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

MATERIALS TRANSACTIONS Vol. 49 (2008), No. 1

  • PREFACE

    Yoshikazu Todaka, Tadanobu Inoue, Zenji Horita

    pp. 1-1

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    DOI:10.2320/matertrans.preface

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  • Evolution of Microstructure and Hardness in Pure Al by Twist Extrusion

    Dmitry Orlov, Yan Beygelzimer, Sergey Synkov, Viktor Varyukhin, Zenji Horita

    pp. 2-6

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    DOI:10.2320/matertrans.ME200724

    Abstract

    High purity Al (99.99%) is subjected to severe plastic deformation (SPD) at room temperature using a process of twist extrusion (TE). The microstructure evolution and the related change in microhardness are examined with respect to imposed strain. It is shown that subgrains develop after the first TE pass with a size of ∼1.6 μm and this size remains essentially the same for further application of TE passes. However, dislocations become less visible within grains and grain boundaries become straight and well-defined with misorientation angle higher as the imposed strain is increased. The hardness increases with imposed strain when the magnitude of the strain is small. However, as the imposed strain is large, the hardness decreases due to a reduction of dislocations within grains. It is confirmed that these results obtained with TE are consistent with those reported using equal-channel angular pressing (ECAP) and high pressure torsion (HPT), indicating that the microstructural change and the variation of related mechanical properties with straining observed in pure Al are not affected significantly by the methods of SPD processing.

  • Influence of High-Pressure Torsion Straining Conditions on Microstructure Evolution in Commercial Purity Aluminum

    Yoshikazu Todaka, Minoru Umemoto, Ayumi Yamazaki, Jun Sasaki, Koichi Tsuchiya

    pp. 7-14

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    DOI:10.2320/matertrans.ME200713

    Abstract

    The influence of heat generation during severe plastic deformation on microstructure evolution was investigated in commercial purity aluminum (Al 1050, CP-Al) by using high-pressure torsion (HPT) process. The microstructure was characterized by the observations of the torsion and the longitudinal planes. CP-Al disks were deformed by HPT-straining up to 20 turns (equivalent strain, εeq, of ∼600) at 0.2 or 5 rpm at room temperature. To prevent the increase in specimen temperature, HPT-straining was also carried out in liquid nitrogen. In the all conditions, the value of Vicker’s microhardness, Hv, was saturated around 0.65 GPa and the microstructure consisted of the equiaxed grains of about 500 nm with quite low dislocation density. The microstructure in the early stage of HPT-straining showed the deformed (sub)structure, and then the equiaxed grain structure with high-angle boundaries formed by grain subdivision, recovery, continuous recrystallization and grain growth with increase in strain amounts and specimen temperature.

  • Microstructural Evolution and Mechanical Properties of High Purity Aluminium Processed by Equal-Channel Angular Pressing

    Jiri Dvorak, Vaclav Sklenicka, Zenji Horita

    pp. 15-19

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    DOI:10.2320/matertrans.ME200719

    Abstract

    High purity (99.999%) aluminium was subjected to Equal-Channel Angular Pressing (ECAP) at room temperature for up to 8 passes through route BC using a die having a channel angle of 90°. Hardness measurements and tensile tests were conducted at room temperature with respect to the number of ECAP passes. It is shown that the hardness and the tensile strength increased to take a maximum at 2 passes. However, unusual behaviour was observed such that further straining by ECAP for more than 2 passes decreased the hardness and tensile strength. Furthermore, the total elongation and the uniform elongation were enhanced with straining by ECAP. Microstructural observation using transmission electron microscopy revealed that straining by ECAP for 2 passes led to the development of a subgrain structure having dislocations within some subgrains and tangled dislocations near subgrain boundaries. After 8 passes, the grains became lager with the grain size of ∼15 μm and with the grain boundaries sharpened and clearly defined. Within such grains, subgrain boundaries were formed with an average separation of ∼3 μm and with the dislocation density very low. It is considered that the high dislocation mobility arising from high stacking fault energy and very low level of impurities were responsible for the unusual tensile behaviour and the difficulty in reducing the grain size to the submicrometer level. It is also considered that the grain refinement occurred due to evolution of subgrain boundaries to high angle boundaries, initially mutual coalescence of dislocations to subgrain boundaries followed by increasing misorientation angles by merging nearby dislocations.

  • Assessment of Strain Energy by Measuring Dislocation Density in Copper and Aluminium Prepared by ECAP and ARB

    Yoshinori Murata, Ippei Nakaya, Masahiko Morinaga

    pp. 20-23

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    DOI:10.2320/matertrans.ME200707

    Abstract

    In order to estimate the strain energies of pure Cu and pure Al prepared by ECAP and ARB processes, dislocation densities remaining in these metals after each process were analyzed by the Warren-Abervach method based on the Williamson-Hall plot from a series of data taken by conventional X-ray diffractometry. The measured dislocation density was higher by several times in pure Cu than in pure Al regardless of ECAP and ARB, resulting in the higher strain energy stored in pure Cu than in pure Al. Also, the dislocation density was found to be lower in the f.c.c. phase in pure Cu than that in the lath martensite phase in high Cr ferritic steels. Furthermore, it was observed that the amount of stored strain energy in pure Al depended considerably on its purity.

