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

MATERIALS TRANSACTIONS Vol. 54 (2013), No. 4

  • PREFACE

    pp. 419-419

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    DOI:10.2320/matertrans.MPR2013901

  • Microstructures and Deuterium-Retention Behavior of Tungsten Exposed to D+(He and/or Be) Mixture Plasmas

    pp. 420-424

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    DOI:10.2320/matertrans.MG201202

    ITER-relevant mixed species (D, He, Be) plasma exposure experiments on microstructural changes and their impacts on D retention behavior in W are reviewed. It was observed that seeding of He into pure D plasma resulted in a significant reduction of D retention and suppression of surface blistering. TEM observations and ellipsometric measurements revealed that nano-sized high-density He bubbles were formed and percolated in the near surface region. Based on the experimental results, a mechanism to explain the reduced D retention was proposed, i.e., implanted D atoms can easily diffuse back to the surface through the percolating bubbles and escape from the surface during plasma exposure. Additional Be seeding to D+He mixture plasma suppressed this He seeding effect. Thus, it is considered that Be seeding has a more dominant influence than He seeding on microstructures and D retention in plasma-exposed W.
  • Clarification of Tritium Behavior in Pb–Li Blanket System

    pp. 425-429

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    DOI:10.2320/matertrans.MG201203

    In order to clarify the tritium behavior in Pb–Li blanket system, the solubility of hydrogen isotopes in the Pb–Li eutectic alloy was experimentally investigated. The effects of (i) Li preferential evaporation from Pb–Li, (ii) thermal convection in a quartz-glass reaction tube and (iii) difference in crucible materials between Al2O3 and W on the solubilities were discussed. Another important issue of the Pb–Li blanket system is to develop a new ceramic coating to decrease tritium permeation rate. Tritium permeation test was performed for Er2O3 coating on F82H. The permeation reduction factor achieved here was higher than 103. Precise experiments were carried out to determine the solubility, diffusivity and permeability of hydrogen isotopes in Pb–Li eutectic alloy by means of a transient permeation method. The isotope effects in solubility and diffusivity between H and D were clarified. No isotope effect in solubility was observed between H and D, and the H diffusivity was 1.4 times larger than the D one. The ratio was near to a value predicted from the classical theory. Experiment using tritium is designed under the collaboration work with INL.
  • Behaviors of Deuterium Retention and Microstructure Change of Tungsten Simultaneously Implanted with Carbon and/or Helium Ions

    pp. 430-436

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    DOI:10.2320/matertrans.MG201205

    Recent studies of deuterium (D) retention and microstructure behaviors of tungsten simultaneously implanted with carbon ion (C+) and/or helium ion (He+) are reviewed. Implantation of deuterium ion (D2+) was performed by simultaneous implantations with C+, He+ and a mixture of C+ and He+ using a triple-ion-implantation system, while D retention behavior was studied by thermal desorption spectroscopy. The D depth profile, microstructure changes, and chemical states of constituent atoms were observed by glow-discharge optical emission spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy, respectively. D retention was observed to be enhanced by both C+ and He+ implantations. After the simultaneous implantation of D2+ and C+, substantial D2 desorption was observed at temperatures higher than 600 K. Following the simultaneous implantation of D2+ and He+, D retention increased to about five times that for D2+ implantation, while the D desorption temperature region was the same. However, in the case of triple-ion implantation, the accumulation of C on tungsten was suppressed, and the retention of D trapped by C was reduced. The D retention in triple-ion-implanted tungsten was considered to be suppressed by He+ implantation. TEM observations suggest that most of the deuterium would be retained at grain boundaries and lattice defects such as vacancy clusters.
  • Retention of Hydrogen Isotopes in Neutron Irradiated Tungsten

    pp. 437-441

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    DOI:10.2320/matertrans.MG201204

    To investigate the effects of neutron irradiation on hydrogen isotope retention in tungsten, disk-type specimens of pure tungsten were irradiated in the High Flux Isotope Reactor in Oak Ridge National Laboratory followed by exposure to high flux deuterium (D) plasma in Idaho National Laboratory. The results obtained for low dose n-irradiated specimens (0.025 dpa for tungsten) are reviewed in this paper. Irradiation at coolant temperature of the reactor (around 50°C) resulted in the formation of strong trapping sites for D atoms. The concentrations of D in n-irradiated specimens were ranging from 0.1 to 0.4 mol% after exposure to D plasma at 200 and 500°C and significantly higher than those in non-irradiated specimens because of D-trapping by radiation defects. Deep penetration of D up to a depth of 50–100 µm was observed at 500°C. Release of D in subsequent thermal desorption measurements continued up to 900°C. These results were compared with the behaviour of D in ion-irradiated tungsten, and distinctive features of n-irradiation were discussed.
  • Irradiation Effect on Tensile Property of F82H IEA and Its Joint in TITAN Project

    pp. 442-445

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    DOI:10.2320/matertrans.MG201207

