The bonding between copper and epoxy resin is investigated using a C–H–Si thin film to achieve highly adhesive and reliable bonding between the resin mold and the copper substrate in power modules. In this study, high-temperature (473 K) testing is performed to improve the reliability of this junction after high-temperature operation. The bond strength decreases and delamination occurs at the film–copper interface after high-temperature testing. The decrease in bond strength may be due to the decrease in the film–copper interfacial strength because of the decrease in the binding force between copper and oxygen upon heating. When Fe and Cr, which have high oxygen bond disassociation energies, are intercalated at the film–copper interface, the bond strength improves after high-temperature testing and a bond strength above 30 MPa was retained after 1000 h.

This study is a novel method of dissimilar bonding based on chemical and physical effects of C–H–Si film existing between copper and resin. Further, it is an effective method of bonding to achieve high-temperature operation of a resin-molded power module, which can contribute to future advancements in power electronics products.

A method for predicting the lifetime of fatigue crack network formation in die-attach joints is considered through experiments on high-speed thermal cycling using a Si/solder/Si joint specimen and the mechanism is identified. Equibiaxial stresses are generated in the solder layer because thermal deformation of the solder is constrained by the Si, which causes continuous initiation and propagation of crisscross-shaped cracks. When the crack density is sufficiently high, crack growth is arrested by collisions between cracks, and the formation of the fatigue crack network is completed. Based on these results, development of the damaged area and arrest of the development by collisions between the cracks is expressed in terms of extended volume theory incorporating crack initiation and propagation functions for solder as well as considering the damage rate equation. The experimental result for the relationship between the damage ratio in the die-attach joint and the number of cycles under each thermal condition are reproduced by the damage rate equation.

The number of radio systems used for global communications or advanced features is increasing in electronic devices such as smart phones. As mobile information and telecommunications terminals are miniaturized and advanced features are added by manufacturers, measures to prevent electromagnetic interference among electronic components on high-density mounting boards are becoming increasingly important. Therefore, it is crucial to develop methods for forming electromagnetic wave shield films in semiconductor devices. Stable sputtering processes are increasingly used for the deposition of shield films. Although metal is often used as a shield film, the mechanism of the adhesion of the sputtered film to the mold resin in semiconductor devices has not been discussed. In this work, sputtering was used to deposit a stainless steel film as a ground film for a copper wave shield film and the factors that affected the adhesion of the stainless steel film to the mold resin were investigated. For the copper film with no ground film, adhesion decreased as the resin filler content decreased. For the copper/stainless steel film, adhesion remained high as the filler content of the mold resin decreased. The argon and nitrogen plasma etching formed carbides and nitrides at the interface of the mold resin and a stainless steel film, whereas argon etching formed carbides. Based on the experimental results, we proposed that the adhesion between the stainless steel film and the mold resin mainly arose from the carbides and nitrides reacting with iron and chromium in the stainless steel film.

Partially filled skutterudite compounds R_xCo₄Sb₁₂ (R = In, Tb and Dy, nominal composition 0.1 ≤ x ≤ 1.5) were synthesized under high pressure using multi-anvil presses. The samples were analyzed by x-ray diffraction and scanning electron microscopy (SEM) with energy-dispersive x-ray spectrometry (EDX). The highest actual filling fraction x of In, Tb and Dy are 0.64, 0.22 and 0.15, respectively. The electron back-scatter diffraction (EBSD) result indicates that the grain size of In_0.8Co₄Sb₁₂ is 2–5 µm. Electrical resistivity, Seebeck coefficient, and thermal conductivity were measured between 2 K and 300 K. The maximum value of the dimensionless figure of merit ZT is 0.10 for nominal composition In_0.8Co₄Sb₁₂ at 300 K. We studied guest ion R dependence of lattice thermal conductivity κ_L for R_xCo₄Sb₁₂ systematically based on a simple model for the rattling. The reduction rate of κ_L of R_xCo₄Sb₁₂ can be scaled by a function of the guest free distance r_GFD (the distance between R and Sb) and the atomic mass of R.