  • Grain Boundary Structure of Ultrafine Grained Pure Copper Fabricated by Accumulative Roll Bonding

    Ken-ichi Ikeda, Kousuke Yamada, Naoki Takata, Fuyuki Yoshida, Hideharu Nakashima, Nobuhiro Tsuji

    pp. 24-30

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    DOI:10.2320/matertrans.ME200715

    Abstract

    Grain boundary structures of ultrafine grained pure copper (Cu) fabricated by the accumulative roll bonding (ARB) have been studied. The atomic structures of grain boundaries in the ARB processed Cu (ARB-Cu) were observed by high resolution electron microscopy. In order to compare the grain boundaries in the ARB-Cu with equilibrium grain boundaries, the grain boundary energy and structure of symmetric tilt boundaries with ⟨110⟩ common axis in pure Cu were computed by molecular dynamics simulation (MD). The low angle boundaries in the ARB-Cu were basically described by conventional dislocation model and simultaneously there were some local structures having certainly high energy configurations. The grain boundaries with large misorientation in the ARB-Cu are basically described by the structural units predicted for the normal grain boundaries by MD. The present results indicate that the atomic structures of the boundaries in the Cu severely deformed by the ARB are rather similar to those of the equilibrium grain boundaries, except for the local distortions.

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  • A Portrait of Copper Processed by Equal Channel Angular Pressing

    Ralph J. Hellmig, Miloš Janecek, Branislav Hadzima, Oleg V. Gendelman, Michael Shapiro, Xenia Molodova, André Springer, Yuri Estrin

    pp. 31-37

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    DOI:10.2320/matertrans.ME200717

    Abstract

    Copper is one of the most important materials used for electrical connectors with a high thermal and electricial conductivity, and there is an ongoing demand to improve its mechanical properties without sacrificing other beneficial properties. A possible approach to achieving this aim is to use the method of severe plastic deformation to obtain an ultrafine grained microstructure. Significant grain refinement obtained by equal channel angular pressing (ECAP) leads to an improvement of mechanical, microstructural and physical properties. This paper gives an overview of a range of the properties that can be achieved by ECAP processing of pure copper including microstructural features, thermal stability, thermal conductivity and corrosion resistance.

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  • Deformation Mechanism of Severely Deformed CP-Titanium by Uniaxial Compression Test

    Eunji Yu, Inyoung Kim, Dong Hyuk Shin, Jongryoul Kim

    pp. 38-40

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    DOI:10.2320/matertrans.ME200723

    Abstract

    Conventional uniaxial compression test, designed to yield an effective strain of ∼1, was carried out to understand the deformation mechanism of severely deformed commercial pure titanium at various temperatures. TEM analysis showed that only [10\\bar11] twins accompanied with ⟨a⟩ slip dislocations were activated on non-basal planes in samples compressed at 573 and 623 K. In particular, the whole area of the sample compressed at 623 K were covered with [10\\bar11] twin bands with a width of 200 nm. However, ⟨a+c⟩ slip instead of [10\\bar11] twinning were found in samples compressed at 673 and 823 K. This indicates that [10\\bar11] twinning plays an important role in accommodating severe plastic strain before the activation temperature of ⟨a+c⟩ slip.

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  • Change in Mechanical Properties and Microstructure of ARB Processed Ti during Annealing

    Daisuke Terada, Masaya Inoue, Hiromoto Kitahara, Nobuhiro Tsuji

    pp. 41-46

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    DOI:10.2320/matertrans.ME200710

    Abstract

    Ultrafine grained commercial purity titanium (CP-Ti) was fabricated by accumulative roll-bonding (ARB) process up to 6 cycles at ambient temperature. The microstructure was composed of the equiaxed grain structure having a mean grain size of 90 nm and the lamellar boundary structure having a mean lamellar spacing of 70 nm. The specimen ARB processed by 6 cycles were subsequently annealed at various temperatures for 1.8 ks. After annealing at 400°C, the ARB specimen showed the partially recrystallized microstructure composed of recrystallized grains with grain size of approximately 0.5 μm and the recovered ultrafine structure. After annealing at 500°C, the microstructure was filled with the equiaxed recrystallized grains having a mean grain size of approximately 2 μm. The mechanical properties of the ARB processed and subsequently annealed specimens were investigated by tensile test. The tensile strength decreased and the total elongation increased continuously with increasing the annealing temperature. It was found that the tensile strength decreased linearly with increasing the total elongation in a strength-ductility balance plot, which was significantly different from the cases of Al and the interstitial free (IF) steel where the strength-elongation balance showed a trade-off relationship. The result indicates that ultrafine grained Ti has an excellent strength-ductility balance compared with Al and IF steel.

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  • Effect of Strain Path in High-Pressure Torsion Process on Hardening in Commercial Purity Titanium

    Yoshikazu Todaka, Minoru Umemoto, Ayumi Yamazaki, Jun Sasaki, Koichi Tsuchiya

    pp. 47-53

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    DOI:10.2320/matertrans.ME200714

    Abstract

    The effect of strain path in high-pressure torsion (HPT) process on hardening was investigated in commercial purity titanium (Ti-0.03Fe-0.03O, mass%). After monotonic HPT (mHPT) straining up to N=10 turns at a rotation speed of 0.2 rpm under a pressure of P=5 GPa, the obtained Vickers microhardness, Hv, was around 3.5 GPa and the microstructure consisted of equiaxed grains of 100∼200 nm with high dislocation density. This Hv value was hardly increased even with further strain and strain gradient (further rotation). To investigate the effect of strain path, cyclic (cHPT) and two-steps HPT (2sHPT) processes were carried out. The cHPT-straining performed by repetitive deformation of N=1⁄2. In comparison with the mHPT process, the Hv was attained rapidly to the saturated value (Hv 3.5 GPa). However, the maximum Hv value was similar to that obtained by mHPT-straining. In the 2sHPT process, first the disk of φ 20 mm in diameter was deformed by HPT-straining. Secondly, the disk of φ 10 mm was cut to contact with the circumference of the deformed disk, and then it was deformed. A higher hardness (Hv 3.8 GPa) was obtained than that by monotonic or cyclic HPT-straining. These results indicate that multi-directional deformations (deformations with different strain paths) contribute to the hardening improvement.