    Under the TITAN project, in order to determine the contributions of different microstructural features to strength and to deformation mode, microstructure of deformed flat tensile specimens of irradiated reduced activation F82H IEA and its joint were investigated by transmission electron microscopy (TEM), following tensile test and fracture surface examination by scanning electron microscopy (SEM). After irradiation, changes in yield strength, deformation mode, and strain-hardening capacity were seen, with the magnitude of the changes dependent on irradiation temperature. Irradiation to F82H IEA at 573 K led to a significant loss of strain-hardening capacity with a large change in yield strength. There was a tendency for a reduction in strain rate to cause a decrease in yield strength and elongation. While, irradiation at 773 K had little effect on strength, but a reduction in strain rate caused a decrease in ductility. SEM revealed fracture surfaces showing a martensitic mixed quasi-cleavage and ductile-dimple fracture in all samples. TEM have exhibited defect free bands (dislocation channels) in the necked region irradiated at 573 K. This suggests that dislocation channeling would be the dominant deformation mechanism in this steel irradiated at 573 K, resulting in the loss of strain-hardening capacity. While, the necked region of the irradiated F82H IEA joint, where showed less hardening than F82H IEA, has showed deformation bands only. From these results, it is suggested that the pre-irradiation microstructure, especially the dislocation density, could affect the post-irradiation deformation mode.
  • Ion-Irradiation Hardening of Brazed Joints of Tungsten and Oxide Dispersion Strengthened (ODS) Ferritic Steel

    pp. 446-450

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    DOI:10.2320/matertrans.MG201212

    Irradiation hardening and microstructural change of the brazed-joint of W and oxide dispersion strengthened ferritic steel (ODS-FS) was investigated by nano-indentation hardness test and transmission electron microscopy after ion irradiation with 6.4 MeV Fe3+ ions at 500°C up to 10 dpa. Dual-beam irradiation of Fe3+ ions and energy-degraded 1 MeV He+ ion was also carried out. A considerable irradiation hardening occurred in the W base metal where dislocation loops and nano-scaled voids or He-bubbles were observed. Dual-beam irradiation enhanced the hardening. No significant hardening was observed in ODS-FS. The hardness of insert material was reduced after irradiation, which is due to the recovery of dislocations generated during joining process.
  • Evaluation of Feasibility of Tungsten/Oxide Dispersion Strengthened Steel Bonding with Vanadium Insert

    pp. 451-455

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    DOI:10.2320/matertrans.MG201213

    A diffusion bonding (DB) technique to reduce thermal expansion coefficient mismatch between tungsten (W) and oxide dispersion strengthened ferritic steel (ODS-FS) was developed by applying a vanadium (V) alloy as an insert material. In order to suppress σ phase precipitation at the interface, DB of ODS-FS and V–4Cr–4Ti was carried out by introducing a Ti insert as a diffusion barrier between V–4Cr–4Ti and ODS-FS, and examined feasibility of W/V/Ti/ODS-FS joint for application to fusion reactor components by comparing the three-point bending strength and microstructure between the joints with and without a Ti diffusion barrier layer. It is shown that the fracture strength of the joint without a Ti insert was decreased by 25% after aging at 700°C for 100 h, but that with a Ti insert shows no change after the aging treatment up to 1000 h. The result indicates that the introduction of a Ti insert leads to the prevention of the formation of σ phase during aging and resultant control of the degradation of the bonding strength.
  • Development of Nanostructured Tungsten Based Materials Resistant to Recrystallization and/or Radiation Induced Embrittlement

    pp. 456-465

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    DOI:10.2320/matertrans.MG201209

    Mitigation of embrittlement caused by recrystallization and radiation is the key issue of tungsten (W) based materials for use in the advanced nuclear system such as fusion reactor applications. In this paper, our nanostructured W materials development performed so far to solve the key issue is reviewed, including new original data. Firstly, the basic concept of mitigation of the embrittlement is shown. The approach to the concept has yielded ultra-fine grained, recrystallized (UFGR) W–(0.25–1.5) mass%TiC compacts containing fine TiC dispersoids (precipitates). The UFGR W–(0.25–1.5)%TiC exhibits favorable as well as unfavorable features from the viewpoints of microstructures and various thermo-mechanical properties including the response to neutron and ion irradiations. Most of the unfavorable features stem from insufficient strengthening of weak random grain boundaries (GBs) in the recrystallized state. The focal point on this study is, therefore, to develop a new microstructural modification method to significantly strengthen the random GBs. The method is designated as GSMM (GB Sliding-based Microstructural Modification) and has lead to the birth of toughened, fine-grained W–1.1%TiC in the recrystallized state (TFGR W–1.1TiC). The TFGR W–1.1TiC exhibits much improved thermo-mechanical properties. The applicability of TFGR W–1.1TiC to the divertor in ITER is discussed.
  • Neutron Irradiation Behavior of Tungsten

    pp. 466-471

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    DOI:10.2320/matertrans.MG201208