Bonding copper in the solid state requires either a high temperature, large deformation, or high vacuum. In the present study, copper was butt-bonded using a K-shaped groove at 298 K under no bonding pressure through copper electrodeposition. The initial gap between the faying surfaces of the copper rods were gradually filled with the electrodeposition of copper from the center to the periphery. No significant defects were observed in the bond layer, and a high joint strength of approximately 240 MPa was obtained. There were three regions in the bond layer: fine columnar grain region, ultrafine grain region, and recrystallized grain region. The size of the ultrafine grains was several tens of nanometers, and the microhardness was larger than that of the base metal. As electrodeposition progressed, there were insufficient additives for electrodeposition near the center of the bond layer, and recrystallization occurred owing to self-annealing.

With their excellent balance of strength, electrical conductivity and ductility, copper alloys are widely applied. However, miniaturized products must be high-strength while maintaining high electrical conductivity. Herein, we analyze whether severe plastic deformation can improve the aging property of copper alloy. We experimentally investigated the aging properties and microstructures of Cu–Cr–Zr alloys after a remarkable torsional plastic deformation (RMACREO) process. The RMACREO-processed specimen exhibited a lower aging temperature than the solution-heat-treated specimen without degrading the Vickers hardness and electrical conductivity. We suggest that the RMACREO process introduced a large amount of distortion in the microstructure and generated a sub-grain structure. The RMACREO process apparently decreased the generation activation energy of the chromium precipitates, thereby decreasing the aging temperature.

In the synthesis of Cu–Sn alloy nanoparticles, we found that the addition of Tin-Ethylhexanoate provided Cu–Sn alloy mesoparticles, which are aggregates of nanoparticles with a diameter of less than 100 nm. HRTEM observation revealed that Cu–Sn alloy constituent nanoparticles contain plane defects and domains with various crystal orientations. Also, mesoparticles have a Cu₂O layer with approximately 4 nm thickness on their surface, in which the Cu₂O (111) plane is parallel to the Cu (111) plane of the lower layer or is inclined slightly. When Cu–Sn alloy mesoparticles with a composition of Sn: 0.42 at% were used as joint materials of the Cu–Cu joint, the maximum shear stress of the joint interface was measured to be more than 11 MPa. In the case of Cu–Sn alloy mesoparticles with 1.5 at% Sn, the maximum shear stress decreased significantly, which is considered to be attributed to the formation of the Cu–Sn intermetallic compound phase. Therefore, mesoparticles with 0.42 at% Sn may be a strong candidate for a low-cost Cu–Cu joint material, which could be used as joint materials for electronic devices under high temperatures than conventional ones.

In the present study, plating films with three-dimensional structures were formed by using a plating bath mixed with Cu sulfate and Ni amidosulfate. Furthermore, the effect of the shape of plating films, which was changed by plating at different potentials, on the adhesive force with epoxy resin was investigated. The results show that when the ratio of the concentration of Ni amidosulfate to that of Cu sulfate was 150:15 (g/L) and the potential was set to −1.0 V to −2.0 V, dendritic plating films were formed. The average peak shear force of the specimen joined with epoxy resin between two Cu plates with the dendritic plating films formed at the potential of −1.5 V, was 180.1 N. The dendritic plating films had higher adhesive force than the smooth plating film through the anchoring effect.

The creep deformation mechanism of Sn–Ag–Cu alloy doped with Bi and Sb (SACBiSb) alloys is theoretically and experimentally analyzed in order to clarify the effect of solid solution additives in Sn–Ag–Cu alloys. The theoretical prediction results and test results are found to mostly agree with each other. The breakaway stress which is the stress at the transition from Class II to Class I in particular is finely reproduced in the theoretical prediction, with a stress of 25 MPa found both by the prediction and test results. In the stress range where the stress is higher than the breakaway stress, the creep strength of SACBiSb is higher than that of Sn–Ag–Cu, while it was predicted that the creep strength of Sn–Ag–Cu would be superior to that of SACBiSb in the range of stress lower than the breakaway stress. The thermal fatigue life of SACBiSb was predicted to be longer than that of Sn–Ag–Cu in the temperature profile mainly used above the breakaway stress. However, in the temperature profile mainly used in the low-stress range, a reversal of the creep strength between SACBiSb and Sn–Ag–Cu was predicted to occur and the loss of the superiority of SACBiSb in the thermal fatigue life was also predicted.