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  • Nano Grain Formation in Tungsten by Severe Plastic Deformation-Mechanical Milling Process

    Eiji Oda, Hiroshi Fujiwara, Kei Ameyama

    pp. 54-57

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    DOI:10.2320/matertrans.ME200704

    Abstract

    The mechanism of nano grain formation in tungsten by mechanical milling, which is one of the methods for severe plastic deformation, was investigated. The powder microstructure was divided in some layered grain structures, and finally consisted of equiaxed nano grains. This nano grain structure was formed by the subdivision of the layered grain. These nano grains have non-equilibrium grain boundary structure. It is considered that such a specific microstructure is formed by extremely heavy plastic deformation by due to the mechanical milling process.

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  • Fracture Toughness Enhanced by Grain Boundary Shielding in Submicron-Grained Low Carbon Steel

    Masaki Tanaka, Naoki Fujimoto, Kenji Higashida

    pp. 58-63

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    DOI:10.2320/matertrans.ME200711

    Abstract

    The enhancement of toughness at low temperatures in fine grained low carbon steel was studied, basing on a shielding theory due to dislocations and grain boundaries. Fully annealed low carbon steel was subjected to an accumulative roll bonding (ARB) process for grain refining. The grain size perpendicular to the normal direction was found to be approximately 200 nm after the ARB process. The fracture toughness of low carbon steel ARB applied was measured at 77 K by four-point bending, comparing with the fracture toughness of those without the ARB process. It was found that the value of fracture toughness at 77 K was increased by grain refining due to the ARB, indicating that the ARB process enhances toughness at low temperatures as reported in interstitial free steel and phosphorus doped interstitial free steel. It also deduces that the brittle-ductile transition (BDT) temperature shifted to a lower temperature. The enhancement of toughness and the decrease of the BDT temperature due to grain refining cannot be explained completely by the dislocation pile-up model of dislocations at grain boundaries. Quasi-two-dimensional simulations of dislocation dynamics, taking into account of crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicated that the BDT temperature is decreased by decreasing the dislocation source spacing. In addition to the simulation, the authors suggest a new concept of accommodating stress intensity at the crack tip due to grain boundaries to explain the enhancement of toughness and the decrease of the BDT temperature in fine grained materials.

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  • Fatigue Crack Propagation Behavior in Commercial Purity Ti Severely Deformed by Accumulative Roll Bonding Process

    Hiromoto Kitahara, Kosuke Uchikado, Jun-ichi Makino, Naomi Iida, Masayuki Tsushida, Nobuhiro Tsuji, Shinji Ando, Hideki Tonda

    pp. 64-68

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    DOI:10.2320/matertrans.ME200716

    Abstract

    Fatigue properties of commercial purity titanium sheets severely deformed by the accumulative roll bonding (ARB) process were investigated. The ARB process was carried out up to 6 cycles (equivalent strain, εeq.=4.8). The sheets ARB processed by 2-, 4- and 6-cycle consist of fine equiaxed grains and elongated lamellar grains. In the sheet ARB processed by 6-cycle, the mean size of fine equiaxed grain was 89 nm, and the mean thickness of the lamellar grains were 67 nm. The tensile strength increased with increasing the number of the ARB cycle. Fatigue crack growth tests were performed to clarify the fatigue properties such as the crack growth rate and threshold stress intensity factor range for crack growth (ΔKth). The ΔKth of the ARB processed specimens were smaller than that of the starting sheet. The ΔKth decreased with increasing the number of the ARB cycle until 4-cycle. However, the ΔKth of 6-cycle specimen was larger than that of the 4-cycle specimen. Fracture surface of the 6-cycle specimen was different from that of the 2- and 4-cycle specimens. Fatigue crack propagation behavior changes between 4- and 6-cycle specimens. On the other hand, the crack growth rate decreases with increasing the number of the ARB cycle.

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  • Mechanical Properties of Magnesium Alloy AZ31 after Severe Plastic Deformation

    J. Xing, X. Yang, H. Miura, T. Sakai

    pp. 69-75

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    DOI:10.2320/matertrans.ME200705

    Abstract

    Grain refinement of magnesium alloy AZ31 was studied in multidirectional forging (MDF) under decreasing temperature conditions. MDF was carried out up to large cumulative strains of 5.6 with changing the loading direction during decrease in temperature from pass to pass. MDF can accelerate the uniform development of very fine- grained structures and an increase of the plastic workability at low temperatures. New grain structures with the minimal grain size of 0.23 μm can be developed by continuous dynamic recrystallization at a final processing temperature of 403 K. As a result, the multidirectional- forged alloy showed excellent higher strength as well as moderate ductility at room temperature, and also a superplastic elongation of over 300% at 423 K. The mechanisms of strain-induced and fine-grained structure development and of the excellent plastic deformation are discussed in detail.