    Tungsten (W) is a candidate for the plasma facing component material of fusion reactors. During fusion reactor operation, not only displacement damage but also transmutation elements such as rhenium (Re) and osmium (Os) are produced in W by neutron irradiation. To understand the irradiation response of W in a fusion reactor, irradiation effects on hardening, microstructure development and electric resistivity of pure W and W–Re–Os alloys are studied using fission reactor irradiation. In the low-dpa region (<0.4 dpa), irradiation hardening was suppressed by Re addition, but significant hardening appeared in W–26Re alloy after high-dpa (>1 dpa) irradiation. The hardening was caused by the irradiation-induced precipitation of WRe (σ–phase) and WRe3 (χ–phase). Os was more effective in the irradiation hardening than Re owing to the similar irradiation-induced precipitate formation even in low-dpa region. On the bases of these results, the alloy design of W for fusion reactor applications is suggested.
  • Silicon Carbide and Silicon Carbide Composites for Fusion Reactor Application

    pp. 472-476

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    DOI:10.2320/matertrans.MG201206

    This paper reviews recent achievements as to “nuclear-grade” SiC composites in particular for materials-system integration. SiC composite component development are reviewed including VHTR control rod scale model and compact intermediate heat exchanger scale mode by current joining and assembly techniques. Joining methods for SiC to metal and results of characterization of joint shear strength by the torsion tests using small specimens were also reviewed. The recent results of neutron irradiation experiments were also reviewed including detailed analysis of mechanical properties, irradiation creep and preliminary results on tritium behavior in SiC.
  • Integrated Material System Modeling of Fusion Blanket

    pp. 477-483

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    DOI:10.2320/matertrans.MG201210

    The blanket of fusion reactors is a multifunctional system that breeds tritium, harvests heat from the burning plasma, and protects the other components and the environment. The common task in the US-J TITAN (Tritium, Irradiation and Thermofluid for America and Nippon) project identifies cross-linked considerations in the blanket system modeling on the basis of the material research in each task. In this paper, we review the main outputs of this task: elucidation of the effect of helium on deuterium retention in the tungsten wall, analysis of tritium transfer in a Pb–Li liquid breeder, experimental and computational studies on the effects of radiation damage on hydrogen trapping, and modeling of the tritium barrier in a double-tube heat exchanger.
  • Enthalpies of Solution in Ti–X (X = Mo, Nb, V and W) Alloys from First-Principles Calculations

    pp. 484-492

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    DOI:10.2320/matertrans.MC201209

    The effects of solute X (Mo, Nb, V and W) on the phase stability of β-Ti alloys were studied from first-principles calculations. The first-principles calculations yielded solution enthalpies for hexagonal close-packed (hcp)-Ti35X1 and hcp-X35Ti1 and body-centered cubic (bcc)-Ti26X1 and bcc-X26Ti1 solid solution alloys. The enthalpy curves for the α (hcp)- and β (bcc)-phases of Ti–X alloys were described as a function of the X concentration by using the calculated solution enthalpies and sub-regular solution model. While the enthalpies of the α-phases increased with increasing concentrations of Mo, Nb, V and W, the enthalpies of the β-phases decreased with increasing concentrations. This is consistent with the experimental results, showing that Mo, Nb, V and W are β-stabilizers. The β-stabilizing strength of solute elements in Ti alloys is gauged using the experimental critical concentration. We found a good linear correlation between the experimental critical concentration and the theoretical metastable equilibrium concentration at which the enthalpy of the α-phase is equal to that of the β-phase. The metastable equilibrium concentration decreased with the increasing lattice stability of the bcc structure with reference to hcp structure.
  • Arrangements of Transition-Metal Atoms in Three Types of Al–Co–Ni Decagonal Quasicrystals Studied by Cs-Corrected HAADF-STEM

    pp. 493-497

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    DOI:10.2320/matertrans.M2012340

    Three types of Al–Co–Ni decagonal quasicrystals in Al72.5Co11Ni16.5, Al71Co14.5Ni14.5 and Al71.5Co16Ni12.5 alloys have been examined by Cs-corrected high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM), which has an enough resolution to represent individual TM atoms as bright dots in observed images of decagonal quasicrystals. On the basis of the structure of a W-(AlNiCo) crystalline approximant, transition-metal (TM) atoms and mixed sites (MSs) of Al and TM atoms are separately distinguished in HAADF-STEM images. Most of TM atoms are arranged with pentagonal tilings of a bond-length of 0.76 nm, and MSs are located inside of pentagonal frames with definite orientations. All of the TM atoms and MSs are located at lattice points of a Penrose lattice of an edge-length of 0.25 nm, and arranged with a bond-orientational order. The present result shows that the structures of Al–Co–Ni decagonal quasicrystals should be characterized as BOO arrangements of transition-metal atoms, instead of previous models of BOO arrangements of 2 nm atom clusters.
  • Annealing Effects on Structures and Morphologies of Fe/Si Core–Shell Clusters

    pp. 498-504

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    DOI:10.2320/matertrans.M2012383