The effect of formic acid surface modification process on the bond strength of the solid-state bonded interface of tin and nickel was investigated by SEM observations of fractured surfaces and interfacial microstructures. Tin and nickel surfaces were modified by boiling in formic acid for 600 s. Diffusion bonding was performed at bonding temperatures of 393–493 K under a load of 7 MPa (bonding time for 1.8 ks). The bond strength increased with increasing bonding temperature irrespective of surface modification process. As a result of surface modification process, bonded joints were obtained at a bonding temperature 70 K lower than that required for non-modified surfaces used, and the bond strength was comparable to that of the base metal. When the surface modification process was not applied, the fracture mode was brittle. As the bond strength increased with increasing bonding temperature, the fracture mode changed to ductile. With surface modification process, this tendency was observed at a bonding temperature 70 K lower than without it. The results suggest that these changes in the fractured surface between tin and nickel were accompanied by expansion of the metal-to-metal contact area, which contributed to the increase in bond strength.

The purpose of this study is to investigate the conditions of fabrication of cellulose nano-fiber (CNF) composited Ni plating film by electroplating method and clarifying the codeposition mechanism. The electroplating was carried out on SUS304H using Watts bath with CNF suspended. The obtained films were evaluated by surface and cross-sectional observation including elemental mapping analysis, X-ray fluorescence analysis and Vickers hardness testing. The obtained film had a double layer structure parallel to the plating surface, and the layer on the substrate side contained a large amount of CNF. This layer is considered to be formed by physical trapping of CNF in the pitting corrosion and the delaminated area inside the substrate caused by the effect of chloride ion and hydrogen gas. It was also suggested that the electrostatic repulsion between CNF and the substrate was one of the factors that resulted in little CNF codeposition within the surface side layer. In addition, the Vickers hardness of the surface was improved by approximately 30% compared to electroplated Ni film without CNF.

The atomistic interactions between screw dislocations and coherent twin boundaries were investigated in face-centered metals of Al, Cu, Ni, Au, Ag, and Pd using molecular dynamics and nudged elastic band method. The interaction mechanism was affected by stable and unstable stacking fault energies predicted by various potentials. In Al, Cu, and Pd, screw dislocation would be absorbed by the twin boundary. However, transmissions were observed in Ni, Au, Ag, and Pd with another potential. It was found that both the critical interaction stress and energy barrier decreased as the reciprocal of the difference between unstable and stable stacking fault energies increased. Activation volumes and strain rate sensitivities were estimated around 14 b³ to 27 b³, where b is the Burgers vector, and 0.011 to 0.023, respectively. The results for Cu and Ni were further compared with experiments and showed good agreements.

Non-equiatomic high-entropy alloys (HEAs) for which the mixing (S_mix), configuration (S_config), and equivalent ideal (S_ideal) entropies satisfy S_mix > S_config = S_ideal were reported for Co–Cr–V–Fe–(Al, Ru, or Ni) systems. Three Co₂₀Cr₂₀Fe₂₀V₁₀X₃₀ (X = Al, Ru, or Ni) alloys (referred to as Al₃₀, Ru₃₀, and Ni₃₀ alloys) were studied here using conventional arc melting and subsequent annealing. The X-ray diffraction profiles revealed that the Al₃₀, Ru₃₀, and Ni₃₀ alloys annealed at 1600 K for 1 h exhibited B2 ordered, hcp, and fcc structures, respectively. A single structure was verified by scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy. Thermodynamic calculations of S_mix normalized by the gas constant (S_mix/R) revealed that Al₃₀, Ru₃₀, and Ni₃₀ alloys at 1600 K had S_mix/R = 0.833, 1.640, and 1.618, respectively, where the latter two alloys exceeded S_config/R = 1.557. A compositionally optimized Al-containing HEA for S_mix with a single bcc structure was computationally predicted and verified experimentally for the Al₆Co₂₇Cr₃₄Fe₁₉V₁₄ alloy (Al₆ alloy). The non-equiatomic Al₆ alloy with S_config/R = 1.480 exhibited S_mix/R of 1.703 at 1600 K, surpassing S_config/R = ln 5 = 1.609 for the exact equiatomic (EE) quinary alloy. The bcc Al₆, hcp Ru₃₀, and fcc Ni₃₀ alloys were regarded as ultra-high mixing entropy alloys (UMHEAs) according to S_mix > S_config. Structure-dependent S_mix and the mixing enthalpy of constituent binary EE alloys are useful for future UHMEAs as a subset of HEAs.