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  • Microstructural and Mechanical Characteristics of AZ61 Magnesium Alloy Processed by High-Pressure Torsion

    Yosuke Harai, Masaaki Kai, Kenji Kaneko, Zenji Horita, Terence G. Langdon

    pp. 76-83

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    DOI:10.2320/matertrans.ME200718

    Abstract

    Experiments were conducted on an AZ61 magnesium alloy to evaluate the microstructural characteristics and the mechanical properties after processing by High-Pressure Torsion (HPT). The results show that processing by HPT produces excellent grain refinement with average grain sizes of ∼0.22 and ∼0.11 μm after processing at 423 K and room temperature, respectively. Tensile testing after HPT revealed the potential for achieving superplastic elongations with a maximum recorded elongation of 620% when testing at a temperature of 473 K. Using microhardness measurements, it is demonstrated that the the microstructure gradually evolves with increasing torsional straining in HPT so that ultimately there is a reasonably homogeneous structure across the disk.

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  • Grain Boundary Sliding in a Superplastic Zinc-Aluminum Alloy Processed Using Severe Plastic Deformation

    Megumi Kawasaki, Terence G. Langdon

    pp. 84-89

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    DOI:10.2320/matertrans.ME200720

    Abstract

    A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Processing (ECAP) to produce a grain size of ∼0.8 μm. Tensile testing at 473 K gave a maximum elongation of ∼2230% at a strain rate of 1.0×10−2 s−1. The significance of grain boundary sliding was evaluated by taking measurements of offsets in surface marker lines at an elongation of 30%. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic elongation. Detailed measurements showed that relatively large offsets occurred at the Zn-Zn and Zn-Al interfaces but there were smaller offsets at the Al-Al interfaces. It is concluded that grain boundary sliding is the dominant flow process during superplastic deformation.

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  • Enhanced Mechanical Properties of Nano/Meso Hybrid Structure Materials Produced by Hot Roll Sintering Process

    Hiroshi Fujiwara, Ryota Akada, Atsushi Noro, Yuki Yoshita, Kei Ameyama

    pp. 90-96

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    DOI:10.2320/matertrans.ME200703

    Abstract

    SUS316L stainless steel and commercially pure titanium powders are processed by Mechanical Milling (MM) which is one of Severe Plastic Deformation (SPD) process. These MM powders are sintered by Hot Roll Sintering (HRS) process. Microstructure of materials produced by HRS process consists of a shell and core hybrid microstructure, that is, a shell structure with nano grains and a core structure with work-hardened coarse grains. All of the HRS materials demonstrate not only superior strength but also good elongation. The mechanical properties are strongly influenced by the shell/core microstructure. The nano/meso hybrid microstructure by HRS process has been proved to be very effective to improve mechanical properties.

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  • Nanostructuring of TiNi Alloy by SPD Processing for Advanced Properties

    Ruslan Valiev, Dmitry Gunderov, Egor Prokofiev, Vladimir Pushin, Yuntian Zhu

    pp. 97-101

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    DOI:10.2320/matertrans.ME200722

    Abstract

    Ultrafine-grained (UFG) alloy Ti49.4Ni50.6 possessing both nano- as well as submicrocrystalline structure has been successfully produced using two techniques of severe plastic deformation (SPD) processing: high pressure torsion and equal channel angular pressing. The features of microstructure, martensitic transformation and deformation behavior of the UFG alloy have been studied in details. The effects of grain size on the mechanical and functional properties of the alloy are discussed.

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  • Change in Magnetic Property of Cu-6.5 mass%Co Alloy through Processing by ECAP

    Kenichiro Suehiro, Shunichi Nishimura, Zenji Horita

    pp. 102-106

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    DOI:10.2320/matertrans.ME200725

    Abstract

    A Cu-6.5 mass%Co alloy containing a fine dispersion of ferromagnetic Co precipitates in the Cu matrix is processed by ECAP and measurements of coercivity are carried out after ECAP with different numbers of passes and different processing routes. Changes in particle size and shape including crystal structures are examined using transmission electron microscopy and X-ray diffraction analysis. It is demonstrated that the application of ECAP for 1 pass leads to an increase in the coercivity by more than twice as much as the as-aged condition without ECAP. Microstructure observation reveals that the increase in the coercivity is due to intense straining by ECAP and fragmentation of the Co particles to finer sizes as 10–30 nm. Magnetic anisotropy is observed when the sample is processed through Route A but it is minor for 4 passes of Route C and very little after 4 passes of Route BC. The anisotropy appears because Co particles are uniaxially elongated by processing via Route A.

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    3. Change in Mechanical Properties and Microstructure of ARB Processed Ti during Annealing MATERIALS TRANSACTIONS Vol.49(2008), No.1

  • Reaction between Magnesia–Chrome Brick/Slag Interface by Electric Furnace Static Slag Corrosion Test

    Min-Hsiung Hon, Chia-Chan Hsu, Moo-Chin Wang

    pp. 107-113

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    DOI:10.2320/matertrans.MRA2007079

    Abstract

    Reaction of magnesia-chrome (MgO-Cr2O3) brick with molten MgO-Al2O3-SiO2-CaO-FetO slag using electric furnace static corrosion test method was investigated by X-ray diffraction (XRD), optical polarized microscopy (OPM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), scanning transmission electron microscopy (STEM) and electron diffraction (ED). From XRD patterns it is found that the crystalline phases at slag-refractory interface are mostly MgO and MgCr2O4 with a minor phase of CaMgSiO4. The MgO content in MgO-Cr2O3 bricks decreases with temperature increasing. The activity coefficient of MgO (γMgO) in the molten MgO-Al2O3-SiO2-CaO-FetO slag also increases with temperature increasing. The diffusion layer of (Mg,Fe) (Cr,Al)2O4 is formed between slag area and MgCr2O4 matrix.

