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

MATERIALS TRANSACTIONS Vol. 61 (2020), No. 4

  • PREFACE

    pp. 561-561

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    DOI:10.2320/matertrans.MPR2020901

  • Microstructure and Mechanical Behavior of Ti–25Nb–25Zr Alloy Prepared from Pre-Alloyed and Hydride-Mixed Elemental Powders

    pp. 562-566

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    DOI:10.2320/matertrans.MT-MK2019001

    A study has been undertaken on the feasibility of the powder-metallurgy manufacturing process to fabricate β-type Ti–25Nb–25Zr alloy (mass%) for biomedical applications. The Ti–25Nb–25Zr alloy was fabricated from a mixture of TiH2 with constituent elemental powders, and from a pre-alloyed Plasma Rotating Electrode Processed (PREP) Ti–25Nb–25Zr powder, separately. It is shown that different processing methods led to different microstructures and mechanical properties. The Ti–25Nb–25Zr compact prepared by pre-alloyed powder exhibits poor strength whereas TiH2 processed Ti–25Nb–25Zr compact exhibits comparatively ultra-fine grained microstructure with significantly improved strength. The proposed fabrication method may have several opportunities to fabricate metallic alloys with enhanced mechanical properties.
  • Development of Co–Cr–Mo–Fe–Mn–W and Co–Cr–Mo–Fe–Mn–W–Ag High-Entropy Alloys Based on Co–Cr–Mo Alloys

    pp. 567-576

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    DOI:10.2320/matertrans.MT-MK2019002

    Co–Cr– and Co–Cr–Mo-based alloys are commercially used in the industry especially for high wear resistance and superior chemical and corrosion performance in hostile environments. These alloys were widely recognized as the important metallic biomaterials. Here, the first development of Co–Cr–Mo–Fe–Mn–W and Co–Cr–Mo–Fe–Mn–W–Ag high-entropy alloys (HEAs) based on Co–Cr–Mo metallic biomaterials is reported. Ingots of six-component Co2.6Cr1.2Mo0.2FeMnW0.27 (Co41.5Cr19.1Mo3.2Fe16Mn16W4.3, at%) HEAs with a minor σ phase and of seven-component Co4.225Cr1.95Mo0.2FeMnW0.2Ag0.5 (Co46.6Cr21.5Mo2.2Fe11Mn11W2.2Ag5.5, at%) and Co2.6Cr1.2Mo0.1FeMnW0.1Ag0.18 (Co42.1Cr19.4Mo1.6Fe16.2Mn16.2W1.6Ag2.9, at%) HEAs without an σ phase were fabricated. The alloy was designed by a taxonomy of HEAs based on the periodic table, a treelike diagram, predicted phase diagrams constructed by Materials Project, and empirical alloy parameters for HEAs. The σ phase formation prevented the formation of solid solutions in Co–Cr–Mo-based HEAs without a Ni element. The σ phase formation in as-cast ingots was discussed based on the composition dependence and valence electron concentration theory.
  • Effect of Elemental Combination on Microstructure and Mechanical Properties of Quaternary Refractory Medium Entropy Alloys

    pp. 577-586

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    DOI:10.2320/matertrans.MT-MK2019003

    The present work reports the effect of elemental combination on microstructure and mechanical properties of quaternary refractory medium entropy alloys (RMEAs) having equi-atomic compositions. As-cast RMEAs ((1) HfNbTaTi, (2) HfNbTaZr, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr) were fabricated by vacuum arc-melting of pure elements under Ar atmosphere, homogenization was then performed at 1150°C for 24 hours with Ar atmosphere. Firstly, microstructures of both as-cast and homogenized RMEAs were observed by SEM-BSE. Three kinds of microstructures consisting of annealed grains (AG), granular morphology (GM) and dendritic morphology (DM) were found to be distributing along solidification direction in the as-cast RMEAs. Inter-dendritic segregation in the as-cast RMEAs was characterized by SEM-EDX. At the same time, grain boundary precipitates were found in the as-cast (2) HfNbTaZr and (4) HfTaTiZr alloys. After homogenization at 1150°C, a fraction of AG greatly increased while that of DM largely decreased. It was also found that the degree of segregation was largely reduced after homogenization. In addition, grain boundary precipitates having equiaxed morphology and HCP structure were observed in the homogenized (2) HfNbTaZr alloy. Subsequently, tensile tests of both as-cast and homogenized RMEAs were performed at room temperature (RT) to characterize mechanical properties of the RMEAs. After homogenization, ductility of (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys was highly improved while (2) HfNbTaZr alloy still showed early brittle fracture. Better ductility of the homogenized (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys could be attributed to the elimination of inter-dendritic segregation as well as grain boundary precipitates through homogenization.
  • Effect of Cobalt-Content on Mechanical Properties of Non-Equiatomic Co–Cr–Ni Medium Entropy Alloys

    pp. 587-595

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    DOI:10.2320/matertrans.MT-MK2019004