    Transmission electron microscopy, X-ray diffraction and electrical resistivity have been observed for annealed Fe/Si core–shell clusters and their assemblies in comparison with those for annealed Fe and Si clusters. Fe clusters are wholly oxidized, where voids are formed in the central regions. Si clusters are not wholly oxidized, keeping a diamond-like structure, where voids are scarcely formed in the central regions because the initially-oxidized surfaces play roles of passive layers and protect the cores from oxidations. In Fe–Si core/Si shell clusters, SiOX thin layers are also formed on Si shell surfaces, protecting Si shells and Fe–Si cores from further oxidations. Fe–Si core regions maintain a bcc structure, where no ordered phase is detected probably due to the chemical heterogeneity among these clusters and the surface segregation of Si atoms. Excess vacancies are accumulated to become voids between Fe–Si cores and Si-shells.
  • Effects of Friction Stir Process and Stabilizing Heat Treatment on the Tensile and Punch-Shear Properties of Mg–9Li–2Al–1Zn Magnesium Alloy

    pp. 505-511

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    DOI:10.2320/matertrans.M2012314

    In this research, Mg–9Li–2Al–1Zn (LAZ-F) magnesium alloy was studied on a friction stir process (LAZ-FSP), and the FSP specimens had stabilized heat treatment (LAZ-FSP-S) to improve the punch-shear properties. LAZ-FSP revealed the network-like structure and particle in the stir zone. Cracks could be found on the subsurface of punch-shear LAZ-F and LAZ-FSP specimens. Notably, specimens with stabilizing heat treatment had no obvious defects on the subsurface after the punch-shear test. It is convincible that stabilizing heat treatment had improved the punch-shear properties of LAZ921. On the other hand, LAZ-FSP shows excellent tensile strength, while LAZ-FSP-S shows a better elongation. Therefore, stabilizing heat treatment at 60°C not only had showed improvements on workability, but also had effectively increased the ductility of magnesium alloy under room temperature.
  • High Temperature Deformation and Dynamic Recrystallization Behavior of Alloy718

    pp. 512-519

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    DOI:10.2320/matertrans.M2012202

    Ni base superalloy is one of the representative heat resistant materials for high temperature environment in various industrial applications. As for cast and wrought material, thermo-mechanical processing during hot forging has a significant role to control and optimize crystal grain structure in order to attain required critical properties. The essential deformation and dynamic recystallization behavior of Ni base superalloy, Alloy718 during hot forging were revealed through hot compression tests and quantitative relations among various parameters that were available for numerical calculation were derived. Dynamically recrystallized grain diameter depended on temperature, strain rate and was independent of initial grain diameter and strain. And the grain diameter was expressed as a function of the temperature compensated strain rate, i.e., Zener–Hollomon parameter quantitatively. Avrami-type equation was available for comprehensive and quantitative expression of dynamic recrystallization transition tendency. And the fraction of dynamic recrystallization was a function of the given strain and the strain for 50% recrystallization which depended on the initial grain diameter and Zener–Hollomon parameter.
  • Formation of Titanium Hydride in Dilute Cu–Ti Alloy by Aging in Hydrogen Atmosphere and Its Effects on Electrical and Mechanical Properties

    pp. 520-527

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    DOI:10.2320/matertrans.M2012423

    We have investigated the specific contributions of titanium-hydride precipitates to the improvement of electrical conductivity and strength for Cu–0.36 mol% Ti alloys aged in a hydrogen atmosphere at 773 K, where Cu4Ti phase is not to appear. During the aging, titanium-hydride particles, which have a composition of H/Ti = 2, are formed by reaction of solute titanium with hydrogen diffused into the alloy. The particles have an octahedral shape with facets parallel to {111} of the matrix. The size and volume fraction of the hydride particles increase with aging time, and then level off after aging for 100 h, while the number density exhibits a maximum at 24 h. The precipitation reduces the concentration of solute titanium in the matrix and leads to significant improvement of the electrical conductivity, to the level comparable to pure copper. The dispersed TiH2 particles gives rise to strengthening with the Orowan mechanism, of approximately 40 MPa at a maximum in yield strength.
  • Effect of ARB Processing on Fatigue Crack Closure in Commercially Pure Titanium

    pp. 528-531

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    DOI:10.2320/matertrans.M2012352