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    2. Synthesis of Mg-Al Alloys by Bulk Mechanical Alloying (BMA) and Their Hydrogen Solubility MATERIALS TRANSACTIONS Vol.49(2008), No.11
    3. Grain Boundary Structure of Ultrafine Grained Pure Copper Fabricated by Accumulative Roll Bonding MATERIALS TRANSACTIONS Vol.49(2008), No.1

  • Theoretical Search of Spintronic Material in Fe2(Cr1−xMnx)Si and (Fe1−xCox)2MnSi

    Shoji Ishida, Daisuke Nagatomo, Sinpei Fujii, Setsuro Asano

    pp. 114-119

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    DOI:10.2320/matertrans.MRA2007163

    Abstract

    Materials suitable for spintronic devices are searched in two systems Fe2(Cr1−xMnx)Si and (Fe1−xCox)2MnSi. The desired characteristics are: high spin polarization, stability of ferromagnetism and high Curie temperature. The first is investigated by considering the density of states and the velocity at the Fermi energy. The second is investigated by comparing the total energies of the paramagnetic, ferromagnetic and antiferromagnetic states. Partial substitution of the Co atoms for the Fe atoms in Fe2MnSi is effective in stabilizing the ferromagnetic state. There may be spintronic materials with the high Curie temperature in (Fe1−xCox)2MnSi.

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  • Dielectric Property of Polycrystalline ZrO2 Substituted BaTi2O5 Prepared by Arc-Melting

    XinYan Yue, Rong Tu, Takashi Goto

    pp. 120-124

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    DOI:10.2320/matertrans.MRA2007621

    Abstract

    ZrO2 substituted polycrystalline BaTi2O5, Ba(Ti1−yZry)2O5, (BT2Z), was prepared by arc-melting and the dielectric property was investigated by AC impedance spectroscopy. The length of a-axis increased from 1.6895 to 1.6952 nm and that of c-axis increased from 0.9411 to 0.9436 nm with increasing y up to 0.06. The b-axis was almost independent of ZrO2 content. The solubility limit of ZrO2 (y) in BT2Z can be 0.06. BT2Z had a strong b-axis orientation at y<0.06. The permittivity of BT2Z at y=0.005 showed the highest peak of 3050 at 725 K and the peak temperature decreased from 750 to 465 K with increasing y from 0 to 0.064. A relaxor-like frequency dependence of permittivity was observed at y>0.06.

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  • Interdiffusion in β Phase of the Ternary Ti-Al-V System

    Tomoshi Takahashi, Yoritoshi Minamino, Masao Komatsu

    pp. 125-132

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    DOI:10.2320/matertrans.MRA2007110

    Abstract

    The interdiffusion in Ti-rich β Ti-Al-V alloys has been investigated in the temperature range from 1323 to 1473 K. The direct interdiffusion coefficients, \\ ildeDAlAlTi and \\ ildeDVVTi, and indirect interdiffusion coefficients, \\ ildeDAlVTi and \\ ildeDVAlTi, are positive in the ternary alloys, and these four interdiffusion coefficients have slight concentration dependence. The values of \\ ildeDAlAlTi are larger than those of \\ ildeDVVTi, and the values of \\ ildeDAlVTi are also larger than those of \\ ildeDVAlTi. The repulsive interactions exist between Al and V atoms in the Ti-Al-V alloys, because the ratio values of indirect coefficient to direct one are positive. On the other hand, the interactions between Ti (solvent) and Al (or V) atoms are attractive in the present alloy, since the ratio of converted interdiffusion coefficients in the ternary alloys shows negative values.

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  • Phase-Field Simulation of the Elastic Effect on the Transformation Kinetics in Precipitation

    Yuxiang Zhang, Jincheng Wang, Yujuan Yang, Gencang Yang, Yaohe Zhou

    pp. 133-138

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    DOI:10.2320/matertrans.MRA2007153

    Abstract

    Phase-field simulations were performed to investigate the effect of elastic field on the transformation kinetics of precipitation for both the single- and the multi-particle systems with different alloy composition c0 and lattice misfits in Ni-Al binary alloys. Simulated results indicate, for both the single- and the multi-particle systems, that there exists a critical value ccrit for the alloy composition. When c0<ccrit, the elastic field delays the transformation, otherwise, accelerates the transformation. Increasing the lattice misfit accelerates the transformation when c0>ccrit, but decelerates the transformation when c0<ccrit. The elastic field also has great effect on the peak value and its occurrence time of transformation rate, and their values are dependent on the alloy composition. The variety of the effect of elastic field on the transformation kinetics is determined by the competition between the self-elastic energy and the strain-induced interaction energy in different systems.

