filed of 275 kA/m under an applied field of 1242 kA/m. These thermal and magnetic properties are nearly the same as those for the corresponding bulk amorphous cylinders of 1 to 12 mm in diameter prepared by copper mold casting. The replacement of Al with B in Nd–Fe–Al amorphous alloy causes a decrease in amorphous-forming ability (AFA), although their hard magnetic properties remain almost unchanged. The reason for the decrease in AFA was analyzed using mixing enthalpy (ΔHmix) and mismatch entropy (Sσ) corresponding the empirical criteria for the achievement of high AFA. Furthermore, amorphous-forming composition regions (AFCRs) were calculated by giving a limitation for formation of an amorphous phase to ΔHmix and Sσ values, which arises from the statistics for 6500 amorphous alloys listed in a database. The calculated AFCR of the Nd–Fe–Al and Nd–Fe–B alloy systems agrees with the experimental data reported previously. The decrease in AFA by the replacement of Al with B in the Nd60Fe30Al10−xBx alloys is interpreted from the result that Nd60Fe30B10 is located near the edge of the AFCR in Nd–Fe–B system.
  • Coercivity and Phase Transitions of Clustered Nd90-xFexAl10 Bulk Hard Magnets

    pp. 1992-1999

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    DOI:10.2320/matertrans.43.1992

    Rapidly and slowly quenched Nd90−xFexAl10 glassy alloys with thicknesses up to 3 mm were investigated comparatively by structural and magnetic measurements in the temperature range 5–800 K and external fields up to 9 T . The glass-forming ability decreases increasing Fe content. Room temperature coercivities of over 0.6 T are observed, depending on composition and external field. The huge increase of the coercive field up to 5.5 T at low temperatures as well as the dependence on the cooling rate are supposed to result from the non-collinear magnetic structures developed in these amorphous alloys. From DC and AC magnetic measurements combined with neutron diffraction results we conclude that the structure, which is quenching conditions dependent, consists of a packing of nanometer-sized clusters. The topological and magnetic structures of Nd90−xFexAl10 melt-spun ribbons and cast rods in a wide range of temperatures and for different x values are modelled using the reverse Monte Carlo method (RMC). The hypotheses of a cluster model, in which exchange interactions, local random anisotropies and thermal effects are competing, are proposed.
  • Soft Magnetic Nanocrystalline Alloys for High Temperature Applications

    pp. 2000-2005

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    DOI:10.2320/matertrans.43.2000

    High temperature inductor applications require improved magnet performance, including simultaneous high magnetization and low core losses at ever higher operation frequencies. Conventional polycrystalline soft magnet alloys are mature technologies with little room for meaningful improvement. Recently, nanocrystalline soft magnetic materials possessing reduced hysteretic losses and offering higher operation frequencies than conventional alloys have been introduced. These nanocrystalline soft magnets with compositions Fe–Co–Zr–B–(Cu) have been offered as alternatives to the conventional alloys. This paper describes the processing, structure, and magnetic properties of these materials.
  • Developments of Aluminum- and Magnesium-Based Nanophase High-Strength Alloys by Use of Melt Quenching-Induced Metastable Phase

    pp. 2006-2016

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    DOI:10.2320/matertrans.43.2006

    The stabilization of supercooled metallic liquid occurs for a number of alloys where the constituent elements have the following three rules, i.e., multi-component of more than three elements, significant mismatches of atomic size above 12% among the main three elements, and negative heats of mixing among their main elements. The use of the stabilized supercooled liquid has enabled us to synthesize a number of novel advanced metallic materials. In this review, we present the alloy components, fabrication processings, structures, mechanical properties and applications of high-strength Al- and Mg-based alloys with glassy, nanocrystalline or nanoquasicrystalline phase which were developed on the basis of the concept of the utilization of stabilized supercooled liquid for the last one decade.
  • Structural Characterisation and Mechanical Properties of Nanocomposite Al-based Alloys

    pp. 2017-2025

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    DOI:10.2320/matertrans.43.2017