    Fatigue crack closure in commercially pure titanium sheets severely deformed using the accumulative roll-bonding (ARB) process were investigated. Sheet processed through 6 cycles of ARB consists of fine equiaxed grains and elongated lamellar grains and shows a high 0.2% proof stress of 790 MPa. Fatigue crack closure was not observed in the ARB processed sheet. The effective threshold stress intensity factor ranges, ΔKth.eff, of the ARB processed sheets and starting sheet are nearly the same. On the other hand, the resistance of the fatigue crack growth rate of the ARB processed sheet is higher than that of the starting sheet. It was found that ARB increases the resistance for the fatigue crack propagation but has no effect on the onset of propagation.
  • Cluster Analysis of Acoustic Emissions Measured during Deformation of Duplex Stainless Steels

    pp. 532-539

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    DOI:10.2320/matertrans.MBW201203

    Aiming at better understanding of the deformation mechanisms involved into plastic deformation of duplex stainless steels, acoustic emission (AE) measurements were performed during room temperature tensile deformation of model steels and alloys with different phase structure including single phase α Fe–30Cr and γ SUS316L steels and duplex α–γ SUS329J4L, Fe–24Cr–6Ni alloys. The real time AE investigations were complemented by microscopic investigations of deformed microstructures. The quantitative AE analysis revealed different AE patterns which were correlated with underlying deformation mechanisms and microstructural transformations. The cluster analysis of AE power spectral densities has been proven effective in revealing specific groups of signals (clusters) which appear on different deformation stages in different steel variants, which can be correlated with involved source mechanisms of plastic deformation and microstructural processes including dislocation slip in either fcc or bcc lattice of γ or α phase, respectively, twinning in the SUS 316L steel and γ–α′ martensitic transformation in the Fe–24Cr–6Ni model alloy.
  • Effect of High Pressure Heat Treatment on Microstructure and Thermal Expansion Coefficients of Cu–Al Alloy

    pp. 540-543

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    DOI:10.2320/matertrans.M2012358

    With different high pressure heat treatments on Cu–Al alloy, the effects on microstructure and thermal expansion coefficients of Cu–Al alloy are studied in the paper by optical microscopy, scanning electron microscopy, transmission electron microscopy and expansion instrument etc. The experimental results show that after heat treatment, the structure of Cu–Al alloy was refined obviously and the compactness increased. The grain refinement effect increases at first and then decreases when pressure is increased. When the pressure is 3 GPa, the most obvious is the thinning grain effect. In addition, the high pressure heat treatment can increase Cu–Al alloy thermal coefficient of expansion, when the Cu–Al alloy is treated by 3 GPa pressure and is at 596.37°C. Thermal expansion coefficient is the biggest, 3.1187 × 10−5°C−1, than that of the same temperature cast state samples increase 99.34%.
  • Phase Relationship for the CaO–SiO2–FeO–5 mass%P2O5 System with Oxygen Partial Pressure of 10−8 atm at 1673 and 1623 K

    pp. 544-552

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    DOI:10.2320/matertrans.M-M2013801

    Dephosphorization process by using multiphase flux requires smaller slag amount than conventional process, while the high refining efficiency can still be achieved, due to the promotion of the CaO utilization. In order to provide proper phase diagram for understanding the reaction mechanism, the phase relationship for the CaO–SiO2–FeO–5 mass%P2O5 quaternary system has been studied with oxygen partial pressure of 10−8 atm at 1673 and 1623 K by using chemical equilibration technique. It has been found that the solid solution exists only as 2CaO·SiO2–3CaO·P2O5, and the ratio between both varies with liquid phase compositions. On the other hand, the liquidus for both 1673 and 1623 K that saturated with above solid solution, moves close to the FeO apex comparing with the CaO–SiO2–FeOx system equilibrated with iron. Besides, the large phosphorus partition ratio between solid solution and liquid slag has also been found, which agrees well with previous works. Based on the regular solution model, both activities and activity coefficients of P2O5 have been discussed.
  • Pitting Corrosion Behavior of Silver-Containing 2205 Duplex Stainless Steel as Secondary Austenitic Phase Existed

    pp. 553-560

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    DOI:10.2320/matertrans.M2012382

    Silver-containing 2205 duplex stainless steel was developed to study its phase transformation, precipitating behavior, as well as corrosion resistance. The result reveals that the doping of silver in 2205 duplex stainless steel decreased the γ2 phase volume fraction. In addition, the solubility of silver in iron was extremely low, and silver particles were distributed randomly both in the matrix and on the boundaries of the material. Silver particles were observed on the material surface and the number of particles on the surface increased with the silver content.
    The pitting corrosion resistance of the test materials decreased with increasing silver content. The pits on the material surface became more abundant when the silver content was increased. The protective function of continuous Cr2O3 film on the surface of the steel was destroyed by the silver particles that weakened the pitting resistance. In addition, the initiation of pitting corrosion usually occurred around the γ2 phase, which had low concentrations of chromium and molybdenum.
  • Electrochemical Behavior of Annealed Soft-Magnetic Fe–Si–B–P–Cu Alloy

    pp. 561-565

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    DOI:10.2320/matertrans.MBW201213