    Non-equiatomic high entropy alloys (HEAs) and medium entropy alloys (MEAs) are expected to have the potential to exhibit good mechanical properties due to abundant composition designs compared to equiatomic alloys. It has been reported that an equiatomic CoCrNi MEA shows better strength-ductility balance than CoCrFeMnNi HEA, and there is a possibility that the mechanical properties can be further improved by changing chemical composition. Among the constituent elements, cobalt (Co) has the effect of decreasing stacking fault energy (SFE). In this study, we clarified the effect of Co-content on mechanical properties of non-equiatomic Co–Cr–Ni MEAs with different amounts of Co through investigating deformation behaviors and deformation microstructures. Cox(CrNi)(100−x) (x = 20 (Co20), 40 (Co40), 60 (Co60) at%) MEAs were processed to very high plastic strains by high-pressure torsion (HPT) and subsequently annealed under proper conditions to obtain FCC single-phase and uniform fine grain sizes. Mechanical properties of the specimens with fully recrystallized microstructures were characterized by tensile tests at room temperature. Their deformed microstructures at different tensile strain levels were observed by electron microscopy. The result of the tensile tests showed that the work-hardening rate was enhanced with increasing the Co-content although early fracture before reaching plastic instability condition occurred in Co60. Planar slip of dislocations and deformation twinning were observed in Co20 (SFE = 30 mJ/m2), while, in addition to them, deformation-induced martensitic transformation to HCP ε-martensite was observed in Co40 having lower SFE (SFE = 10 mJ/m2), leading to higher work-hardening rate. By increasing Co-content (decreasing SFE) further, phase fraction of ε-martensite greatly increased in Co60 (SFE = 0 mJ/m2) compared with Co40, and early fracture occurred due to stress concentration at intersects between martensite and grain boundaries. The present results suggested that the mechanical properties of the present materials could be effectively designed by controlling the SFE.
  • Characterization of Growing Dendrites in CrMnFeCoNi High-Entropy Alloy by Time-Resolved and In-Situ Tomography

    pp. 596-604

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    DOI:10.2320/matertrans.MT-MK2019006

    Time-resolved tomography (4D-CT) and X-ray diffraction (XRD) were combined to observe growing dendrites and to measure their crystallographic orientation in a CrMnFeCoNi high-entropy alloy with an FCC structure. The evolution of the dendritic grains cooling at 0.083 K/s was reconstructed using 200 projections over a 180° rotation every 4 s from 4D-CT and a phase field filter. The voxel size was a 6.5-µm cube. Simultaneously, the crystallographic orientations of the dendritic grains were measured by XRD. The dendrite arms grew preferentially along the 〈100〉 direction, corresponding with typical FCC alloys. The specific solid–liquid interfacial area, which was normalized by the total volume, was evaluated as a function of solid fraction. The interfacial area reached a maximum at a solid fraction of 0.55. The interfacial area was compatible with the reported values of Al–Cu and Mg–Sn alloys. The secondary arm spacing was on the same order of magnitude as the spacing of conventional alloys. Overall, it appears that solidification in this high-entropy alloy can be analyzed by using models developed for binary or pseudo-binary alloys.
  • An Atomistic Modeling Study of the Relationship between Critical Resolved Shear Stress and Atomic Structure Distortion in FCC High Entropy Alloys — Relationship in Random Solid Solution and Chemical-Short-Range-Order Alloys —

    pp. 605-609

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    DOI:10.2320/matertrans.MT-MK2019007

    The relationship between the critical resolved shear stress (CRSS) at T = 0 K and the atomic structure distortion was studied using molecular dynamics (MD) simulation with atomic distortion (root-mean-square-atomic-displacement (RMSAD)) controlled Lennard-Jones (LJ) interatomic potentials for different face-centered-cubic (FCC) high entropy alloy (HEA) systems, such as ternary, quaternary, and quinary alloy systems. We demonstrated that an almost universal linear relationship exists between CRSS and RMSAD for the random solid solution (RSS) of these alloy systems. The universality was also confirmed by a more realistic embedded atom method (EAM) potential. However, alloy systems that have a chemical-short-range-order (CSRO) do not follow this universal linear relationship.
  • Thermal Vacancies in High-Entropy Alloys

    pp. 610-615

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    DOI:10.2320/matertrans.MT-MK2019008

    The thermodynamic description of vacancies in binary alloys was expanded to high-entropy alloys (HEAs) within the framework of the CALPHAD method and was applied to estimate the vacancy fraction and the vacancy formation enthalpy in the HEA CoCrFeMnNi. Using the average value of the vacancy formation enthalpy in the pure elements and the excess Gibbs energies in the binary subsystems, the formation enthalpy in the HEA was found to be 1.7 eV. This value is in good agreement with the experimental value of 1.72 eV determined by the positron annihilation technique. This result suggests that the sluggish diffusion cannot be explained from the viewpoint of vacancy formation enthalpy.
  • Doping of Interstitials (H, He, C, N) in CrCoFeNi High Entropy Alloy: A DFT Study

    pp. 616-621

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    DOI:10.2320/matertrans.MT-MK2019009

    The dissolution behavior of H, He, C and N in CrCoFeNi high entropy alloy (HEA) is studied by density functional theory calculation. Results show that the site preference of H, C and N in CrCoFeNi HEA is the octahedral site, while the site preference of He is the tetrahedral site. The dissolution energy of H in CrCoFeNi HEA is the lowest and that of He is the highest. The high dissolution energy of He is considered to be due to its closed-shell electronic structure. Moreover, results suggest that the H, He and N are more stable at the site with more Cr atoms, and C is more stable at the site with less Ni atoms. Furthermore, It is found that H, He, C and N have the effect in decreasing the magnetic moments of solutes in CrCoFeNi HEA. Electronic structure analysis shows that, for C and N, there is strong hybridization between them and solutes. It implies the chemical bonding between C, N and solutes is strong.
  • Hydrogenation Character and Crystal Structures of Hydrides in RMgNi4 (R = Nd, Gd, Er)

    pp. 622-625

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    DOI:10.2320/matertrans.MT-M2019224