    Nanocomposite Al-based alloys can be obtained with a combination of amorphous, crystalline and quasicrystalline phases. In order to understand the correlation between the nanostructure and the mechanical behaviour, four nanocomposite alloys with different characteristics were studied: two alloys from the Al–Fe–Cr–Ti system consisting of a spherical nanoquasicrystalline phase in an α-Al matrix; one alloy from the Al–Fe–V–Ti system consisting of a mixture of amorphous and α-Al phases; and one alloy from the Al–Mn–Cr–Cu system consisting of nanocrystalline particles embedded in an α-Al matrix. Melt-spun samples were prepared and the structure was characterised by means of X-ray diffraction and transmission electron microscopy. Differential scanning calorimetry was used to study the thermal stability and the transformation processes. Tensile tests, fractographic analysis and Vickers microhardness at room temperature were performed in order to evaluate the mechanical behaviour. A combination of solid solution, particle dispersion and grain refinement strengthening was responsible for the high strength of the alloys. The microstructure of the alloy Al93Fe3Cr2Ti2 (at%) remained acceptably stable up to 703 K, due to the slow coarsening rate of the icosahedral phase.
  • Strengthening Mechanisms in Al-Based and Zr-Based Amorphous Nanocomposites

    pp. 2026-2030

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    DOI:10.2320/matertrans.43.2026

    Devitrified Al-based and Zr-based amorphous alloy nanocomposites with nanoscale precipitates of element, compound or quasicrystalline particles are regarded as new prospective structural materials with good mechanical properties. In order to analyse the strengthening behaviour of the partially devitrified amorphous nanocomposites, a phase mixture model is presented, in which the partially crystallised Al-based or the Zr-based amorphous alloys is regarded as a nanocomposite of nanoscale particles and the remaining amorphous matrix. Most attention is paid to the change of solute concentration in the matrix. The element, compound or quasicrystalline particles are treated as perfect materials. The matrices are treated as amorphous materials, in which the solute concentrations change depending on the solute concentration and volume fraction of precipitate particles. Investigating the solute concentration changes associated with overall mechanical properties could prove that the phase mixture model can successfully describe the strengthening mechanism in the devitrified Al-based and Zr-based amorphous nanocomposites.
  • Composed Phases and Microhardness of Aluminium-Rich Aluminium-Iron Alloys Obtained by Rapid Quenching, Mechanical Alloying and High Pressure Torsion Deformation

    pp. 2031-2038

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    DOI:10.2320/matertrans.43.2031

    Aluminium-based Al–Fe alloys with Fe content of 2, 4, 5, 8 and 11 mass% were examined using X-ray diffraction and Mössbauer spectroscopy. The alloys were prepared by various techniques: remelting, accelerated cooling by centrifugal casting, rapid quenching from the melt at a rate of 106 K/s, and mechanical alloying of pure elements in high-energy planetary ball mill. It is shown that the crystalline structure refinement and the composed phases of the alloys essentially depend on the techniques used for the sample preparation. Phase transformations by severe plastic torsion deformation of the alloys prepared by various techniques were studied. The highest supersaturation of Fe in the aluminium-based solid solution can be reached using two subsequent techniques of alloy treatment: rapid quenching and high-pressure torsion.
  • Amorphous Phase and Compact Solid Formation of Ti-(37.5-X) at%Si-X at%Fe (X=0-10) Powders by Mechanical Alloying and Pulse Current Sintering

    pp. 2039-2043

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    DOI:10.2320/matertrans.43.2039

    Ti–(37.5−X) at%Si–X at%Fe (X=0–10) powder mixtures have been mechanically alloyed in a planetary ball mill under an argon atmosphere for 180 ks. The milled alloy powders become amorphous and crystallization temperature is 843 K for X=5, being 90 K higher than that for X=0. Milled powders have been consolidated using a pulse current sintering process under 1.5 GPa. The porosity of an amorphous compact solid consolidated at 813 K is estimated to be 7.5% for X=5, being much lower than 24% for X=0. Fe addition to Ti–37.5 at%Si powder mixtures in mechanical alloying improves the stability of an amorphous phase and forming-ability of compact solid powders.
  • A Study of Local Nanocrystalline Structure of 0Cr16Ni22Mo2Ti Steel in Bond Area of Flash Welding

    pp. 2044-2045

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    DOI:10.2320/matertrans.43.2044

