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

MATERIALS TRANSACTIONS Vol. 62 (2021), No. 2

  • Metals and Medicine

    Takao Hanawa

    pp. 139-148

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    DOI:10.2320/matertrans.MT-M2020268

    Abstract

    The research and development of metallic biomaterials and their future prospects are overviewed. Approximately 80% of implant devices and 95% of orthopedic devices are still made of metal, and metallic materials therefore continue to play an important role in medical treatment. Current research efforts in metallic biomaterials can be summarized into the following categories: the elucidation of interfacial reactions between metals and tissues (including evaluations of safety and corrosion resistance); the development of new surface treatment techniques (including the control of surface morphology); the development of new alloys; and the development of new manufacturing processes. Interfacial reactions between metals and living tissues are discussed from the viewpoints of biocompatibility and biofunction, corrosion resistance, and calcium phosphate formation on the surface oxide film. The transitions taking place in the surface treatments for osteogenesis, soft tissue adhesion, antibacterial property, and antithrombotic property are summarized, followed by a discussion of their future prospects. In addition, the concept of a dual-functional surface is explained. A review is done of zirconium alloys that decrease magnetic resonance imaging (MRI) artifacts, Ni-free austenitic stainless steel, high-pressure torsion and sliding processing, and additive manufacturing. Finally, the future of biomaterials research is considered. This Paper was Originally Published in Japanese in Materia Japan 59 (2020) 252–259.

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  • Interband Contribution to Thermoelectric Properties of Al–Cu–Ir Quasicrystalline Approximant

    Koichi Kitahara, Kaoru Kimura

    pp. 149-154

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    DOI:10.2320/matertrans.E-M2020864

    Abstract

    Thermoelectric properties of quasicrystalline approximants have been reasonably described by the semiclassical Boltzmann transport theory under the empirical constant-diffusivity approximation. To investigate why the approximation could provide such a reasonable description, the properties of an Al–Cu–Ir cubic approximant at temperatures between approximately 400 K and 1000 K were reinvestigated on the basis of a quantum-mechanical transport theory that takes into account two effects missing in the Boltzmann theory, i.e., the interband contribution and the lifetime broadening. The present theory describes the properties as the constant-diffusivity approximation. However, 45(5)% to 66(4)% of electrical conductivity is accounted for by the interband contribution. Spectral diffusivity is less energy-dependent within the electron lifetime necessary for describing the measured properties of the approximant [2.2(3) fs to 3.8(6) fs], which is why the constant-diffusivity approximation could reasonably describe the properties even though the interband contribution is not explicitly taken into account.

  • Time-Resolved Observation of Phase Transformation in Fe–C System during Cooling via X-ray Absorption Spectroscopy

    Yasuhiro Niwa, Kei Takahashi, Kouhei Ichiyanagi, Masao Kimura

    pp. 155-160

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    DOI:10.2320/matertrans.MT-M2020301

    Abstract

    The kinetics of phase transformation in the Fe–C system during cooling are of great importance for controlling its microstructure and mechanical properties. However, real-time observation of the phase transformation has been a challenging task because of experimental difficulties. Here, we developed an analytical technique for time-resolved observation of the phase transformation of an Fe–C system during cooling via X-ray absorption spectroscopy with a time resolution of 200 µs. The technique was applied to model specimens: Fe, Fe–0.044C, and Fe–1.24C. The incubation time before phase transformation and the multiple steps of phase transformation from γ-Fe to α-Fe (+ Fe3C) could be clearly observed by the proposed technique, and the behaviors were significantly different among the specimens. Through the developed technique, change in atomic structures at a short-range scale was detected, which is complementary to X-ray diffraction that detects atomic structures only in long-range order. The developed technique can detect structure changes at early stage of phase transformations, where structures changes often begin at a short-range scale, and provide inevitable information on the kinetics.