    Whether the pseudo-binary RNi2 compounds consistent with the criterion rA/rB = 1.37 for HIA. The hydrogenation performances and crystal structures of the pseudo-binary RMgNi4 (R = Nd, Gd, Er) were investigated at fixed R/Mg ratio equal to 1. The pseudo-binary RMgNi4 (R = Nd, Gd, Er) compounds can repetively absorb and desorb hydrogen without the occurrence of HIA and decomposition during hydrogen absorption and desorption cycles at 298 K, 6 MPa. The HIA criterion rA/rB of pseudo-binary RMgNi4 alloys should be higher than 1.37 at fixed R/Mg equal to 1.
  • Microstructure and Thermal Conductivity of Carbon Nanotube Block-Reinforced Aluminum Matrix Composite

    pp. 626-631

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    DOI:10.2320/matertrans.MT-M2019180

    To solve the problem of pore formation caused by the aggregation of carbon nanotubes (CNTs) in metal matrix composites, unidirectionally aligned CNT (CNT blocks) were used as the reinforcement material. CNT block preforms with high porosities were fabricated via a spacer method using electroless Cu-plated CNT blocks. During the preform manufacture, the thickness of the Cu layer was varied while maintaining a constant CNT block volume fraction (10%). In addition, CNT block-reinforced Al matrix (CNT block/Al) composites were manufactured using a low-pressure infiltration method at 0.1 MPa. The interface between the CNT blocks and the Al matrix and the reactivity of the Al matrix with the Cu layer were investigated. The composites with the CNT block:Cu ratios ≤6.8:3.2 showed an improved CNT block/Al interface. At the CNT block:Cu ratio of 3.7:6.3, an intermetallic compound, Al2Cu, was formed by the reaction between Cu and Al. Furthermore, the thermal conductivities of the fabricated CNT block/Al composites were determined.
  • High Temperature Deformation and Microstructure Evolution of Ni–Co Base Superalloy TMW-4M3

    pp. 632-640

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    DOI:10.2320/matertrans.MT-MBW2019002

    The high temperature deformation and microstructure evolution of Ni–Co base superalloy TMW-4M3 during the isothermal forging process were studied. A uniform compression test of TMW-4M3 where both the strain rate and compression temperature were controlled showed dynamic recrystallization flow stress. The peak stress and steady stress of the deformation resistance curve were characterized with the Zener-Hollomon parameter. The average grain size after dynamic recrystallization was also correlated with the Zener-Hollomon parameter, but this relationship changed with compression temperature. We found that this temperature dependency was related to the pinning effect of the γ′ precipitates in the γ matrix and proposed a new prediction model for dynamic recrystallization grain size considering not only the Zener-Hollomon parameter but also the volume fraction of the γ′ precipitates. This enables us to calculate the average grain size after isothermal forging within an error of 12%.
  • Online Roll Force Model for Non-Oriented Electrical Steel Considering Temperature-Transformation-Roll Force Coupling Effect

    pp. 641-646

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    DOI:10.2320/matertrans.MT-M2019230

    During the finish rolling process of non-oriented electrical steel, the transformation from austenite to ferrite will occur. The uneven temperature along width and thickness direction will lead to the difference of phase transformation. Meanwhile, the latent heat accompanying transformation will affect the distribution of temperature. Due to the high-temperature deformation characteristic in dual-phase region is exactly opposite to that in single-phase region, the transformed fraction should be considered in the calculation of roll force. In order to make up for the deficiency of the traditional roll force prediction model. The transformation kinetics model, latent heat model and deformation resistance model for different phase regions which are established in our past work are adopted. The online prediction model of roll force of non-oriented electrical steel considering Temperature-Transformation-Roll force coupling effect are eventually obtained by taking the above basic models into the temperature model based on two-dimensional alternating difference method, which is able to calculate the distribution of temperature field and phase field and predict the roll force of each stand in different phase regions. After comparison and verification, the prediction errors of finish rolling temperature can be controlled within 10°C by new model proposed. The position of transformation is consistent with the abnormal wave observed. The proportion of rolled coils whose prediction error over 10% is decreased from 37.1% to 8.5% and the proportion of rolled coils whose prediction error less than 5% is increased from 15.8% to 41.2%. The computing time is only 68 ms which proves the feasibility of online application.
  • High-Temperature Plastic-Deformation Behavior of Mg–(Y/Zn) Supersaturated Solid-Solution Alloys and the Resulting Dislocation Structures

    pp. 647-656

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    DOI:10.2320/matertrans.MT-M2019271

    High-temperature plastic deformation behavior and the resulting microstructures of magnesium (Mg)-based supersaturated solid-solution alloys containing zinc (Zn) and/or yttrium (Y) have been thoroughly examined in a comparative study by means of various electron microscopy combined with microanalytical techniques. According to the results of compression tests measured for the alloys at constant testing temperatures ranging from room temperature (RT) to 300°C, it is admittedly found in common to the respective alloys that twinning of {1012}-tensile type dominates the deformation at lower temperatures but this gives way to dislocation-slip with a rise in temperature. Above all, Mg–Y–Zn ternary solid-solution alloys yield remarkably higher levels of flow stresses capable of withstanding high temperatures than binary counterparts. The solid-solution alloy of Mg–0.6Y–0.3Zn (at%) subjected to compression at 300°C, in fact, has many deformation-induced stacking-faults on the (0001) basal planes significantly decorated by Y/Zn-solute segregation, providing the definite evidence that Suzuki effect actually contributes to a substantial enhancement of the flow stresses at elevated temperatures. This study demonstrates that the Suzuki effect is measurably activated in Mg-based solid-solution alloys, especially when those containing an adequate amount of combined solutes of Y and Zn, e.g. 0.6∼1 at%Y and 0.3∼0.5 at%Zn, are plastically-deformed at a temperature of 300°C and at a strain rate of 1.0 × 10−3 s−1.
  • Preparation of High-Coercivity Magnetic Powder via Heat Treatment of a Rapidly Quenched Amorphous Starting Compound with a ThMn12 Structure

    pp. 657-662

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    DOI:10.2320/matertrans.MT-M2019356