    A nanocrystalline layer was fabricated in bond area of 0Cr16Ni22Mo2Ti steel using flash welding. The average grain size near bond line is about 30 nm and the farther the distance from the bond line, the larger the size of the nanocrystallites. The thickness of the nanocrystalline layer is about 80 \\micron. The formation mechanism of the nanocrystallites may be that the metal in liquid state in the bond area is solidified under both high undercooling and high density d.c. electric current that can refine the microstructure of metal solidification.
  • Thermo-Kinetic Anomalies across Rigidity Threshold in GexSe1-x

    pp. 2046-2049

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    DOI:10.2320/matertrans.43.2046

    We have investigated the glass-transition kinetics of nine GexSe1−x glasses by differential scanning calorimetry. Relation between the drive (heating-rate q) and response (heatflow shift ΔH at Tg) is seen to be strictly linear only for GeSe4, known to signify the bulk-rigidity threshold for this series. From an Arrhenius analysis the activation energies for glassy relaxation are estimated, and point to the existence of different thermokinetic phases below and above the threshold composition. Series behaviour of the kinetic activation is conciled to a concurring one seen in the size of cooperatively diffusing regions. The anomalies are attributed to structural crossovers with Ge doping; first from the parent uniform Se-chains to that of backbones out-branching at corner-shared Ge(Se1⁄2)4 tetrahedral clusters, and subsequently interconnecting by edge-shared configurations to realize a random pearl-necklace 3-D covalent network.
  • THE FORTY-SEVENTH GOLD MEDALIST OF THE JAPAN INSTITUTE OF METALS, 2002 Martensitic Transformations: Microstructures and Uniaxial Stress, Magnetic Field and Hydrostatic Pressure Effects

    pp. 2050-2057

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    DOI:10.2320/matertrans.43.2050

    Microstructures in martensites and effects of uniaxial stress, magnetic field and hydrostatic pressure on martensitic transformations, which were investigated by the author, are briefly summarized. Unsolved problems related to this investigation are pointed out and some ideas are proposed to solve the problems. Interactions between lattices and electrons should be clarified in order to establish the cause of martensitic transformation. In particular, the exact electronic band structure at Brillouin zone boundaries should be delineated, and whether martensitic nuclei are indeed grown from precursory strained regions in matrix phases should be clarified. How the precursory strained region is related to the lattice invariant strain considered in the phenomenological crystallographic theory for martensitic transformations and actually observed by TEM should also be clarified. Some other future research subjects, such as the effects of uniaxial stress, magnetic field and hydrostatic pressure on martensitic transformations, and various related functions, such as shape memory effect and superelasticity, are further pointed out.
  • Annealing Effect on the Optical Properties of a-SiC:H Films Deposited by PECVD

    pp. 2058-2062

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    DOI:10.2320/matertrans.43.2058

    Effects of annealing temperature (Ta) on the structure of hydrogenated amorphous silicon carbide (a-SiC:H) films prepared by RF plasma chemical vapor deposition (CVD) method are investigated by using Fourier Transform-Infrared Spectrometry (FT-IR), X-ray Photoelectron Spectroscopy (XPS) and UV-VIS spectrophotometer techniques. It is found that an annealing process results in structural rearrangement and evacuation of hydrogen atoms from CHn and SiHn bonds. The emission of hydrogen bonded to silicon and carbon responsible for the decrease of optical band gap (Eopt) by 0.70 eV in the range of Ta from 573 to 873 K . This hydrogen loss is interpreted in terms of hydrogen molecule formation and outerdiffusion. In addition, the surface morphology of films was investigated by atomic force microscopy (AFM).
  • Tensile Properties at Room Temperature to 823 K of Mg-4Y-3RE Alloy

    pp. 2063-2068

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    DOI:10.2320/matertrans.43.2063

    This paper describes tensile properties of a peak-aged Mg–4Y–3RE alloy at room temperature to 823 K with 10−5–10−1 s−1. The Mg alloy exhibited high strength (> 250 MPa) at room temperature to 473 K . However, the strength rapidly decreased at 573 K . It is suggested that a large decrease in strength at 573 K is attributed to grain boundary sliding. Also, elongation increased rapidly at 723–823 K . This is likely to arise from the relatively high strain rate sensitivity of about 0.3 due to the glide-controlled dislocation creep.
  • Solvent Extraction Separation of Co, Mn and Zn from Ni-rich Leaching Solution by Na-PC88A

    pp. 2069-2072

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    DOI:10.2320/matertrans.43.2069