  • A New Approximate Solution for Diffusional Growth and Dissolution of Spherical Precipitates

    Akira Seki, Kotarou Hayashi

    pp. 161-166

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    DOI:10.2320/matertrans.MT-M2020310

    Abstract

    A new approximate solution for diffusional growth and dissolution of spherical precipitates has been proposed. The exact analytical solutions of the time-dependent diffusion equation with the flux balance at the interface is often difficult to obtain. The approximate solutions are useful even when the exact solutions are obtainable. The present approximation is the modified version of the conventional linearized gradient approximations, which are tractable for the spherical precipitates growth but does not agree accurately with the exact solution. It is shown that the present model is better than the conventional approximations. The present model is also applicable to the spherical precipitates dissolution, which has no exact solution. The present model gives an ordinary differential equation for dissolution, which can be solved numerically.

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  • In Situ Synchrotron X-ray Analysis: Application of High-Pressure Sliding Process to Ti Allotropic Transformation

    Zenji Horita, Daisuke Maruno, Yukimasa Ikeda, Takahiro Masuda, Yongpeng Tang, Makoto Arita, Yuji Higo, Yoshinori Tange, Yasuo Ohishi

    pp. 167-176

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    DOI:10.2320/matertrans.MT-M2020314

    Abstract

    In this study, severe plastic deformation through high-pressure sliding (HPS) was applied for in situ high-energy X-ray diffraction analysis at SPring-8 in JASRI (Japan Synchrotron Radiation Research Institute). Allotropic transformation of pure Ti was examined in terms of temperatures, pressures and imposed strain using a miniaturized HPS facility. The true pressure applied on the sample was estimated from the peak shift. Peak broadening due to local variation of pressure was reduced using white X-rays. The phase transformation from α phase to ω phase occurred at a pressure of ∼4.5 GPa. Straining by the HPS processing was effective to promote the transformation to the ω phase and to maintain the ω phase even at ambient pressure. The reverse transformation from ω phase to α phase occurred at a temperature of ∼110°C under ambient pressure, while under higher pressure as ∼4 GPa, the ω phase remained stable even at ∼170°C covered in this study. It was suggested that the reverse transformation from the ω phase to the α phase is controlled by thermal energy.

  • Effect of Thermal History on Hot Workability of Fe–Cr Multiphase Steel during Hot Working

    Shunsuke Sasaki, Tatsuro Katsumura, Hiroki Ota

    pp. 177-184

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    DOI:10.2320/matertrans.P-M2020863

    Abstract

    In the hot working of multiphase steels, the thermal history before hot working affects the phase balance. The aim of this study is to investigate the effect of the thermal history immediately before hot working on hot workability by performing hot uniaxial tensile tests. Duplex stainless steel was chosen as the test material, as this material has a typical dual-phase structure consisting of δ ferrite and austenite, and its phase balance changes with the working temperature. The phase ratio of δ ferrite is higher at high temperatures, and the ratio of the austenite phase increases as the temperature decreases. To clarify the effect of the cooling rate immediately before hot working on hot workability, a hot tensile test was carried out with two extremely different cooling rates (0.3°C/s, 10.0°C/s) from the heating temperature of 1250°C. The hot working test temperature was in the range of 650°C to 1150°C. Hot workability was higher under the rapid cooling condition than under the slow cooling condition in the hot working temperature range of 850°C to 1150°C, but this tendency was eliminated or reversed at temperatures below 750°C. The reason for this change was clarified by investigating both the phase balance immediately before hot working, which depends on the cooling rate, and the strength of each single phase. Therefore, in order to accurately measure the hot workability of a multiphase material, it is important to consider the thermal history immediately before hot working. This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 340–345.