    The nanocrystalline soft-magnetic Fe83.3Si3B10P3Cu0.7 ribbon alloy with a width of 10 mm prepared through annealing amorphous Fe83.3Si3B10P3Cu0.7 ribbon alloy at 698 K had a saturation magnetization flux density of 1.82 T and a coercivity of 11.8 A m−1. The nanohetero-amorphous microstructure consisted of α-Fe nanocrystals with an average size of 22 nm and amorphous neighbor phases. The results of polarization curves and electrochemical impedance spectra obtained in 0.5 M Na2SO4 and 0.5 M NaCl solutions demonstrated the positive shift of the corrosion potentials, the decrease in the passive current density, the increase in the breakdown potentials and the increase in the polarization resistance for the nanocrystalline Fe83.3Si3B10P3Cu0.7 ribbon alloy, compared with its amorphous counterpart alloy. The corrosion performance was improved via annealing, which is thought to result from the enrichment of B2O3 constituent in the surface oxide layer.
  • Comparison of Bond Order, Metal d Orbital Energy Level, Mechanical and Shape Memory Properties of Ti–Cr–Sn and Ti–Ag–Sn Alloys

    pp. 566-573

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    DOI:10.2320/matertrans.MC201213

    Bond order (\overline{B0}) and metal d orbital energy level (\overline{Md}) values were calculated for our recently developed Ti–(6,7)Cr–3Sn and Cr substituted Ti–(6,7)Ag–3Sn alloys developed in this study. Position of these alloys in the \overline{B0}-\overline{Md} phase stability map developed by Morinaga and co-workers was then determined. Phase constitution, mechanical and shape memory properties were co-related with the position of Ti–(6,7)Cr–3Sn and Cr substituted Ti–(6,7)Ag–3Sn alloy in the \overline{B0}-\overline{Md} phase stability map. Cr substitution with Ag has resulted drastic decrease in the bond order (\overline{B0}) and metal d orbital energy level (\overline{Md}) values, resulting drastic decrease in the strength, fracture strain and shape memory properties. Position of Ti–(6,7)Cr–3Sn alloys was within the twinning region of \overline{B0}-\overline{Md} phase stability map and these alloys exhibited good mechanical and shape memory properties due to stress induced martensitic transformation. It is concluded that position of developed Ti–Cr–Sn and Ti–Ag–Sn alloys in the \overline{B0}-\overline{Md} phase stability map not only affect the phase constitution but also the mechanical and shape memory properties.
  • Development of High Modulus Ti–Fe–Cu Alloys for Biomedical Applications

    pp. 574-581

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    DOI:10.2320/matertrans.M2012361

    Ti–Fe–Cu alloys with higher Young’s modulus, hardness and compressive mechanical properties than those of the existing Ti alloys were developed using a d-electrons alloy design method in order to improve Young’s modulus, hardness and compressive properties of Ti and existing Ti alloys for use as metallic stents. Their microstructures, Young’s modulus, hardness and compressive mechanical properties were investigated both before (as-cast) and after heat-treatments performed under a high-purity argon atmosphere at 1173 K for 21.6 and 86.4 ks.
    The studied Ti–Fe–Cu alloys consist of the β-Ti phase and dendritic TiFe intermetallic phase. Moreover, the area fraction of the TiFe intermetallic phase increases with increasing atom ratio (Fe + Cu)/Ti of the alloys and with the heat-treatment time.
    The Young’s modulus of the studied Ti–Fe–Cu alloys increases from 110 GPa (Ti78Fe18Cu4 alloy) to 145 GPa (Ti68Fe30Cu2 alloy) with increasing atom ratio (Fe + Cu)/Ti of the alloys and the area fraction of the TiFe intermetallic phase. However, the Young’s modulus is saturated or slightly decreased when the area fraction of the TiFe intermetallic phase is more than 34%. The Vickers hardness of the as-cast alloys increases from 490 HV (Ti78Fe18Cu4 alloy) to 550 HV (Ti63.4Fe30Cu6.6 alloy) with increasing atom ratio (Fe + Cu)/Ti of the alloys and area fraction of the TiFe intermetallic phase. On the other hand, the Vickers hardness of the heat-treated alloys is lower than that of the as-cast alloys, despite the increase in the area fraction of the TiFe intermetallic phase after the heat-treatment. The heat-treated alloys have better compressive properties than those of the as-cast alloys and the reported Ti–Fe–Cu alloys. The compressive strength and strain of the heat-treated Ti67Fe27Cu6 alloys reach to 2131 MPa and 24.5%, respectively.
  • Methanol Oxidation Activity and Chemical State of Platinum Oxide Thin Film Treated by Electrochemical Reduction

    pp. 582-587

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    DOI:10.2320/matertrans.MBW201215