    Solvent extraction experiments for separation of impurities from Ni-rich solution were carried out for manufacturing of high purity Ni compounds from acid leaching solution of spent Ni–Cd secondary battery. Artificial and leaching solutions were used as aqueous phases and PC88A saponified by sodium in kerosene were used as organic phase. The extraction order is Zn>Mn>Co>Ni and extraction percentage of metal ions was increased with increase of the concentration of extractant, initial pH of aqueous phase and ratio of O/A . The separation of cobalt, zinc and manganese from nickel was effectively accomplished at the condition of extraction stage=1, O/A=1 and initial pH 5.0 with 1.0 mol/dm3 PC88A saponified to 50% with NaOH.
  • Solar Light Absorption Property of Nano-Structured Silver Layer and Application to Photo-Thermal Energy Conversion Coating

    pp. 2073-2079

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    DOI:10.2320/matertrans.43.2073

    Nano-structured Ag layers with the thickness in a range of dAg=0.9–24 nm sandwiched between MgO layers are formed by rf sputtering. The morphology of nano-structured Ag is observed by the transmission electron microscope. Then the optical property is measured in a range of wavelength of 0.24–2.6 \\micron, which covers the solar energy spectrum. Several characteristics are recognized. For example, the nano-structured Ag layer having the intermediate morphology between the nano-particle and the continuous film shows the effective optical property in absorbing solar spectrum, such as large absorptance not only at visible region but also at near-infrared region. Moreover, it is found that the nano-structured Ag layers with dAg≤5.4 nm and with dAg≥24 nm are thermally stable at 673 K, but those with dAg=6.3–13.5 nm are thermally unstable. In order to utilize the advantageous characteristics of the nano-structured Ag layers for the solar absorber coating, the optical constant of each layer is experimentally evaluated. Then the selective solar absorber coating with functional graded structure made of all nano-structured Ag layers is designed. The fabricated coating shows the high-performance such as the photo-thermal conversion efficiency of 0.72 for air mass 1.5 at an operating temperature of 373 K, the thermal stability as well.
  • Semi-Continuous Production of Tantalum Powder by Electronically Mediated Reaction (EMR)

    pp. 2080-2086

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    DOI:10.2320/matertrans.43.2080

    An electronically mediated reaction (EMR) has been explored to produce tantalum powder semi-continuously by the calciothermic reduction of tantalum chloride (TaCl5). TaCl5 feedstock and reductant calcium alloy were charged into electronically isolated locations in a molten CaCl2 bath at 1123 K . The feed was freshly recharged three times, but the bath was reused without replenishment during four repeated reduction experiments. The current flow through an external path between the feed (cathode) and reductant (anode) locations was monitored. A current between 0.2 and 0.9 amps was measured during the reaction in the external circuit connecting cathode and anode. Tantalum powder was readily obtained after each experiment, but its purity decreased from 95% to 87%Ta as the run number increased. Tantalum powder obtained from EMR was fine compared with that of metal powder metallothermic reduction. Tantalum powder with low aluminum and nickel content was obtained although liquid Ca–Al–Ni alloy was used as the reductant. The results demonstrate the possibility of semi-continuous production of tantalum powder by the EMR without the direct physical contact between the feed and reductant. The mechanism of the calciothermic reduction of TaCl5 in the molten salt was discussed, using an isothermal chemical potential diagram.
  • Strengthening of Carbon Fibers by Imposition of a High Magnetic Field in a Carbonization Process

    pp. 2087-2091

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    DOI:10.2320/matertrans.43.2087