    Amorphous (Sm0.8Zr0.2)1.1(Fe0.9Co0.1)11.3Ti0.7 samples prepared by the rapid-quenching method were annealed under various conditions (848–1223 K and 1–60 min) in a pure Ar atmosphere (PO2 < 10−2 Pa). The powders obtained at low annealing temperatures (<1073 K) were predominantly composed of the 1-9 phase (SmFe9), whereas those obtained at high annealing temperatures (>1123 K), were mostly composed of the 1-12 phase (ThMn12). However, the highest coercivity (Hc) of more than 400 kAm−1 (5 kOe) was observed for samples containing a mixture of the 1-9 and 1-12 phases, which were obtained by annealing at intermediate temperatures of 1123–1173 K for 10–30 min. Domain structure observations revealed that the sample with the highest Hc (magnetic anisotropy field (Ha) of approximately 6 MAm−1) was composed of single-domain particles. Moreover, Hc was decreased to approximately 160 kAm−1 (2 kOe) by the formation of a domain structure (approximately 500 nm in width) in samples of the 1-12 mono-phase, despite high Ha of approximately 9 MAm−1. A mechanism for these coercivity variations is proposed.
  • Effect of Co and P on the Discontinuous Precipitation Behavior in High Concentration Corson Alloy

    pp. 663-667

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    DOI:10.2320/matertrans.MT-M2019152

    The effects of Co and P on the discontinuous precipitation (DP) behavior of Cu–Ni–Si system alloys have been investigated using Cu–4.5 mass%Ni–1.1 mass%Si alloy (base alloy), Cu–4.0 mass%Ni–0.5 mass%Co–1.1 mass%Si alloy (Co alloy) and Cu–4.0 mass%Ni–0.5 mass%Co–1.1 mass%Si–0.05 mass%P alloy (Co + P alloy) aged at 430, 460 and 490°C. In all the alloys, slightly before attaining peak age-hardening within grains, DP cells nucleated at grain boundaries and grew into the grains ahead of reaction fronts. The growth rates of DP cell for Co alloy were slower than those for Base one. Moreover, the trace addition of P to Co alloy (Co + P alloy) considerably retarded DP reaction. The kinetic analyses of DP using the Turnbull (T) and Petermann and Hornbogen (P-H) models yielded grain-boundary diffusion data. The activation energies for the base and Co alloys obtained using the T model were nearly identical and 125 and 124 kJ mol−1, respectively. The values for both alloys determined from the P-H model were slightly larger than those obtained from the T model and about 130 kJ mol−1. These results strongly suggested that the growth of DP cells in base and Co alloys was controlled by the boundary diffusion of Ni, and Ni or Co in Cu matrix, respectively. The smaller growth rate of DP cells in the Co alloy was ascribed to the higher number density of continuous δ precipitates than that of base one, which successively suppressed the boundary migration. This Paper was Originally Published in Japanese in Journal of the Japan Institute of Copper 57 (2018) 101–106.
  • Changes in States of Carbon and Mechanical Properties with Aging at 50°C after Quenching in Low Carbon Steel

    pp. 668-677

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    DOI:10.2320/matertrans.MT-M2019351

    The changes in the states of carbon (C) together with hardness and the tensile properties of low C steel (0.045C–0.34Mn in mass%) quenched from 710°C and aged at 50°C were investigated as a function of aging time using TEM and atom probe tomography. Vickers hardness increases at about 1.1 × 104 s, exhibits significant increase at 5.8 × 104 s (16 h) and maintains peak hardness untill 8.6 × 105 s (10 d) followed by a decrease after further aging time. At the start of peak aging, C clusters form with an irregular shape that resembles a sphere about 10 nm in diameter. The number of C atoms is about 700, and the C content is in the range of 1–2 at% at 1.0 × 105 s (28 h), where no enrichment of elements except for C is observed. At the end of peak aging, the plate-shaped precipitates (about 1 nm wide and 12 nm long) having a C content greater than 10 at% are distributed with the {100} habit plane, thus confirming the transition from C clusters to fine carbides. Lower yield strength (LYS) is the lowest for the specimen with solute C, and significantly increases for the specimen with C clusters and fine carbides in this order. LYS is determined presumably by the cutting mechanism for the C cluster specimen and the Orowan mechanism for the fine carbide specimen. The work hardening for the solute C and C cluster specimens is high, while the carbide specimen shows less work hardening. The C cluster is assumed to be decomposed into solute C through shearing by dislocations, causing work hardening and relatively good uniform elongation. Post uniform elongation (l-El) was the lowest for the C cluster specimen followed by the fine carbide specimen with the same strength level. This is because dynamic strain aging caused by solute C promotes the strain localization leading to the deterioration in l-El. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 353–362.
  • Work-Hardening Mechanism in High-Nitrogen Austenitic Stainless Steel

    pp. 678-684

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    DOI:10.2320/matertrans.H-M2020804