    The methanol oxidation activity of Pt oxide can be significantly improved by heat reduction in a H2 atmosphere or by electrochemical reduction using a cathodic current. Therefore, the Pt oxide is expected to be a novel anode catalyst for DMFC. In this study, measurement of the electrochemical active surface area by CO stripping voltammetry, analysis of the chemical bonding state by XPS and determination of the O/Pt atomic ratio by EPMA were carried out to investigate the cause of the improvement in the methanol oxidation activity of the Pt oxide by the reduction treatment. Although the methanol oxidation current remarkably increased by electrochemical reduction treatment of the Pt oxide thin film prepared by reactive sputtering in 100% O2, the effect of the reduction on the methanol oxidation activity was not recognized at all for the Pt thin film prepared in 100% Ar. For the Pt oxide thin films, it was shown that there was a close correlation between the methanol oxidation current and the electrochemically active surface area obtained by CO stripping voltammetry. Moreover, the quantitative analysis by EPMA clearly showed that the O/Pt atomic ratio of the Pt oxide thin film having a high activity for the methanol oxidation reaction decreased to about 0.1 by the electrochemical reduction. These results together with the XPS analysis suggested that the residual oxygen related to the Pt–O bond was activating the methanol oxidation for the electrochemically reduced Pt oxide thin films.
  • High Temperature Mechanical Properties of Al–Si–Mg–(Cu) Alloys for Automotive Cylinder Heads

    pp. 588-594

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    DOI:10.2320/matertrans.M2012285

    To improve the fuel efficiency and reduce automobile emissions, there has been growing demand of more durable alloys for engine components with the improved thermal and fatigue resistance. This study examined the effect of alloying elements on the high mechanical behavior of Al–Si–Mg–(Cu) casting alloys for cylinder heads. Depending on the alloying elements affecting the strength of the matrix, the thermal expansion coefficient decreased with increasing Mn and Cu content at high temperatures with a concomitant increase in the elastic modulus, hardness and tensile strength. Quantatative analysis showed that the mechanical properties of the Al2Cu precipitate hardened alloy were maintained at temperatures over 250°C, whereas the degradation of mechanical properties of the Mg containing alloy occurred at 170°C due to coarsening of the Mg2Si precipitation phase. The LCF (low cycle fatigue) lives decreased with increasing alloy content according to the Coffin-Manson relation due to the smaller elongation. On the other hand, an analysis of the fatigue lives with the hysteresis loop energy, which consists of both strength and elongation, showed that the fatigue lives were normalized with an alloy of the same strengthening mechanisms regardless of the test temperature.
  • Development of Intermetallic Compounds Reinforced Al Alloy Composites Using Reaction of Porous Nickel and Aluminum

    pp. 595-598

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    DOI:10.2320/matertrans.MBW201214

    A new process is proposed to fabricate an intermetallic compound reinforced aluminum alloy matrix composite using the reaction between porous nickel and molten aluminum. The intermetallic compound reinforced aluminum alloy composite was manufactured with the infiltration process method. Porous nickel reacted with molten aluminum at 1023 K, and the intermetallic compound of Al3Ni was generated on the surface of the porous nickel. The generated intermetallic compound Al3Ni, was delaminated according to the difference of thermal expiation coefficient with nickel, and moves in the direction of aluminum matrix. The effects of processing variables, such as processing temperature, applied pressure and specific surface area of porous nickel on the formation and dispersion behavior of Al3Ni were investigated.
  • Synthesis of Conductive Nano Ink Using 1-Octanethiol Coated Copper Nano Powders in 1-Octanol for Low Temperature Sintering Process

    pp. 599-602

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    DOI:10.2320/matertrans.M2012363

    A conductive nanoink was prepared using copper nanoparticles (CNPs). In order to prevent the oxidation of CNPs, the particles were coated by 1-octanethiol using Vaporized Self-Assembled Multi-layers (VSAMs) method. The coating of 1-octanethiol onto CNPs was confirmed by transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Dispersion stability of CNPs was checked by monitoring its viscosity over 6 weeks. The sheet-type of patterned samples using the fabricated ink was sintered at 350 and 230°C, respectively. Electrical resistivity was measured to be 5.69 × 10−8 Ω·m when sintered at 350°C and 7.67 × 10−8 Ω·m when sintered at 230°C. These results were 3.4 and 4.6 times than the value for bulk copper. During the sintering process, the optimum temperature of removing organic materials was found to be 200°C. This removal temperature dramatically influenced necking and the density of samples. Therefore, 1-octanethiol VSAMs and 1-octanol ink were used successfully for the low-temperature sintering process to fabricate conductive Cu patterns by finding the optimum temperature of 200°C for complete removal of organic materials prior to the sintering process.
  • The Surface Properties of Plastic Mold Steel Subjected to Nitriding Treatment

    pp. 603-608

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    DOI:10.2320/matertrans.M2012386