Al–Mn–Mg alloys have been considered as the most suitable basket material of transport and storage casks for spent nuclear fuels. However, the Mg content should be defined with consideration of their prospective heat histories; e.g. exponentially decreasing in temperature from about 200°C to 100°C during the fuel storage up to 60 years. In this research, precipitation behavior of β phase (Al₃Mg₂) in Al–Mn–Mg alloys was analyzed at various temperatures, focusing on the effects of the amount of Mg. Time-temperature-precipitation (TTP) diagrams for these alloys were depicted by a model which had been developed based on classical nucleation theory. The TTP diagrams were further rebuilt to “degree of supersaturation-diffusion length-precipitation (SLP) diagram”, where the degree of supersaturation (S) and diffusion length multiplied by cubic root of Mg concentration (L^*′) were involved for predicting the starting condition of precipitation. From the SLP diagram, Mg content of 1.0 mass% was found to be able to maintain sufficiently the solid solution strengthening for 60 years.

This Paper was Originally Published in Japanese in J. JILM 70 (2020) 43–50.

In this study, we systematically investigated the effects of short-time heat treatment on the mechanical properties and fatigue strength of Ti–6Al–4V alloy produced by selective laser melting (hereafter SLM material). The short-time heat treatment was composed of the following two heat treatments: the first treatment was short-time solution treatment (ST treatment) in which the SLM material was heated at 1173–1298 K for 60 s and water-quenched; the second treatment was short-time aging treatment (AG treatment) in which the ST-treated materials were reheated at 823 K for 40 s and air-cooled. Before the heat treatments, the microstructure of the SLM material was composed of the acicular α′ martensite phase and the metastable β phase. When the ST treatment was conducted at 1173 K and 1223 K lower than the β transformation temperature of Ti–6Al–4V alloy (1271 K), the α′ phase was transformed to the stable α phase during heating and the new fine α′ phase was generated in the metastable β phase by water quenching. These microstructural changes reduced the static strength but increased the ductility. When the ST treatment was carried out at 1298 K higher than the β transformation temperature, the microstructure was almost the same as that of the SLM material and the mechanical properties were not greatly altered. AG treatment of the ST-treated materials induced the precipitation of the small α phase and reduced the volume fraction of the metastable β phase. This treatment improved the static strength but reduced the ductility. The fatigue strength of the SLM material was much lower than that of the wrought material (63%) because the molding defects formed inside caused stress concentration and accelerated the initiation of fatigue cracks. However, the ST treatment at 1298 K and its combination with AG treatment effectively suppressed the initiation of fatigue cracks and markedly improved the fatigue strength to the same level as that of the wrought material. To improve the fatigue strength, the ST treatment at 1298 K was more effective than its combination with AG treatment because the volume fraction of the metastable β phase was higher and the compressive residual stress near the surface was higher.

In order to clarify the effect of deformation prior to nitriding and alloying elements on nitriding behavior in Fe–M binary alloys, pure iron and Fe–M (M = Al, Cr) binary ferritic alloys, which had been deformed by high pressure torsion (HPT) process, were plasma-nitrided at 843 K for 3.6 ks, then the microstructure and hardness of them were investigated. It was found that the effects of deformation on hardness of nitriding diffusion zone in pure iron and Fe–1Cr alloy were small. In contrast, the hardness of diffusion zone in Fe–1Al alloy was greatly increased by deformation because deformation promoted the precipitation of AlN more extensively than non-deformed specimen. In addition, deformation resulted in more homogeneous growth of the compound layer with the finer grain size of γ′ on the compound layer. This feature was observed most remarkably in Fe–1Al alloy, where needle-like γ′ compound layer was formed in the non-deformed specimens.

This Paper was Originally Published in Japanese in J. Japan Soc. Heat Treatment 60 (2020) 239–246. The captions of Fig. 5, 7 are slightly modified.

In this study, A2017-T4 aluminum alloy was plated with electroless Ni–P with different phosphorus content and rotary bending fatigue test was conducted to investigate the effect of hydrogen by plating on fatigue properties. The fatigue strength of the low-phosphorus type Ni–P plated specimen was higher than that of the untreated specimen, while that of the high-phosphorus type plated specimen was much lower. It is clear that the fatigue strength differs greatly depending on the phosphorus content in the plating film. The decrease in fatigue strength of the high phosphorus type plating specimen was attributed to hydrogen induced by plating from hydrogen analysis. Thus, despite the previous report that 2000 series aluminum alloys do not exhibit hydrogen embrittlement in slow strain rate tensile tests under wet condition, it was found that A2017-T4 aluminum alloy undergo hydrogen embrittlement when the alloy is plated with high-phosphorus type electroless Ni–P and fatigue-tested on rotary bending machine.