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    3. Change in Mechanical Properties and Microstructure of ARB Processed Ti during Annealing MATERIALS TRANSACTIONS Vol.49(2008), No.1

  • Effects of Manganese and/or Carbon on the Grain Refinement of Mg-3Al Alloy

    Jun Du, Jian Yang, Mamoru Kuwabara, Wenfang Li, Jihua Peng

    pp. 139-143

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    DOI:10.2320/matertrans.MRA2007196

    Abstract

    The effects of manganese and/or carbon on the grain refinement of Mg-3Al alloy have been investigated in the present study. Significant grain refinement was obtained for the Mg-3Al alloy modified with either carbon or Al-Mn master alloy. There existed an optimal content of 0.1∼0.2 mass%Mn to obtain refining grain size for the Mg-3Al alloy. The Al-Mn intermetallic particles with molar ratio of 1:1 were considered as potent nuclei for Mg grains. The addition of Mn had no obvious effect on the grain size for the Mg-3Al alloy which was refined by 0.2 mass%C. The Al-C-O-Mn compounds were considered as the potent nuclei for Mg grains. A new hypothesis that the particles of Al-Mn compounds with Al4C3 coating film can act as potent nuclei for Mg grains was proposed.

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  • Fatigue Crack Initiation and Propagation in Lotus-Type Porous Copper

    Hironori Seki, Masakazu Tane, Hideo Nakajima

    pp. 144-150

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    DOI:10.2320/matertrans.MRA2007623

    Abstract

    We studied fatigue crack initiation and propagation in lotus-type porous copper with cylindrical pores aligned in one direction. For fatigue loadings in the direction parallel to the longitudinal axis of pores, stress field in the matrix is homogeneous. Therefore, slip bands are formed all over the specimen surface. On the other hand, for the perpendicular loadings, slip bands are formed only around pores in which stress highly concentrates. Since the localized slip bands form fatigue crack, fatigue fracture occurs even when the total plastic strain range is small. Stress field in the matrix of lotus copper affects the direction of crack propagation. For the parallel loadings, a crack propagates along a straight line as well as nonporous copper. On the other hand, for the perpendicular loading, a crack propagates along a path in which stress highly concentrates. Since stress highly concentrates around anomalously large pores, fatigue cracks are preferentially formed around the large pores and cracks propagate by crossing the large pores.

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  • Deformation and Rotary Driving Characteristics of a Shape-Memory Alloy Thin Strip Element

    Hisaaki Tobushi, Toshimi Sakuragi, Yoshiki Sugimoto

    pp. 151-157

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    DOI:10.2320/matertrans.MRA2007214

    Abstract

    If a shape-memory alloy (SMA) thin strip is applied to elements subjected to torsion, a rotary driving element with a simple mechanism can be developed. With this purpose in view, torsion, recovery torque and torsion fatigue tests were carried out for a TiNi SMA thin strip, and the basic torsion characteristics of the strip were obtained. It was found that torque and recovery torque both increase in proportion to the angle of twist and temperature, and that the recoverable strain energy increases in proportion to temperature, while dissipated work decreases slightly with an increase in temperature. Fatigue life decreases in proportion to the angle of twist, and the fatigue life of a heat-treated material is longer than that of an as-received material. The angle of twist at the fatigue limit for the heat-treated material is almost the same as for the as-received material. As a practical application for a rotary driving element of this type, a means of opening and closing a door with an element driven by an SMA thin strip is demonstrated.

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  • Microstructure and Electrical Conductivity of CaRuO3 Thin Films Prepared by Laser Ablation

    Akihiko Ito, Hiroshi Masumoto, Takashi Goto

    pp. 158-165

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    DOI:10.2320/matertrans.MRA2007175

    Abstract

    CaRuO3 (CRO) thin films were prepared by laser ablation at substrate temperatures (Tsub) ranging from room temperature to 1073 K in a high vacuum (10−6 Pa) atmosphere and at oxygen pressures (PO2) of 0.013 to 130 Pa. The effects of deposition conditions on the microstructure and electrical conductivity (σ) were investigated. Rectangular-shaped CRO island grains grew at 0.013Pa<PO2<130Pa and Tsub>873 K. At PO2=0.13 Pa and Tsub=973 K, as well as at PO2=13 Pa and Tsub=873 K, well-connected island grains were observed. While the composition of island grains was nearly stoichiometric independently of Tsub, the Ca fraction of film matrix increased with increasing Tsub. CRO thin films with σ less than 104 S·m−1 showed semiconducting behavior. At Tsub=973 K and PO2=0.13 Pa, CRO thin films exhibited metallic conduction with the highest σ of 1.5×105 S·m−1 at room temperature.

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  • Wear and Mechanical Properties, and Cell Viability of Gas-Nitrided Beta-Type Ti-Nb-Ta-Zr System Alloy for Biomedical Applications

    Toshikazu Akahori, Mitsuo Niinomi, Masaaki Nakai, Hideki Nishimura, Yukiko Takei, Hisao Fukui, Michiharu Ogawa

    pp. 166-174

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    DOI:10.2320/matertrans.MRA2007210