  • Subsurface Crack Propagation from Internal Defect in Rolling Contact Fatigue of Railway Wheel Steel

    Takanori Kato, Takashi Fujimura, Shinichiro Hiramatsu, Yuichiro Yamamoto

    pp. 185-190

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    DOI:10.2320/matertrans.Z-M2020860

    Abstract

    Rolling contact fatigue failure is one of the main fatigue damages in railway wheels caused by cyclic rolling contact with rails. This fatigue failure is caused by internal defects such as nonmetallic inclusions or voids and may occur in heavy haul freight car wheels. The subsurface crack propagation behaviors caused by the internal defects are evaluated by twin-disc-type rolling contact fatigue tests using test specimens with artificial defects. Finite element analyses involving simulated rolling contact fatigue tests are also conducted. The subsurface cracks are more likely to propagate in the test specimens with larger artificial defects. Moreover, cracks initiating from the trailing side of the defects propagate faster than those from the leading side. Shear mode equivalent stress intensity factors obtained from the finite element analyses correspond well to the results of the rolling contact fatigue tests. The test specimen models with larger defects have larger equivalent stress intensity factor ranges than the test specimen models with smaller defects in both the leading and trailing sides. The results of the finite element analyses also suggest that the crack propagations are affected by the deformation of the artificial defects, which leads to higher resistance to crack propagation in test specimens with smaller internal defects. This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 68 (2019) 904–909.

  • Fatigue Characteristics of DP780 Steel Spot Welding Joints with Different Static Fracture Modes

    Lihua Xie, Bowen Shi, Zhenkang Xiao, Jiangwei Ren, Dong Li

    pp. 191-197

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    DOI:10.2320/matertrans.MT-M2020214

    Abstract

    Two specimens made of DP780 dual phase steels were produced by resistance spot welding and the quality was evaluated by the static tension-shear test and low cycle fatigue test. The fracture characteristics, macrostructure, microstructure, microhardness distribution and fatigue morphology were studied. The results show that the nugget size and microhardness gradient in the heat affected zone affect the static load-bearing performance and the fracture mode. Meanwhile, applied load, microhardness gradient in the heat affected zone and inclined angle on faying surface caused by macro deformation have influence on the low cycle fatigue life. Compared with the specimen which has high static load-bearing capacity, samples with lower static load capacity has a higher fatigue life and plastic deformation characteristics under the same applied load of 40% of its static load-bearing capacity, which indicates that the applied load is the controlling factor in the low cycle fatigue test.

  • A New Model for Predicting the Creep-Fatigue Life under Stepped-Isothermal Fatigue Loading

    Liange He, Lan Zhou, Jianqiang Su, Bin Zhang, Wenjun Shi

    pp. 198-204

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    DOI:10.2320/matertrans.MT-M2020266

    Abstract

    This paper proposes an isothermal fatigue life prediction model to predict the fatigue life of the creep-fatigue damage interaction. The fatigue life is caused by high-frequency mechanical loads and low-frequency temperature loads, i.e. stepped-isothermal fatigue loads. The model is based on continuum damage mechanics (CDM), in which the interaction between creep and fatigue damage is considered nonlinear. To verify the proposed model, the cast aluminum alloy is subjected to fatigue tests at 200–350°C. The results show that the predicted life of the model can reach a good consistency with the experimental data.

  • Thermal Fatigue Life Prediction of BGA Solder Joints Using a Creep Constitutive Equation Incorporating Microstructural Coarsening Effect

    Kouichi Morooka, Yoshiharu Kariya

    pp. 205-212

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    DOI:10.2320/matertrans.MT-M2020313

    Abstract

    A thermal fatigue life prediction method is proposed for Sn–Ag–Cu solder joints that incorporates the effect of creep strength reduction due to microstructural coarsening of the solder during thermal cycling. The proposed method was used to predict the thermal fatigue life of solder joints in a BGA (ball grid array) semiconductor package. In the study, the thermal fatigue life was calculated through as follows. A creep constitutive equation for the solder that incorporates the strain-enhanced growth of the intermetallic compounds in the solder was used to update the constitutive equation at each given cycle and the relationship between inelastic strain energy density range and the number of cycles of the solder joint was obtained to calculate the fatigue life. As the number of thermal cycles increased, the intermetallic compounds grew, and in conjunction with this, the inelastic strain energy density range increased, causing the thermal fatigue life of the solder joint to decrease compared with that without the microstructural coarsening effect. Since the growth of intermetallic compounds under field conditions with a long dwell time is large compared with that during accelerated tests, the decrease in thermal fatigue life due to microstructural coarsening was found to be significant under field conditions with a long dwell time.