    The homogeneous microstructure, precise texture, and weldability of pre-hardened AISI P21 plastic mold steel are ideal for mirror-finished plastic mold cores and cavities. This study applies a nitriding heat treatment and oxynitriding to AISI P21 plastic mold steel, using various processing parameters to determine the influence that nitriding treatment has on the surface characteristics. Regarding the prepared specimen, an optical microscope was used to analyze the microstructure and thickness of the nitride layer, X-ray diffraction (XRD) was performed to examine the structure, a 3D surface profile meter was employed to analyze the surface morphology and roughness, and polarization corrosion was conducted to assess the corrosion resistance. A Rockwell C test was also applied to examine the hardness. The results show that applying nitriding treatment to AISI P21 plastic mold steel can effectively improve the surface hardness, and the addition of an oxynitride surface layer increases the corrosion resistance. The nitrogen element under the surface layer, which although relatively hard is brittle, produces ferrous nitrides such as Fe3N and Fe4N that maintain the precise shape of the plastic mold and extend its service life.
  • Effects of β-Mg2Si Precipitates on Semi-Solid Microstructures of Wrought Al–Mg–Si Based Alloys Produced by Deformation-Semi-Solid-Forming Process

    pp. 609-617

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    DOI:10.2320/matertrans.M2012339

    The Deformation-Semi-Solid-Forming (D-SSF) process has been developed to produce superior semi-solid microstructures of fine α-Al grains and refined intermetallic compounds. The wrought Al–Mg–Si (6xxx alloys) alloys are generally considered to be complicated in controlling the semi-solid forming process because of the high temperature sensitivity to the liquid fraction. Additional alloying elements to the 6xxx series aluminum alloys are usually required to produce second-phase particles in order to refine the spheroidized α-Al grains during the reheating process. Generally, the β-Mg2Si phase is precipitated in the supersaturated Al matrix with Mg and Si atoms at various heat treatments in the manufacturing process of Al–Mg–Si alloys. In this study, the annealing process was performed for the precipitation of the equilibrium β-Mg2Si phase. By applying mechanical deformation, the stored energy is much increased by the presence of the rod-like β-Mg2Si precipitates, which accelerates recrystallization during heating to the semi-solid state. Fine α-Al grains with the average size of 104 µm were achieved by annealing and 60% cold-rolling in the alloy without additional alloying elements. The coarsened harmful Fe-intermetallic compounds were fragmented and became effective particles to refine the α-Al grains. Moreover, the combination of the fragmented Fe-intermetallic compounds and β-Mg2Si precipitates can further refine the α-Al grains with the average size of 79 µm by annealing and 40% cold-rolling.
  • Thermoelectric Characteristics of n-Type Bi2Te3 and p-Type Sb2Te3 Thin Films Prepared by Co-Evaporation and Annealing for Thermopile Sensor Applications

    pp. 618-625

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    DOI:10.2320/matertrans.M2013010

    A thermoelectric thin-film device consisting of n-type Bi2Te3 and p-type Sb2Te3 thin-film legs was prepared on a glass substrate by using co-evaporation and annealing process. The Seebeck coefficient and the power factor of the co-evaporated Bi2Te3 film were −30 µV/K and 0.7 × 10−4 W/K2·m, respectively, and became substantially improved to −160 µV/K and 16 × 10−4 W/K2·m by annealing at 400°C for 20 min. While the Seebeck coefficient of the co-evaporated Sb2Te3 thin film was 72 µV/K, it increased significantly to 165–142 µV/K by annealing at 200–400°C for 20 min. A maximum power factor of 25 × 10−4 W/K2·m was achieved for the co-evaporated Sb2Te3 film by annealing at 400°C for 20 min. A thermopile sensor consisting of 10 pairs of n-type Bi2Te3 and p-type Sb2Te3 thin-film legs exhibited a sensitivity of 2.7 mV/K.
  • Integration of Temperature, Stress State, and Strain Rate for the Ductility of Ductile Metals

    pp. 626-629

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    DOI:10.2320/matertrans.M2012389

    The ductility of ductile metals, evaluated in terms of fracture strain, is strongly affected by temperature, stress state and strain rate. A parameter integrating the three parameters was proposed, which can be used as a single variable representing the fracture strain of ductile metals.
  • Editor’s Announcement

    pp. 630-630

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    DOI:10.2320/matertrans.EAM2012604

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