    Carbon fibers produced from PAN (polyacrylonitrile) as a precursor are generally subjected to the three heat treatment processes of stabilization and carbonization followed by graphitization. Stabilized fibers and high-temperature heat-treated fibers were carbonized in a high magnetic field imposed parallel to the fiber axis under a temperature of 1773 K and a tension of 8N per 12000 pieces of fibers. The carbon fibers produced from the stabilized fibers in a magnetic field of 5 T showed higher tensile strength than those done in no magnetic field, and the high-temperature heat-treated fibers processed in the magnetic field resulted in reverse. It is found that the fibers processed in the magnetic field have a larger crystallite size than those treated in no magnetic field. The mechanism of increase in the crystallite size due to the imposition of a high magnetic field has been discussed on the basis of an intermolecular cross-linking reaction model, in which the radical pair theory is modified by taking account of magnetic field.
  • Electron Energy-Loss Spectroscopy of Carbon Films Prepared by Electron-Cyclotron-Resonance Plasma Sputtering

    pp. 2092-2096

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    DOI:10.2320/matertrans.43.2092

    A protective carbon film, prepared by electron-cyclotron-resonance (ECR) plasma sputtering, has attracted considerable interest because the film can possess both the high wear durability and the high conductivity without doping. Such properties can be obtained, for example, when a negative bias voltage VB (−40≤VB≤−140 V) is applied to a substrate made of silicon. Little is known, however, about the atomic structure and bonding state of the ECR films for such a bias region, so that the physical origins of the macroscopic properties are not fully understood. In the present study, electron energy-loss spectroscopy (EELS) in a transmission electron microscope has been applied to investigate such ECR films that were deposited on a sodium chloride substrate for three different bias voltages: 0 V, −75 V and −120 V . The physical density of each ECR film was found to be lower than that of graphitized carbon and that of amorphous carbon. For each of the ECR films, the fraction of sp2 bonding was estimated to be more than 90%. The carbon clusters with sp2 bonding were considered to be more ordered for VB=−75 V and −120 V than for VB=0 V . The averaged density of valence electrons did not change much for each film, but the band structure is considered to vary depending on a local area. For the film prepared by the ECR technique, the macroscopic properties of the film such as electronic and mechanical ones may be controlled by controlling the ordering of sp2 clusters.
  • Whisker-Like Goethite Nanoparticles Containing Cobalt Synthesized in a Wet Process

    pp. 2097-2103

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    DOI:10.2320/matertrans.43.2097

    Acicular α-FeOOH particles are formed through aging of ferric oxyhydroxide colloidal solution formed by the neutralization of FeCl3 aqueous solution by NaOH . The effects of cobalt ion addition in the sol–gel–sol process on the formation rate and morphology of α-FeOOH nanoparticles have been investigated. The magnetic properties of metal nanoparticles prepared from the obtained particles have also been examined. The dependence of reaction rate constants in the formation of α-FeOOH in the wet process and the crystallite size of (110) on Co content in α-FeOOH has been made clear. Both the rate constant and crystallite size decrease with increasing Co content in α-FeOOH particles. The negative zeta-potential of the formed α-FeOOH particles increases slowly with increasing pH of the electrolytic solution. The negative zeta-potential of α-FeOOH particles containing Co is larger than that of α-FeOOH particles without Co in the range of pH6–pH12. The relationship between the morphology of the formed α-FeOOH particles and the cobalt content in α-FeOOH particles has been investigated. The length of major axis increases and the length of minor axis decreases with increasing Co content, and the aspect ratio increases remarkably. It is likely that Fe(OH)4 complex ion precipitates preferentially on the c plane, and the particles grow remarkably in the direction of c axis. The interplanar spacing of the α-FeOOH decreases with increasing Co content. The formed α-FeOOH grows abnormally like whisker in the direction of major axis: c axis in the case of significantly much Co content: 27 at%, and also the whisker like nanoparticles are observed to grow with curving. The magnetic metal nanoparticles produced have excellent dispersivity and high aspect ratio. The metal nanoparticles have magnetic properties enough for the magnetic recording media.
  • First-Principles Investigation of L10-Disorder Phase Equilibrium in Fe-Pt System

    pp. 2104-2109

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    DOI:10.2320/matertrans.43.2104

    First-principles study is attempted to investigate L10-disorder phase equilibrium in Fe–Pt system. The present study consists of electronic structure total energy calculations by FLAPW for the ground state and statistical mechanics calculations by Cluster Variation Method for finite temperatures. It is revealed that the magnetism plays a crucial role in the phase stability. The spin polarized FLAPW calculation confirms that the most stable magnetic state for FePt3 (L12) is anti-ferro, and the incorporation of anti-ferro magnetic state is critical to reproduce the experimental L10-disorder transition temperature. Thermal vibration effects considered based on Debye-Gruneisen model further improve the calculated transition temperature.
  • Semi-Solid Consolidation Processing of Rapidly Solidified Powders for Fabrication of High Strength AZ91 Magnesium Alloys

    pp. 2110-2114

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    DOI:10.2320/matertrans.43.2110