    The remarkably high work-hardening rate in high-nitrogen austenitic stainless steels is generally believed to be due to the promotion of dislocation accumulation by nitrogen addition. However, analysis of dislocation accumulation behavior by the modified Williamson-Hall/Warren-Averbach method reveals that no difference to the increment of the dislocation density during deformation exists between austenitic steels with and without nitrogen. Since cross slipping is markedly suppressed in high-nitrogen steels, the moving dislocations are back-stressed by the planar dislocation arrays. This induces the deformation resistance and the high work-hardening rate. This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treat. 59 (2019) 222–229. In this article, we have used the expressions of ‘High-N steel’ and ‘Low-N steel’ instead of ‘High N steel’ and ‘Low N steel’ in the original Japanese article.
  • Comparative Three-Point Bending Flexural Test of Dental Ceramics Using Standard and Self-Adjustable Specimen Support Fixtures

    pp. 685-690

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    DOI:10.2320/matertrans.MT-M2019307

    The flexural strength of dental ceramic specimens was comparatively measured by using the standard fixture and the self-adjustable specimen support fixture which carefully prepared for this work. One hundred and twenty dental ceramic specimen bars fabricated in accordance with the BS EN ISO 6872 standard were ground, polished and classified into 6 groups with opposing surface parallelism differences of 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05 mm. Each group contained 20 specimens which ten specimens were tested by using the standard fixture and the other ten specimens were tested by using the self-adjustable specimen support fixture. The flexural strength and Weibull modulus of both specimen groups were analyzed using two way analysis of variance (ANOVA) and Weibull analysis. The fracture surfaces of both specimen groups were investigated under the scanning electron microscope and analyzed for their fracture behavior. Results from the study found that the self-adjustable specimen support fixture for the three-point flexural test was able to improve the force distribution on the surface of specimens under test. As a result, the actual flexural strength of materials under test with the higher value and reliability was achieved. The flexural strength from this study was found to be comparable to that measured with the bi-axial test. The strength values increased significantly from 79.1 ± 5.1 MPa to be 90.1 ± 3.4 MPa when tested with the standard fixture and the self-adjustable specimen support fixture respectively. The 10% failure stress and Weibull modulus of specimens tested with the self-adjustable specimen support fixture also improved obviously. The fracture surface characteristics of specimens tested with the standard fixture indicated most crack origin occurred at the specimen edge. In the specimens tested with the self-adjustable specimen support, the crack origin was observed as the appearance of the arrest lines or Wallner lines at the inner area beyond the specimen edge.
  • Composition-Processing-Property Correlation Mining of Nb–Ti Microalloyed Steel Based on Industrial Data

    pp. 691-699

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    DOI:10.2320/matertrans.MT-M2019172

    Modeling strength of hot rolled strip based on industrial data may cause misleading predictions because of the high dimension, low quality and unbalanced original data. Industrial data processing is essential to building a successful composition-process-property corresponding relationship model. In current work, the Pauta criterion was implemented to eliminate abnormal data, and uniform grid technique was applied to select out the represented data for obtaining the balanced training data set. Prior to modeling by Bayesian regularization neural network, principal component analysis was applied to alleviate the effect of correlation variables on the modeling. The yield strength prediction model of Nb–Ti microalloyed steel was established with the relative error of ±8%, indicating a good agreement between predicted value and measured value. Finally, the relationship of chemical composition, process parameters and yield strength of hot rolled strip under specific process parameters was analyzed for a further investigation.
  • Nonlinear Multiple Regression Model and Optimization of Process Parameters for Weld Bonding of DP780 High Strength Steel

    pp. 700-707

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    DOI:10.2320/matertrans.MT-M2019295

    Based on the Box-Behnken Design (BBD) method, the experiment on weld bonding for DP780 high-strength steel was carried out. The failure load, nugget diameter and energy absorption value of the joints were viewed as the target quantities. The welding current, welding time, electrode pressure and interaction between the parameters were defined as the factors to influence the target quantities. The nonlinear multiple regression model of the weld-bonded joints for DP780 high strength steel was established. The experimental verification shows that the model has high saliency and high degree of fitting, which can effectively predict the failure load, nugget diameter and energy absorption value of the joints. With the increase of welding current and time, the failure load of the weld-bonded joints, the diameter of the nugget and energy absorption value increase, while they decrease with the increase of electrode pressure. The optimal process parameters are welding current 8 kA, welding time 150 ms, electrode pressure 0.3 MPa, which are obtained by the regression model, and the actual value of joint’s failure load under the shear test is 17669 N. The ultrasonic C-scan image is used to identify the gasification zone of the adhesive layer outside the weld nugget. When the welding time is small, the increase of the welding current will provide more heat input, which will lead to an increase in the burning area of the adhesive layer and reduce the static properties of the joints.
  • Influence of Strain Rate on Mechanical Behaviours of Gradient-Structured Copper

    pp. 708-717

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    DOI:10.2320/matertrans.MT-M2019373