    Abstract

    Frictional wear resistance is one of the important properties of metallic biomaterials. Surface hardening treatments such as oxidizing, nitriding and ion implantation tend to be applied for improving the wear resistance of titanium and its alloys. The simple gas nitriding process is expected to further improve the wear resistance of newly developed beta-type Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) for biomedical applications. However, there is a possibility for the mechanical properties such as tensile and fatigue strength of TNTZ to be degraded through gas nitriding process. Therefore, the gas nitriding process was carried out in this study to improve the wear resistance of TNTZ and alpha+beta-type Ti-6Al-4V ELI alloy (Ti64), which is one of the representative titanium alloys practically applied for biomedical applications, in simulated body fluid (Ringer’s solution). Their tensile and fatigue properties and cell viability was also investigated in order to confirm the reliability as biomedical materials.
    The Vickers hardness near the specimen surface of nitrided TNTZ and Ti64, where TiN and Ti2N forms, increases significantly as compared to that of their matrices. The wear resistances of TNTZ and Ti64 are improved significantly in Ringer’s solution by nitriding process as compared to those of as-solutionized TNTZ (TNTZST) and Ti64 (Ti64ST). The tensile strength of nitrided TNTZ increases by around 90 MPa as compared to that of TNTZST. The tensile strength of nitrided Ti64 does not change significantly at all nitriding temperatures. On the other hand, the elongation decreases with increasing the nitriding temperature. The run out (plain fatigue limit) of TNTZ subjected to a nitriding process at 1123 K, which has relatively good balance between wear resistance and tensile properties, is around 300 MPa, and is nearly equal to that of Ti64 subjected to a nitriding process at 1123 K, although the tensile strength of the nitrided TNTZ is around 200 MPa smaller than that of the nitrided Ti64. The cell viabilities of nitrided TNTZ and Ti64 range from 1.4 to 1.6 against that of control (cell disc), and are a little higher than that of TNTZST and Ti64ST. The cell viabilities of nitrided TNTZ and Ti64 after removing the oxide layer on their surfaces are similar to that of control and are not significantly degraded.

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  • Change of Magnetic Properties and Structure in Fe3O4 Films on Si Substrates with Annealing Temperature

    Sayeeduzzaman Syed, Yasushi Endo, Takanobu Sato, Yoshio Kawamura, Ryoichi Nakatani

    pp. 175-178

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    DOI:10.2320/matertrans.MRA2007200

    Abstract

    We have investigated the magnetic properties and structure of Fe3O4 films on Si substrates before and after annealing at various temperatures, Ta. The saturation magnetization and the coercivity in the Fe3O4 films are observed at all Ta. They markedly increase with increasing Ta up to 873 K, and their values become maximum at Ta=873 K. Furthermore, they slightly decrease above Ta=873 K. The Fe3O4 phase mainly exists in the films at all Ta. In addition, the α-Fe2O3 phase (hematite) is also formed in the films above Ta=873 K. On the basis of these results, it is found that the growth of Fe3O4 phase by annealing is mainly dominant in the change of magnetic properties in the Fe3O4 film up to Ta=873 K and that the formation of α-Fe2O3 phase influences the change of magnetic properties in the Fe3O4 film above Ta=873 K.

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  • Relationship between the Weldability and the Process Parameters for Laser-TIG Hybrid Welding of Galvanized Steel Sheets

    Cheolhee Kim, Woongyong Choi, Jeonghan Kim, Sehun Rhee

    pp. 179-186

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    DOI:10.2320/matertrans.MER2007159

    Abstract

    In the lap welding of zinc-coated steel, porosity formation is one of most significant weld defects. It is caused by zinc vapor generated between the steel sheets. Various solutions have been proposed in the past but development of more effective method remains a valuable subject to be investigated. In this study, laser-TIG hybrid welding was applied to the lap welding of zinc-coated steel without a gap. The weld defects could be eliminated by laser-TIG hybrid welding, as the leading TIG arc partially melted the upper sheet, and the coated zinc on the lapped surfaces were vaporized or oxidized before the trailing laser irradiated on the specimen.
    Optimization of the process parameters for laser-arc hybrid welding process is intrinsically sophisticated because the process has three types of parameters–arc, laser and hybrid welding parameters. In this paper, the relationship between weldability and the process parameters of the laser beam-arc distance, welding current and welding speed were investigated using a full factorial experimental design. Weld quality was evaluated using the weight of the spatter, as porosity formation is a major weld defect in the lap welding of zinc-coated steel sheets. It was found that the weld quality was increased as the laser beam-arc distance and welding current increased, and that this decreased as welding speed increased.

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  • Electromotive Force of Gas Concentration Cell Using Alumina-Rich Nonstoichiometric Magnesium Aluminate Spinel as the Solid Electrolyte

    Yuji Okuyama, Noriaki Kurita, Norihiko Fukatsu

    pp. 187-192

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    DOI:10.2320/matertrans.MER2007243

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  • Effect of Hot Isostatic Pressing on Microstructure and Mechanical Properties of CM-681LC Nickel-Base Superalloy Using Microcast

    Chao-Nan Wei, Hui-Yun Bor, Li Chang

    pp. 193-201

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    DOI:10.2320/matertrans.MER2007088

    Abstract

    This study investigates how hot isostatic pressing (HIP) affects the microstructure and fracture modes of CM-681LC superalloy. As-cast test bars with grain size of 80 μm were prepared using the fine-grain process followed by HIP. Experimental results indicate that micropores formed during solidification and contraction degrade the tensile strengths and elongations of the fine-grain CM-681LC superalloy before HIP. The area fraction of micropores was reduced from 0.2% to 0.06% following HIP. Scriptlike MC carbides decompose into particlelike M23C6 carbides during HIP, revealing that HIP refines and spheroidizes the carbides. Eliminating the micropores and refining the carbides increases the mechanical strength by up to about 9% and the elongation by over 10% in room- and high-temperature tensile tests. The fracture analyses after tensile tests of the fine-grain test bars reveal that the microporosity and the scriptlike carbides at the grain boundaries are the main causes of the fracture of the test bars before HIP. The fracture mode of the fine-grain test bars following HIP, according to the tensile test, is typically intergranular because the micropores are eliminated and the carbides are refined. Since the elimination of the micropores and refinement of the carbides by HIP effectively improves the tensile strength and elongation, the fine-grain casting yields favorable mechanical properties.