    Rapidly solidified AZ91 magnesium alloy was fabricated by a single roll technique. Microstructure of the alloys were composed of a fine-grain supersaturated α–Mg solid solution with a small amount of β-Mg17Al12 precipitates dispersed. The ribbons were successively crushed into fine powders using mechanical grinding apparatus and preformed at 673 K using pulse current sintering machine. The preformed compacts were extruded into a bar using semi-solid consolidation process at 723 K . The pressure needed to extrude preformed AZ91 compacts was 110 MPa about 40% lower than that of as-cast alloys. Semi-solid consolidated specimen after heat treatment showed high tensile strength 350 MPa. These were mainly attributed to grain refinement. The process proposed in this study led to the improvement of formability and also tensile strength for AZ91 alloys.
  • The K Value Distribution of Liquid Phase Sintered Microstructures

    pp. 2115-2119

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    DOI:10.2320/matertrans.43.2115

    A complex geometry of a powder compact in the three-dimensional multiple particle arrangement was generated by a Monte Carlo method, and the K value (a constant links the two-dimensional connectivity to the three-dimensional coordination number) distributions of liquid phase sintered systems, parameters linked the two-dimensional connectivity to the three-dimensional coordination number, were solved by the numerical computation. With this probability model, irregular packing of particles in three-dimensional space could be formulated with the variations of the particle size distribution. Unlike previous analytical models in which the grain boundary energy was set to a constant value, a relationship between the grain boundary energy and the misorientation angle between neighboring grains could be incorporated into this model in a probability manner. Simulation results indicate that the mean K value in the initial stage of liquid phase sintering increases with an increasing volume fraction of liquid phase, an increasing ratio of the mean base particle size to the mean additive particle size, and a decreasing standard deviation of the particle size distribution. The findings of the simulation are favorably compared with previous experimental observations on W–Ni–Fe alloys.
  • Solubilities and Equilibrium Distribution Coefficients of Oxygen and Carbon in Silicon

    pp. 2120-2124

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    DOI:10.2320/matertrans.43.2120

    Solubilities of oxygen and carbon in solid silicon at 1673 K were determined by using a chemical equilibrium technique. Solid silicon was heated at 1673 K in an oxygen atmosphere for 1800 ks, and then oxygen content in the solid silicon equilibrated with silica was measured by the inert gas fusion-IR absorption method. Carbon content in the solid silicon equilibrated with silicon carbide after heating at 1673 K in an Ar–5%CO atmosphere for 4860 ks was measured by the combustion-IR absorption method. Comparing these solubility values in solid silicon with those in liquid silicon that were previously reported by the present authors, the equilibrium distribution coefficients of oxygen and carbon in silicon at the melting point were evaluated to be 0.85±0.08 and 0.30±0.16, respectively.
  • 111Cd Time Differential Perturbed Angular Correlation Study of Deformed Ni

    pp. 2125-2129

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    DOI:10.2320/matertrans.43.2125

    Using a time differential perturbed angular correlation (TDPAC) method with 111In which decays to 111Cd, the lattice defects induced by cold rolling in Ni metal at 77 K have been studied by the magnetic hyperfine fields observed at the 111Cd probe nuclear sites. To investigate the dependence of annealing temperature, we have measured TDPAC spectra at 298 K during 1 hour isochronal annealing up to 1273 K in Ar+7%H2 gas atmosphere. After 1273 K annealing, only one component which shows ωL=98.0±0.5 Mrad/s corresponding to magnetic hyperfine field of 6.68±0.02 T has been observed at 300 K . This frequency is interpreted as a hyperfine field at 111Cd nucleus which occupies substitutionally the Ni lattice sites. We have also investigated the interaction between probe nucleus and lattice defects from the measurements of 111Cd TDPAC spectra. After deformation, distinctive changes have not been observed. But after 473 K annealing, ωL=40.60±1.03 Mrad/s corresponding to magnetic hyperfine field of 2.77±0.07 T was observed in addition to the component originating from the substitutional 111Cd atoms. This frequency seems to be due to the component from the trivacancy trapping site. Because 111In−V3 site might transform immediately to 111In−V4 complex, and shows the cubic environment. This component disappeared after annealing above 873 K, and after 1273 K annealing the spectrum completely returned to the original substitutional 111Cd spectrum before the cold rolling.
  • Influence of Shear Height on Shear Strength of Tin-Lead Solder Ball Bonding

    pp. 2130-2136

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    DOI:10.2320/matertrans.43.2130