  • Temperature-Dependent Deformation Behavior of Al–Mg–Sc Alloys Fabricated by Multi-Directional Forging at Room Temperature

    Shunsuke Yamazaki, Syutaro Sawa, Chihiro Watanabe, Ryoichi Monzen, Tomoya Aoba, Hiromi Miura

    pp. 213-220

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    DOI:10.2320/matertrans.MT-M2020287

    Abstract

    Ultrafine-grained Al–Mg–Sc alloys were fabricated by multi-directional forging (MDF) with different number of forging passes of 3, 9 and 15, i.e., to cumulative strains of ΣΔε = 1.2, 3.6 and 6.0, at room temperature. The achieved average grain sizes were 950, 680 and 360 nm at 3, 9 and 15 passes, respectively. Peak-aging treatments at 473 K for 172.8 ks were adopted for a portion of specimens after MDF in order to obtain finely dispersed Al3Sc precipitates. Grain coarsening did not take place in all the specimens during the aging. The activation volume for plastic deformation was estimated from the strain-rate jump tensile tests before and after the aging. Aging-free 3- and 9-pass specimens showed positive temperature dependence of the activation volume, while that of the aging-free 15-pass one bearing the smallest grain size exhibited a negative temperature dependence. Contrary to these results, values of the activation volume in the peak-aged specimens were approximately identical regardless of grain size or deformation temperature. These results strongly suggested that, due to the precipitation of Al3Sc, the rate-controlling process of deformation was changed from interaction between forest dislocations and mobile dislocations for the aging-free 3- and 9-pass specimens to interaction between mobile dislocations and Al3Sc precipitates, or from bowing-out of dislocations from grain boundaries for the aging-free 15-pass specimen to the interaction between the mobile dislocations and precipitates. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 208–215.

  • Dislocation Structures Formed inside Dislocation Channels of Rapid-Cooled and Tensile-Deformed Aluminum Single Crystals

    Ryoma Fukuoka, Kazushige Tokuno, Masatoshi Mitsuhara, Kohei Yamakoshi, Shinnosuke Tsuchida, Junji Miyamoto, Masahiro Hagino

    pp. 221-228

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    DOI:10.2320/matertrans.MT-M2020255

    Abstract

    Dislocation structures inside the cleared dislocation channels in rapid-cooled and tensile-deformed aluminum single crystals were investigated by using transmission electron microscope (TEM). The present study especially focused on the dislocation structures at their early formation stage. In their very beginning stage, arrays of prismatic dislocation loops of the primary slip system were essentially formed elongating along [1 2 1] direction and each prismatic loop stacked to [1 0 1]. With the progress of plastic deformation, the number of the prismatic loops composing the array increased and produced tangled structures with dislocations of the primary coplanar slip system. The tangled structures may act as strong obstacles against the following primary dislocations and become a triggering factor for the creation of the cell structure.

  • Improvement of Mechanical Properties by Microstructural Evolution of Biomedical Co–Cr–W–Ni Alloys with the Addition of Mn and Si

    Kosuke Ueki, Soh Yanagihara, Kyosuke Ueda, Masaaki Nakai, Takayoshi Nakano, Takayuki Narushima

    pp. 229-238

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    DOI:10.2320/matertrans.MT-M2020300