This Paper was Originally Published in Japanese in J. Jpn. Inst. Light. Met. 71 (2021) 450–454.

In order to assess the susceptibility to reheat cracking of high-strength austenitic stainless steels of KA-SUS304J1HTB and KA-SUS310J1TB, we established a reheat cracking simulated test method by using a stress relaxation test. A simulated HAZ (Heat Affected Zone) test specimen of KA-SUS304J1HTB fractured during stress relaxation at 650°C when the strain exceeded 16.6%. On the other hand, an as-received test specimen of KA-SUS310J1TB fractured during stress relaxation at 650°C when the strain was more than 10.3%. Subsequently, local reduction of area (RA) of the fractured simulated HAZ and as-received test specimens was measured. No appreciable local RA was observed in the vicinity of the fractured surface on both test specimens. Besides, test specimen with larger grain size was found out to be fractured in shorter time, as compared to test specimen with small grain size. These findings were in close accordance with the previously reported characteristics of the reheat cracking cases. We then calculated the stress relaxation processes on the basis of experimental results by using Norton’s creep law. The calculation results showed that simulated HAZ specimen of KA-SUS304J1HTB would not generate reheat cracking for more than 100,000 hours when the initial stress was equal to or below 300 MPa. However, reheat cracking would occur on KA-SUS310J1TB after only about 40,000 hours when the initial stress was 300 MPa. Compared to KA-SUS304J1HTB, KA-SUS310J1TB was more susceptible to reheat cracking.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 69 (2020) 105–110

Hot rolling combined with inter-pass annealing is an important method for increasing plastic deformation and refining the grain size of magnesium alloys. In this study, we statistically analyzed the influence of the number of annealing treatments and the annealing holding time (5–20 min) between hot rolling passes on the microstructure of AZ31 alloy after four rolling passes and inter-pass annealing. In addition, an exponential model was proposed to predict the functional relationship between the average grain size and the number of annealing treatments and holding time. After a single rolling pass, the average grain size increased exponentially with increasing holding time. However, increasing the annealing holding time to more than 15 min resulted in a nonhomogeneous microstructure owing to grain coarsening and secondary recrystallization. The grain refinement effect weakened with the increasing number of rolling passes, whereas the microstructural uniformity was significantly improved by multi-pass rolling deformation under the same annealing conditions. The higher amount of grain boundary energy accumulated during multi-pass rolling clearly increased the grains size of the dynamic recrystallized in the early stages with increasing holding time in the subsequent annealing processes. The average grain size was determined to be an exponential function of both the number of annealing treatments and holding time. The predicted average grain diameters were in good agreement with the measured results, thus validating the proposed method for establishing the average grain size in hot deformation processes.

The fabrication process of titanium (Ti) ingots generates a large number of Ti scraps contaminated by oxygen (O). In the past, various direct deoxidation methods from Ti for recycling of the Ti scraps have been developed, although they have not been industrialized yet. In this study, we developed a novel deoxidation process utilizing cerium (Ce), which is an inexpensive and abundant rare-earth element, as a deoxidant for Ti. Based on the thermodynamic analysis utilizing the available thermochemical data with some theoretical assumptions for cerium oxychloride (CeOCl), it is expected that the production of Ti with extremely low oxygen concentration is feasible by a deoxidation reaction of Ti through the formation of CeOCl. In this study, we demonstrated that Ti samples can be deoxidized by Ce metal in KCl fluxes to 1000 mass ppm O or less through the formation of CeOCl. Furthermore, we also demonstrated that Ti samples were deoxidized by Ce metal in CeCl₃ fluxes down to 100 mass ppm O or less under the Ce/CeOCl/CeCl₃ equilibrium. In some experiments, Ti samples without physical contact with Ce metal and CeCl₃ fluxes were also deoxidized. This result suggests the possibility of developing a new deoxidation process by supplying the deoxidant and flux through the gas phase. This new deoxidation technique using Ce metal and chloride fluxes enables Ti scraps containing a large number of oxygen impurities to be recycled.