    The crack generation energy U1 and the crack progress energy U2 of eutectic Sn–37 mass%Pb solder ball and Sn–36 mass%Pb–2 mass%Ag one were surveyed by the shear test. Both balls were bonded at various reflow cooling rates (10–200 K/min). The shear test was carried out under the condition of two kinds of the shear height, Z=0 \\micron and Z=200 \\micron. U1 and U2 were calculated by multiplying the shear strength by the shear distance. Though U2 was independent on the cooling rate, the ball composition and the shear height, U1 changed depending on these parameters. Only U1 of Sn–36 mass%Pb–2 mass%Ag ball bonding cooled at 200 K/min dropped sharply though U1 of both ball bonding was almost the same and increased with the faster cooling rates in case of Z=0 \\micron. U1 of Sn–36 mass%Pb–2 mass%Ag ball bonding was higher than that of the eutectic ball at each cooling rate as a result of the shear test at Z=200 \\micron. The needle shape Ag3Sn intermetallic compound in Sn–36 mass%Pb–2 mass%Ag ball and near the interface contributed mainly to the lower U1 at Z=0 \\micron because Ni3Sn4 reaction layer formed at 200 K/min was thin. The higher U1 at Z=200 \\micron was due to the fine lamellar structure (Sn phase/Pb phase) in Sn–36 mass%Pb–2 mass%Ag ball. The shear property of the same ball depended on the shear height in the present study.
  • Effects of Chromium on the Glass Formation and Corrosion Behavior of Bulk Glassy Fe-Cr-Mo-C-B Alloys

    pp. 2137-2142

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    DOI:10.2320/matertrans.43.2137

    Bulk glassy Fe60−xCrxMo15C15B10 (x=0, 7.5, 15, 22.5 and 30 at%) alloys with high thermal stability were synthesized and the effects of chromium on the glass formation and corrosion behavior were clarified. The maximum diameter for glass formation is 2–2.5 mm for the 7.5 and 15 at%Cr alloys and 1 mm for the other alloys. In the present glassy alloy system, the temperature interval of the supercooled liquid region (ΔTx) changed with chromium and was enlarged from around 70 K at x=0, 22.5 and 30 at% to over 80 K at x=7.5 and 15 at%. Both corrosion rate and anodic current density by potentiodynamic polarization in HCl solutions decreased with an increase of chromium content in the alloys. For the Cr-free alloy, molybdenum was significantly concentrated in the surface film after immersion in 1 N HCl solution. The bulk glassy alloys containing chromium was immune to corrosion by the formation of protective passive film enriched with chromium during immersion in the solution.
  • Surfactant Effect of Oxygen Atoms on Epitaxial Growth of fcc Ultra Thin Films on Cu(001)

    pp. 2143-2147

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    DOI:10.2320/matertrans.43.2143

    Surface active agent (surfactant) effect of the adsorbed oxygen atoms on epitaxial growth of fcc ultra thin films (Co, γ-Fe, Cr) on Cu(001), a Co/γ-Fe/Co trilayer on Cu(001) and a [Co/Cu] multilayer was investigated. The surfactant effect of oxygen atoms on the growth of fcc Co on Cu(001)–O reconstructed structure of (2\\sqrt2×\\sqrt2)R45° was found by the RHEED observation. The non-equilibrium fcc structure of Fe was stable until 45 ML on the Cu(001)–O surface formed on Cu(001) single crystal, but 20 ML on the Cu(001)–O surface formed on Cu(001) buffer layer deposited on MgO(001). The surfactant effect was not observed for the non-equilibrium fcc Cr. The surfactant effect of oxygen atoms on the Co/γ-Fe/Co trilayer and [Co/Cu]20 multilayer were observed. The surface reconstruction structure of Cu(001)–O(2\\sqrt2×\\sqrt2)R45° is necessary for the oxygen surfactant effect on the epitaxial growths of fcc metals on Cu(001).
  • Optimizing Drilling Conditions for AZ61A Magnesium Alloy

    pp. 2148-2156

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    DOI:10.2320/matertrans.43.2148