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  • Recovery of Molybdenum from Spent Acid by Ammonia Gas Neutralization

    Jong-Jin Pak, Jong-Oh Jo, Chul-Hwan Park, Jin-Goo Kang, Dong-Hyuk Shin

    pp. 202-207

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    DOI:10.2320/matertrans.MER2007193

    Abstract

    The present work reports the commercialization of a recycling process of spent acid to recover molybdenum. The process consists of ammonia gas neutralization of spent acid containing molybdenum, crystallization and filtration of ammonium molybdate, roasting and hydrogen reduction of molybdic oxide to produce a commercial grade molybdenum metal powder. The mother liquor, highly nitrogeneous residual solution after the filtration of ammonium molybdate, can be utilized to produce chemical fertilizers.

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  • Thermal Synthesis of Nanocrystalline (CoxNi1-x)yFe1-y KOVAR Alloy through Gaseous Reduction of Mixed Oxides

    M. Bahgat, Min-Kyu Paek, Chul-Hwan Park, Jong-Jin Pak

    pp. 208-214

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    DOI:10.2320/matertrans.MER2007229

    Abstract

    KOVAR is a very interesting and widely applied Fe-Ni-Co alloy. Nanocrystalline KOVAR alloy is successfully synthesized in the present investigation. The effect of hydrogen reduction on composition, microstructure and magnetic properties of produced Fe-Ni-Co alloy is investigated. A molar ratio of (99.9%) nickel oxide, cobalt oxide and ferric oxide for synthesis of 29% Ni-17% Co-Fe (KOVAR) alloy were thoroughly mixed and compressed into compacts. The dried compacts were reduced partially and completely at 500, 600, 700 and 800°C in a constant flowing hydrogen gas atmosphere (100% H2). The course of reduction was followed up using the thermogravimetric technique to study the reduction behavior and kinetics reaction mechanism. The initial dried powder and the various reduction products were characterized by XRD, VSM, FE-SEM, EDX and reflected light microscope. At the higher reaction temperature (700–800°C) complete reduction was achieved with synthesis of nanocrystalline (13.9 nm) Fe-Ni-Co alloy. The activation energy values were calculated from the Arrhenius equation, and the approved mathematical formulations for the gas solid reaction were applied. It was found that the initial and final reduction stages are controlled by the combined gaseous diffusion and interfacial chemical reaction mechanisms.

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  • Globularization Behavior of ELI Grade Ti-6Al-4V Alloy during Non-Isothermal Multi-Step Forging

    Jin Young Kim, Kyung-Tae Park, In Ok Shim, Soon Hyung Hong

    pp. 215-223

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    DOI:10.2320/matertrans.MER2007224

    Abstract

    The globularization behavior of ELI grade Ti-6Al-4V alloy having different initial lamellar structures during multi-step forging under non-isothermal condition was investigated. The samples with either a thin or thick lamellar structure, which were produced from β annealing followed by different cooling conditions, were upset and stretched repeatedly at forging start temperatures of 940°C and 900°C, and subsequently air cooled. The microstructural changes after non-isothermal multi-step forging were analyzed with respect to globularization of α lamellae. After multi-step forging at 940°C, the initially thin lamellar structure changed to homogeneous equiaxed α globules, but elongated α globules with high aspect ratio were obtained from the initially thick lamellar structure. By forging at 900°C, the initially thin lamellar structure was changed to a mixed structure of both equiaxed and elongated α globules, while severely deformed lamellae were preserved inside distorted colonies with little globularization for the initially thick lamellar structure. Globularization of the samples by forging at 900°C with both initially thin and thick lamellar structures was less effective compared to that by forging at 940°C. A quantitative microstructural analysis revealed that, in addition to the initial microstructure and forging start temperature, globularization of α lamellae was significantly affected by the instantaneous microstructural development by thermal fluctuation during non-isothermal multi-step forging.

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  • Method of Fast Forecasting Mold Filling Capacity of Al-Si Alloy by Surface Tension

    Dequan Shi, Dayong Li, Guili Gao, Lihua Wang

    pp. 224-226

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    DOI:10.2320/matertrans.MRP2007215

    Abstract

    Based on the theoretical analysis, a new method for fast forecasting the mold filling capacity of Al-Si alloy by its surface tension has been proposed. Through many experiments and statistical analysis, a specific relation between the mold filling capacity of Al-Si alloy and surface tension has been got. Depending on this relation, the mold filling capacity of Al-Si alloy can be fast forecasted by surface tension before being poured.

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  • The Structure of Guinier-Preston Zones in an Mg-2 at%Gd-1 at%Zn Alloy Studied by Transmission Electron Microscopy

    Masahiko Nishijima, Kenji Hiraga, Michiaki Yamasaki, Yoshihiko Kawamura

    pp. 227-229

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    DOI:10.2320/matertrans.MEP2007257

    Abstract

    Planar Guinier-Preston zones (GP-zones) parallel to a basal plane of an Mg hexagonal lattice are found together with β′ and β1 precipitates, which have been found in Mg-Gd alloys, in an Mg-2 at%Gd-1 at%Zn (Mg97Gd2Zn1) alloy aged at 200°C for 150 hrs. From the combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark field scanning transmission electron microscopy (HAADF-STEM), we can determine the structure of the GP-zone, in which Gd and/or Zn atoms occupy at ordered positions in two close-packed planes sandwiching a close-packed plane of Mg atoms.

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  • © 2008 The Japan Institute of Metals and Materials

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