In recent years, stainless steel has been used as a separator in polymer electrolyte fuel cells (PEFCs). An electrophoretic deposition (EPD) method has been developed as a surface treatment, in which titanium nitride (TiN) is coated on stainless steel to impart conductivity. Using the EPD method, the surface of SUS316L steel was coated with three types of TiN–styrene-butadiene rubber (SBR) with different TiN particle diameters and impurity contents to improve the contact resistance. A smaller impurity content in the TiN resulted in a lower contact resistance. Furthermore, when the TiN particle size was between 50 nm and 1.5 µm, a larger particle size provided a lower contact resistance. In the polarization curve measurements, no deterioration of the corrosion resistance was observed for TiN–SBR. At the potential of an actual PEFC environment, low current densities were maintained in both the anodic and cathodic environments. Furthermore, scanning electron microscopy and electron probe microanalysis before and after the polarization test confirmed that there was no TiN detachment in the PEFC environment. The above results suggest that TiN-coated SUS316L steel is a promising separator for PEFCs.

Polarization tests of the β-phase (Mg₁₇Al₁₂) in Mg–Al alloys were conducted using a 0.1 M NaCl solution (pH 11); E_corr of the β-phase was −1.377 V. When Ag or Zn was added to the β-phase, E_corr increased depending on the standard potential of the additive elements. However, the additive elements showed no significant difference in passive behavior during the test. This suggested that the additive elements did not affect the dissolution of the protective surface film. The same polarization test was conducted on pure Mg, and the results were compared with those of the β-phase. The anodic regions of the two polarization curves had different profiles. It was confirmed that the protective surface films formed on the pure Mg and β-phase had different properties.

Effects of anodization by using a phosphate solution and via laser irradiation on the adhesive properties of an AZ91D magnesium alloy were evaluated to produce multi-materials for the purpose of fabrication of the lightening materials. AZ91D sheets were anodized, and then post-treatment was carried out by dipping the sheets in nitric acid solution to improve the adhesive property. The anodized film suppressed the reflection of laser beam and improves laser workability. It was possible to obtain a joining strength of more than 10 MPa by drilling the AZ91D sheets treated with anodization by optimal laser irradiation. Furthermore, the anodization in phosphate solution modifies the surface of the magnesium alloy to an inactive state, thus suppressing the deterioration of adhesiveness due to oxidation that occurs in the untreated material.

This Paper was Originally Published in Japanese in J. JILM 71 (2021) 241–245.

In the present study, we solidified JIS A7204 and A6022 (7204 and 6022 in short hereafter) aluminum alloys using an electromagnetic vibration (EMV) technique as a function of vibration frequency. The solidified structures were qualitatively observed, and then average grain size was quantitatively measured using the Image-Pro Plus^® software by a centroid method. Similarly to our previous observations of other alloy systems, the average grain size versus vibration frequency in both alloys exhibits a “V-shaped” relation; it reaches minimum of approximately 50 µm at the frequency of f = 1000 Hz in 7204 alloys and around 61 µm from the frequency of f = 500 Hz to f = 750 Hz in 6022 alloys. The microstructure formation was discussed when considering the substantial difference in electrical resistivity between the primary aluminum solid solution and the remaining liquid, which resulted in an uncoupled movement between the primary mobile solid and remaining sluggish liquid. Possible influences of different solute elements on solidification structures were briefly presented.

The packing density of powder bed is one of the critical parameters affecting the quality of the final parts fabricated via powder bed fusion additive manufacturing. In this study the packing density of the first layer of the powder bed was experimentally estimated from the packing densities of recoated powder with different number of layers. It is found that the packing density of the first layer is much lower than the apparent density of powder and the macro-scale packing density increases as the number of recoated layer increases. Furthermore, recoating simulation using discrete element method (DEM) was conducted to investigate the deposition mechanism of the powder at the particulate-scale. The simulation results showed the packing density of powder bed increases as the number of recoated layer increases, similar to the experimental results. This is caused by the rearrangement of the powder in the powder bed stimulated by the supplied powder. Also, the packing density of the powder bed was not uniform in the thickness direction, and the top surface layer which affects the quality of manufactured parts had almost the same packing density as that of the first recoated layer independently of the number of recoated layers.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 68 (2021) 457–463.