    Abstract

    We investigated changes in the microstructure and mechanical properties of biomedical Co–20Cr–15W–10Ni alloys (mass%) containing 8 mass% Mn and 0–3 mass% Si due to hot forging, solution treatment, cold swaging, and static recrystallization. The η-phase (M6X–M12X type cubic structure, M: metallic elements, X: C and/or N, space group: Fd-3m (227)) and CoWSi type Laves phase (C14 MgZn2 type hexagonal structure, space group: P63/mmc (194)) were confirmed as precipitates in the as-cast and as-forged alloys. To the best of our knowledge, this is the first report that reveals the formation of CoWSi type Laves phase precipitates in Co–Cr–W–Ni-based alloys. The addition of Si promoted the formation of precipitates of both η-phase and CoWSi type Laves phase. The solution-treated 8Mn+(0, 1)Si-added alloys exhibited TWIP-like plastic deformation behavior with an increasing work-hardening rate during the early to middle stages of plastic deformation. This plastic deformation behavior is effective in achieving both the low yield stress and high strength required to develop a high-performance balloon-expandable stent. The 8Mn+2Si-added alloy retained the CoWSi type Laves phase even after solution treatment, such that the ductility decreased but the strength improved. Additions of Mn and Si are effective in improving the ductility and strength of the Co–Cr–W–Ni alloy, respectively.

  • Creep Fracture of Aluminum and Copper Tricrystals Having 〈110〉-Tilt Σ3, 3, 9 Grain Boundaries

    Tatsuya Okada, Hiromu Hisazawa, Akihiro Iwasaki, Katsuya Kawaguchi, Hiroki Morimoto, Kazuki Nakao, Tomoyuki Ueki, Takuro Tomita

    pp. 239-245

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    DOI:10.2320/matertrans.MT-M2020325

    Abstract

    The major objective of the present study was to investigate the effect of the constraint of grain-boundary sliding and its accommodation at triple junctions on the creep fracture of tricrystals. Tricrystals of pure aluminum and copper having 〈110〉-tilt Σ3, 3, 9 boundaries were grown by the Bridgman method. Creep tests were carried out for tensile specimens in which the Σ9 boundary made an angle of 45° with the tensile axis at temperatures above 0.80 TM, where TM stands for the melting temperature on the absolute temperature scale. In both aluminum and copper tricrystals, dominant grain-boundary sliding occurred along the Σ9 boundary. The aluminum tricrystal fractured along a plane across grains, far separated from the triple junction. In contrast, the copper tricrystal fractured via a complete separation along the Σ9 boundary followed by a tear-off of the remaining portion. This difference is accounted for by the different local deformation behaviors around the triple junctions to release the stress concentration induced by the constraint of Σ9 sliding, e.g., grain-boundary sliding along the Σ3 boundary in aluminum and crack formation along the Σ9 boundary in copper.

  • Quantitative Analysis on Influence of Structural Conditions on Thermoelectric Properties of Tilted Rod-Ni/Mg2Si Composites Based on Finite Element Simulation

    Takashi Itoh

    pp. 246-251

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    DOI:10.2320/matertrans.Y-M2020862

    Abstract

    The rod-Ni/Mg2Si composites with anisotropy in the thermoelectric properties were focused on, and the influence of the structural conditions (volume fraction and tilt angle of Ni rod) in the composites on the thermoelectric properties was investigated using a finite element simulation. The mixture laws in the thermoelectric properties were verified from the simulation results. The relations between the thermoelectric properties of the tilted rod-Ni/Mg2Si composites and the structural conditions were quantitatively clarified. Furthermore, the thermoelectric performance of the tilted composites was calculated using the clarified quantitative relations, and the optimum structural conditions bringing the maximum thermoelectric performance were predicted. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 65 (2018) 713–718.

  • Statistical Evaluation of Mechanical Properties of Thermally Sprayed Alumina Coatings by Nanoindentation Method

    Kosuke Sanami, Kentaro Shinoda, Atsushi Yumoto, Jun Akedo

    pp. 252-260

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    DOI:10.2320/matertrans.T-M2020866