    The mechanical properties of gradient-structured (GS) pure Cu processed by surface mechanical attrition treatment (SMAT) were investigated by tensile tests at different strain rates in the range of 5 × 10−4 s−1 to 5 × 10−2 s−1 at room temperature. The yield strength (YS), ultimate tensile strength (UTS) and ductility (uniform elongation, UE) of the gradient-structured Cu are simultaneously increased with increasing strain rate, especially for the sample processed by longer SMAT-treating time, while the coarse-grained (CG) Cu showed no obvious strain rate effect on strength and ductility. In addition, the strain rate sensitivity (m) and hetero-deformation induced (HDI) stress of the gradient-structured samples also increased with increasing strain rate. The increase in strength is mainly attributed to the gradient-structured layers and HDI stress strengthening. On the other hand, the increase in ductility can be attributed to an increase in strain rate sensitivity (m) and the strain hardening induced by the accumulation of a large number of geometrically necessary dislocations (GNDS) caused by a large stress gradient (HDI hardening).
  • The Influence of Carbon Nanotubes on the Properties of Sn Solder

    pp. 718-722

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    DOI:10.2320/matertrans.MT-M2019369

    The microstructure and properties of Sn lead-free solders with various carbon nanotubes (CNTs) contents (0, 0.025, 0.05, 0.075, 0.1, 0.125, wt. %) were studied. The results indicate that the wettability and the mechanical properties of solder joints both were effectively improved when the content of CNTs was 0.075 wt. %. Furthermore, adding 0.075 wt. % CNTs can effectively refine the structure of solder and inhibit the formation of brittle intermetallic compounds (IMCs). The solder joints fracture mode of the specimen was mainly ductile fracture when the addition of CNTs was 0.075 wt. %.
  • Microstructure and Mechanical Property Improvement of Laser Additive Manufacturing Ti–6Al–4V via the Niobium Addition

    pp. 723-728

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    DOI:10.2320/matertrans.MT-M2019370

    Titanium alloys with high specific strength have widespread applications within the aerospace industry. However, the direct fabrication of titanium alloys with excellent performance is very difficult using traditional manufacturing techniques. In the present work, we adopted advanced laser additive manufacturing (LAM) technology to fabricate Ti–6Al–4V titanium alloy via the addition of Niobium (Nb), which result in the improvement of mechanical properties. The effects of Nb additions on microstructure, phase composition, and mechanical properties of Ti–6Al–4V alloys were investigated. Research results showed that the addition of Nb not only significantly refined the original β-Ti and the primary acicular α-Ti structure, but also increased the content of β-Ti to some extent, which contributed to improving both the strength and plasticity of the material. The tensile strength, yield strength, the elongation, and the section shrinkage of the Ti–6Al–4V+Nb increased by 15.2%, 6.9%, 2.6%, and 7.4%, respectively, compared with Ti–6Al–4V, which exceeded the required forging standard.
  • Molding and Mold Properties of Spherical Artificial Sand Coated with Inorganic Binder

    pp. 729-733

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    DOI:10.2320/matertrans.F-M2020802

    In recent years, molds made by additive manufacturing (AM) are increasingly applied to build prototypes and small-quantity casting products. Technological development aiming to use mass-produced castings is advancing with the emergence of high speed AM molding technology. Mass production is expected to improve the potential of the entire casting product, such as the realization of complicated internal structures, thinness, and weight reduction by the improvement of cavity accuracy.Regarding binders applied to AM sand molding technology, so far organic self-hardening systems are mainly applied and their use is spreading. On the hand, in the case of aluminum alloy castings, the application of hardening system with is required from the viewpoint of the clean working environment. The purpose of this study is to develop an AM sand molding technology utilizing a hardening system with an inorganic binder and artificial sand. The developed hardening system is composed of sand coated with an inorganic binder and inkjettable hardening catalyst. First, the hardening characteristics of the developed hardening system were evaluated using hand molded test pieces, and it was found that it is effective for increasing the initial hardening rate compared with conventional methods. Next, the ability to mold with this hardening system was tested using a binder jetting AM molding apparatus. The gas component generated by heating was measured and the coefficient of linear expansion by the molded test pieces was tested to evaluate the characteristics of the molded test. It was confirmed that harmful gas can be reduced and the mold has low thermal expansion. This Paper was Originally Published in Japanese in J. JFS 90 (2018) 286–291. Figure 3 and Table 2 were slightly changed.
  • Additive Manufacturing Technology by Furan Sand Mold Using Sintered Artificial Sand Coated with Solid Catalyst

    pp. 734-739

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    DOI:10.2320/matertrans.F-M2020801

    In recent years, casting molds made by additive manufacturing (AM) are increasingly applied to build prototypes and small quantity casting product. Devices and materials have been developed for accelerating the molding speed to achieve mass production using AM technology. Conventional AM molding methods using furan type hardening system adopt a combination of a liquid catalyst and binder. The sand wet by the liquid catalyst makes it difficult to increase the molding speed and apply artificial sands which have excellent refractoriness to the sand molding using AM technology.This study developed a solid catalyst coated sand to improve the flowability of the sand and catalyst and to increase the binder hardening rate. The solid catalyst coated sand was made by covering sintered artificial sand with the solid catalyst of meta-xylene sulfonic acid after covering with the fine powder of anhydrous magnesium sulfate.First, this study confirmed that the solid catalyst coated sand has good fluidity in fluidity tests. The reduction of the surface tension by the bonding of the powder anhydrous magnesium sulfate and the sulfate of the solid catalyst resulted in the dry state of the solid catalyst sand. Consequently, the catalyst sand showed good fluidity.Secondly, in mold hardening tests, the solid catalyst coated sand hardened the mold quicker than conventional method. During the hardening reaction of the binder, water generated when furfuryl alcohols were bonded to each other was converted into a hydrate by anhydrous magnesium sulfate and absorbed. As a result, the reaction rate greatly improved compared with conventional methods that only release water outside the system.Molds using sand additive manufacturing have been used for a long time, but the molding speed and size needs to be improved in order to apply high refractory granular material.Finally, the ability to mold with this hardening system was tested with the AM molding apparatus using the binder jetting. The results of evaluating the characteristics of the mold showed that it has low thermal expansion and the unhardened part of the solid catalyst coated sand could be reused. This Paper was Originally Published in Japanese in J. JFS 90 (2018) 280–285. Fig. 5, Fig. 7, Fig. 9, Table 1–Table 3 were slightly changed.
  • Possibility of As-Cast Applications on β-Type Titanium Alloys Proposed in the Newly Expanded Area of Bot-Mdt Diagram

    pp. 740-749

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    DOI:10.2320/matertrans.F-M2020803