    For magnesium alloys, optimizing the machining conditions is necessary to prevent ignition of chips. In this study, effects of point angles of drill bits and drilling parameters on surface roughness and cutting resistance forces were measured and studied. Surface roughness and cutting resistance forces are increased following the increase of point angle and material removal rate. Point angle (2p) descends from 118° to 55° producing the smoothest machined surface and minimum variance in the measured roughness. In addition, effect of drilling operation on varying microstructure of AZ61A was also investigated in this study. The drilled sample showed a minimum extent of deformation twinning layer, when the drill bit adopted a point angle of 55°. The drilled sample developed a superior surface roughness and a short extent of twinning layer generated on the matrix of machined sample, if a 5%NaOH solution was used as lubricant and a 55° point angle was used.
  • High-Strength and High-Speed Bonding Technology using Thick Al-Ni Wire

    pp. 2157-2160

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    DOI:10.2320/matertrans.43.2157

    In order to create high-strength and high-speed thick Al wire bonding technology, high-temperature bonding at 423 K using Al–Ni wire has been investigated. It was found that Al–Ni wire bonds exhibit higher bonding strength than those of Al wire bonds. Al–Ni wire bonds joined at 423 K for 40 ms exhibited high-strength comparable to those of Al–Ni wire bonds joined at RT for 180 ms. It was found that high-temperature, high-speed bonding substantially reduces the occurrence of Si damage. High-temperature and high-speed bonding with Al–Ni wire can be considered as a promising bonding technology for the mass production of low cost power modules.
  • Hydrogenation of Body-Centered-Cubic Titanium-Chromium Alloys Prepared by Mechanical Grinding

    pp. 2161-2164

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    DOI:10.2320/matertrans.43.2161

    The C15 and C14 intermetallic TiCr2−x (x=0, 0.2 and 0.5) compounds were subjected to grinding in a high-energy ball mill. The X-ray diffraction profiles showed that the crystal structure transformed from C15 and C14 to bcc after mechanical grinding for 57.6 ks. The hydrogenation properties of the TiCr2−x (x=0, 0.2 and 0.5) samples were examined by differential thermal analysis and pressure-composition isotherm measurements. The sample reacted with hydrogen at 5 MPa and 523 K by maintaining the bcc structure. An higher hydrogen content was observed for the sample with the higher Ti content. The maximum hydrogen content of TiCr2.0, TiCr1.8 and TiCr1.5 was found to be about 0.32, 0.36 and 0.47 H/M at 313 K, respectively, at 8 MPa.
  • Hydrogen Pressure-Composition Isotherms for Ti45Zr38Ni17 Amorphous and Quasicrystal Powders Produced by Mechanical Alloying

    pp. 2165-2168

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    DOI:10.2320/matertrans.43.2165

    Pressure-composition isotherms (PCTs) for amorphous and icosahedral (i) quasicrystal powders produced by mechanical alloying of Ti45Zr38Ni17 powder mixtures were measured at temperatures of 473 K and 523 K at low-hydrogen pressures, lower than 0.1 MPa. Sloping plateau-like features on PCTs were observed at equilibrium hydrogen pressures lower than 1 kPa, below an H/M (hydrogen to metal atom ratio)≈1.2 and ≈1 for the amorphous and i-phase powders respectively. The plateau-like region for the i-phase powder was steeper and narrower than that for the amorphous powder, implying some small differences between the local structures of the i-phase and the amorphous phase. After the PCT measurements, an increase in the nearest-neighbor atom spacing and an expansion of the quasilattice were observed for the amorphous and i-phase powders respectively. Impurities from some unsynthesized elemental material and a Ti2Ni type phase were also present. These also absorbed hydrogen, shown by an expansion of their crystal lattices. However, no crystal hydride formation was observed in any of the powders.

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