Active screen plasma nitriding (ASPN) is a nitriding method that avoids the edge effects and arcing that occur during conventional direct current plasma nitriding (DCPN). Furthermore, applying voltage to a sample during ASPN allows the nitriding rate to be increased (S-DCPN). While steel is the predominant screen material, there are few reports of non-ferrous material screens. Therefore, in this study, we investigated the effect of a Ni screen on ASPN and S-DCPN. Low carbon steel S15C was treated by ASPN and S-DCPN using a Ni screen. A steel plate cold commercial (SPCC) screen was also prepared for comparison. Plasma nitriding was performed at 773 K for 240 min under an atmosphere of 75% N₂ + 25% H₂ at a gas pressure of 200 Pa. After the nitriding treatment, X-ray diffraction (XRD), glow discharge optical emission spectrometry (GD-OES), cross-sectional microstructure observation, surface microstructure observation, Vickers hardness test, and corrosion test were performed. As a result, when the Ni screen was used for S15C steel nitriding, more nitrogen atoms diffused into the sample than that when the SPCC screen was used; furthermore, nickel atoms diffused within the samples treated by both ASPN and S-DCPN using the Ni screen. ASPN-treated samples had less surface hardness and were higher corrosion resistance when prepared using the Ni screen than the SPCC screen. S-DCPN-treated samples had greater surface hardness and were less corrosion resistance when prepared using the Ni screen than the SPCC screen.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 430–438.

Ti–6Al–7Nb alloy with and without heat treatment was processed by high-pressure torsion (HPT), and subsequently its microstructures and mechanical properties were investigated. Herein, the development of the microstructure by HPT processing significantly depended on the initial microstructures even though their minimum grain size was approximately equal and less than 100 nm. The needle-like structure was fragmented into several grains by HPT processing with a small number of revolutions, and grain refinement was more easily achieved than the equiaxed structure. The Ti–6Al–7Nb alloy with a duplex microstructure comprising equiaxed and needle-like structures was obtained with an adequate balance of tensile strength (1280 MPa) and elongation to fracture (22%). The results show that the combination of the initial duplex microstructure of equiaxed α grain, needle-like structure of the α′ phase, and the resultant inhomogeneous microstructure obtained by HPT processing through a moderate number of revolutions is effective in achieving a better balance of mechanical properties in the Ti–6Al–7Nb alloy.

WC–Co ultrafine-grained cemented carbide with the combined addition of Ti(C,N) and Cr₃C₂ was studied for microstructure and mechanical properties. In particular, the strength (transverse rupture strength) of the alloy was examined in detail comparing it with other kinds of cemented carbides. The WC grain size of the alloy with the single addition of Ti(C,N) or Cr₃C₂ became finer with increasing the amount of the additive. The combined addition of Ti(C,N) and Cr₃C₂ made the WC grain size further finer and the microstructure more homogeneous. The hardness of Ti(C,N)–Cr₃C₂ added alloys increased with decreasing the average grain size of WC or the mean free path of the binder phase, but the fracture toughness decreased. The strength of Ti(C,N)–Cr₃C₂ added alloy varied depending on the amount of the additive. It was noted that the strength of 3 vol%Ti(C,N)–0.5 vol%Cr₃C₂ alloy was the best and reached 4.6 GPa on average and 5.0 GPa at the maximum. It was observed that such a high strength alloy was shattered after the transverse rupture strength test and so that it was difficult to detect the fracture origin. Based on the limiting strength that is generated in normal-grained cemented carbide, it was considered that a high strength level exceeding the limiting strength of normal-grained alloy was achieved in the ultrafine-grained cemented carbide obtained in this study. The WC–Co ultrafine-grained cemented carbide with Ti(C,N)–Cr₃C₂ was superior in strength compared to conventional cemented carbide. New applications can be expected to take advantage of these characteristics.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 67 (2020) 10–17.

This paper introduces the 11 best papers awarded in 2021 by Materials Transactions. Their brief summaries are given to overview current research areas. The best papers were carefully selected from the 6 different research areas such as (1) materials physics, (2) microstructures of materials, (3) mechanics of materials, (4) materials chemistry, (5) materials processing, and (6) engineering materials and their applications. Six out of the 11 best papers are those specially selected for young scientists whose ages are 35 or below. An important trend in the year of 2020 is that four out of the 11 best papers are concerned with high-entropy alloys, reflecting current research based on a national project.