    Ten types β-type titanium alloys for as-cast applications were proposed by using mainly ubiquitous elements in the area greatly differ from conventional alloys in compositions, in the diagram consisting of bond order (Bo) and d-orbital energy level (Md). The object was to develop β-type titanium alloys with ultimate tensile strength value of 1000 MPa and fracture strain value of 10% in as-cast condition without the post-treatments after cold crucible levitation melting, and their microstructures were controlled by adjusting its process parameters. The same microstructure and mechanical properties were also indicated on both as-cast and solution treated alloys, which mean the unnecessary of solution treatment in proposed alloys. Ti–5.2Cr–5.5Mn–2.2Fe–5.5Zr, Ti–11Cr–6Mn–4.5Zr–0.5Al and Ti–13Zr–6Mn–6Cr–5V alloys with a β mono phase achieved objective values of mechanical properties in as-cast condition, which lead to development of alternative materials to the conventional β-type titanium alloys, since they reduce energy consumption and 25% production costs by omitting complex post processing procedures. Moreover, the phase boundary between the mono β phase and dual β plus intermetallic compound phases were predicted for the first time in β-type titanium alloys, according to the ten proposed alloys in newly expanded compositional area of the Bot-Mdt diagram.
  • Molecular Dynamics Simulation of Nucleation from Undercooled Melt of Nickel–Aluminum Alloy and Discussion on Polymorphism in Nucleation

    pp. 750-757

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    DOI:10.2320/matertrans.MT-M2019353

    Nucleation from undercooled melt of Ni–Al alloy is investigated by molecular dynamics (MD) simulation. Multiple nucleation of NiAl nuclei with B2 structure appears from undercooled melt of Ni–50 at%Al, which forms a fine microstructure of B2-NiAl. On the other hand, stepwise phase transition happens from undercooled melt of pure Ni, which is known as Ostwald’s step rule. That is, body-centered-cubic (BCC) phase appears first from the undercooled melt and then face-centered-cubic (FCC) nucleation occurs from the inside of previously existing BCC nucleus. Origin of the polymorphism in stepwise nucleation of Ni and the preferential nucleation of B2-NiAl from melt of Ni–Al alloy is discussed on the basis of classical nucleation theory.
  • Development of a New Titanium Powder Sintering Process with Deoxidation Reaction Using Yttrium Metal

    pp. 758-765

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    DOI:10.2320/matertrans.MT-M2019340

    Oxygen (O) contamination in titanium (Ti) is difficult to control using conventional Ti powder-metallurgy technologies, owing to the strong affinity between Ti and O. In this study, we developed a new sintering process that can remove O from Ti by placing Ti green and yttrium (Y) in molten salt. This study demonstrates that the O concentration in Ti can be reliably controlled in the range of 200–2000 ppm O by varying aY in the Y/Y2O3 equilibrium at 1300 K in NaCl–KCl (l), such that the sintering reaction of Ti powder simultaneously proceeds. Furthermore, it is also shown that the O concentration in Ti can be reduced to 30–60 ppm O in YCl3 (l) in the sintering process, when the Y/YOCl/YCl3 equilibrium is employed. This study demonstrates the feasibility of a new sintering process that can control the O concentration in Ti to approximately 30–2000 ppm O. The process ensures economical rationality because the cost of Y metal is negligibly small in recent years. By developing this process, inexpensive high-O-concentration Ti powder can be applied for fabricating the desired low-O-concentration Ti products.
  • Effects of Size of the Largest Crack and Size Difference among Cracks on Critical Current of Superconducting Tape with Multiple Cracks in Superconducting Layer

    pp. 766-775

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    DOI:10.2320/matertrans.MT-MBW2019001

    The effects of the size of the largest crack and size difference among cracks on critical current of superconducting tape with multiple cracks of different sizes in the superconducting layer were investigated by a model analysis and a Monte Carlo simulation, using the specimens consisting of a series circuit of local sections where each section has one crack of different size from each other. It was revealed that, with increasing distribution-width of crack size, the increase in the size of the largest crack acts to reduce the critical current and the increase in the crack size-difference among the sections acts to raise the critical current, and these conflicting two effects are summed up and determine the critical current value. To describe this feature quantitatively, we expressed the statistics of the size of the largest crack in specimens with the Gumbel’s extreme value distribution function. Also we monitored the effect of the difference in crack size among the sections on specimen’s critical current, using the number of the sections equivalent to the largest crack-section at the critical voltage for determination of specimen’s critical current. In this monitoring, small number of the sections equivalent to the largest crack-section corresponds to large difference in crack size among the sections. With the present approach, the effect of the increase in size of the largest crack, which acts to reduce critical current, and the effect of the increase in difference in crack size among the sections, which acts to raise critical current, with increase in distribution-width of crack size, could be estimated separately.
  • Design of Zirconium Quaternary System Alloys and Their Properties

    pp. 776-781

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    DOI:10.2320/matertrans.MT-M2019315

    Two multicomponent β-type Zr alloys were designed using the d-electron alloy design method, and their mechanical properties, magnetic susceptibility, and Young’s modulus were evaluated. A phase stability (BoMd) map was constructed by performing theoretical calculations and was subsequently used to determine alloy compositions (Zr–14Nb–5Ta–1Mo and Zr–14Nb–10Ta–1Mo mass%) based on the results previously obtained for Zr–Nb, Zr–Mo, and Zr–Ta ternary alloys. The designed alloys were fabricated via arc melting and casting methods. They consisted of the β-phase and a small volume fraction of the ω-phase. Both alloys exhibited similar mechanical properties; however, a higher strength of 796 MPa and an elongation of 15% were obtained for the Zr–14Nb–5Ta–1Mo alloy. Furthermore, the fabricated Zr–14Nb–5Ta–1Mo and Zr–14Nb–10Ta–1Mo alloys were characterized by low magnetic susceptibilities of 16.96 × 10−9 and 17.34 × 10−9 m3 kg−1, respectively, and Young’s moduli of 61 GPa and 58 GPa, respectively. In conclusion, the designed alloys demonstrated a good balance of mechanical properties with low Young’s moduli and magnetic susceptibility.
  • Magnetic Properties of Nd–Fe–B Sintered Magnets Produced by Reduction-Grain Boundary Diffusion Process with Heavy Rare-Earths Compounds and Ca Metal Vapor

    pp. 782-786

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    DOI:10.2320/matertrans.MT-MBW2019006

    The grain boundary region of NdFeB sintered magnets were modified by reduction-grain boundary diffusion (r-GBD) process using Heavy Rare-Earths (HRE = Dy or Tb) metal which generated via reduction of HRE compounds such as oxide or fluoride with Ca metal vapor. After modification, magnetic properties were measured accurately by superconducting magnet-based vibrating sample magnetometer (SCM-VSM). The coercivity of DyF3 and TbF3 treated magnets with the assistance of reduction by Ca metal vapor (HcJ = 1451 kA/m and 1778 kA/m, respectively) were effectively enhanced compared to untreated magnets (HcJ = 1003 kA/m) without severe decreasing of remanence value. The resultant coercivity was higher than that of the magnets modified with DyF3 (HcJ = 1319 kA/m) and TbF3 (HcJ = 1580 kA/m) without using any reducing agent such as Ca metal vapor.
  • Temperature- and Catalyst-Dependent Transformation from Reaction Rate-Limited to Diffusion Rate-Limited Hydrogenation of Mg with Nb2O5 Catalyst

    pp. 787-794

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    DOI:10.2320/matertrans.MT-MAW2019002

    The hydrogen consumption behavior during the hydrogenation of magnesium (Mg) with niobium oxide (Nb2O5) catalyst was analyzed via the volumetric Sieverts method, and categorized into two types of consumption kinetic processes: increasing consumption rate with temperature at lower temperatures and almost constant consumption rate at higher temperatures. The consumption rates were measured as the hydrogen absorption rates of Mg, and were analyzed based on a first-order reaction for two types of absorption processes, reaction rate-limited and diffusion rate-limited. The apparent activation energies in the hydrogen absorption rate increased with temperature at low temperatures, and were 49.5–53 kJ/mol for Mg with 5, 10, and 20 mass% Nb2O5 catalyst, which were lower than the activation energy for pure Mg. The apparent activation energies in the absorption rate were almost constant at higher temperatures, ranging from 0–9.6 kJ/mol. The transformation of the absorption reaction process from a reaction rate-limited to diffusion rate-limited process with an extremely lower apparent activation energy is similar to that observed during the combustion of charcoal.
  • Effect of the Pulse Duty Cycle on the Microstructure and Properties of a Jet Electrodeposited Nanocrystalline Copper Coating

    pp. 795-800

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    DOI:10.2320/matertrans.MT-M2019364

    Nanocrystalline copper coatings were jet electrodeposited using a square-wave pulse current with three duty cycles (30%, 50% and 70%) and a direct current condition. The effect of the duty cycle on the surface morphology, microstructure, grain growth and mechanical performance of the copper coatings was examined. The experimental results revealed that a decrease in the duty cycle significantly improved the coating surface morphology and microstructure. It was shown that a pulse current at a low duty cycle during jet electrodeposition effectively generated a nanocrystalline structure in the coating and improved the mechanical properties. At a low (30%) duty cycle during pulse current electrodeposition, coatings with a fine and smooth surface and dense microstructure were produced with a minimum grain size of 25 nm, microhardness of 2.37 GPa and tensile strength of 712 MPa.
  • Fabrication and Enhanced Vickers Hardness of Electrodeposited Co–Cu Alloy Film with High Composition Gradient

    pp. 801-804

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    DOI:10.2320/matertrans.MT-M2019334

    By applying an oscillating potential wave, we obtained an electrodeposited Co–Cu alloy film in which the Co concentration changed periodically at a short modulation wave length. A triangular Co concentration modulation was observed along the film growth direction. The local composition gradient became as high as 50 at%/µm. The Vickers hardness of the composition gradient film was 380 HV. Because this value was much higher than that of simple electrodeposited Co–Cu alloy films, contribution of the composition-gradient structure to hardness was suggested.
  • Erratum

    pp. 805-805

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    DOI:10.2320/matertrans.ERM202001

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