    Abstract

    Additional better-quality homogeneous microstructures can be rendered available by the use of finer ceramic particles in thermal spraying, which in-turn requires more precise and advanced evaluation approaches for assessing their microstructures and properties, such as a nanoindentation method. However, it is important to examine whether the required microstructure and phase information can be accurately obtained using the nanoindentation method. In this study, the hardness and Young’s modulus were measured by the nanoindentation method and were statistically evaluated by the Weibull distribution. As a case study, alumina coatings deposited by atmospheric plasma spraying (APS) and high-velocity oxyfuel-flame spraying (HVOF) were examined. When medium-sized alpha-alumina powder was sprayed by APS, the coating consisted of alpha- and gamma-alumina, and the Weibull plot of the hardness showed a bimodal distribution. Conversely, in the case of small-sized powder sprayed by APS, the coating exhibited a gamma-phase and a unimodal distribution. When finer alpha-alumina powder was sprayed using HVOF, it consisted of alpha, gamma, and non-crystalline phases, and the Weibull plot revealed a bimodal distribution. The gamma and non-crystalline phases were considered to appear from molten states and as the alpha phase was believed to originate from the unmolten states of the particles. Therefore, the unimodal distribution was ascribed to the molten state of the particle, while the bimodal distribution to the molten and unmolten states. This Paper was Originally Published in Japanese in J. Jpn. Therm. Spray Soc. 56 (2019) 154–161. The corrigendum was also published in Japanese in J. Jpn. Therm. Spray Soc. 57 (2020) 179. The content of the corrigendum has been reflected to this paper. The caption of Fig. 5 is slightly modified.

  • In Situ Observations of the Formation of Surface Fine Crevice Structures Created by Laser Irradiation

    Jaebong Yeon, Peiyuan Ni, Masashi Nakamoto, Toshihiro Tanaka

    pp. 261-270

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    DOI:10.2320/matertrans.MT-M2020293

    Abstract

    We used laser irradiation to create surface fine crevice structures, which induced region-selective super-spread wetting. These crevice structures enabled joining of Cu–Cu. We recently reported that laser irradiation can form surface fine crevice structures, not only on Cu but also on Fe surfaces. However, the formation mechanism of such structures is not yet understood. To clarify this mechanism, we performed in situ observations of Cu and Fe surfaces during laser irradiation, with the use of a high-speed camera and laser illumination. In this way we directly observed swelling and spattering caused by ejection of molten metal from laser-irradiated regions. We conclude that the formation of the surface fine crevice structure was caused by the accumulation of ejected molten metal, that is, entanglement of protuberances and spatter.

  • Comparative Investigation of the Mechanical and Wear Properties of Multilayer Si-DLC/DLC Films

    Yukio Kobayashi, Akio Nishimoto

    pp. 271-277

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    DOI:10.2320/matertrans.MT-M2020261

    Abstract

    In this study, multilayer diamond-like carbon films 1 µm thick with different numbers of multilayer repetitions were deposited onto austenitic stainless steel SUS304 substrates using different source gases. The influence of the gas and the difference in the number of multilayer repetitions on the adhesion strength and wear resistance of the films was subsequently investigated. The samples were subjected to cross-sectional microstructure observations, elemental analysis by glow-discharge optical emission spectroscopy, nano-indentation tests, Rockwell indentation tests, friction and wear tests, and delamination tests. The nano-indentation tests showed that the films prepared using C2H2 gas were harder than those prepared using CH4 gas. In addition, irrespective of the gas used, the film hardness was improved when four layers rather than two layers were deposited. However, the hardness of the eight layers film decreased. The Rockwell indentation tests showed no improvement in adhesiveness when the gas or the number of multilayer repetitions was varied. The wear tests revealed that the friction coefficient of the films prepared using C2H2 gas was smaller than that of films prepared using CH4 gas. No difference was observed with increasing number of multilayer repetitions. The delamination tests showed that the distance until delamination of the films prepared using C2H2 gas was longer than that of the films prepared using CH4 gas. In addition, the distance until delamination was improved by changing the number of multilayer repetitions from two layers to four layers for both gases but decreased when the number of repetitions was extended to eight layers. In this study, the value of H/E (hardness/Young’s modulus) increased and various characteristics improved with increasing number of multilayer repetitions.

  • Energy Absorption Behaviors of Laser Additive Manufactured Aluminium Alloy Thin-Walled Tube Tailored by Heat Treatment

    Xu Niu, Ruixian Qin, Yunzhuo Lu, Bingzhi Chen

    pp. 278-283

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    DOI:10.2320/matertrans.MT-M2020271

    Abstract

    Thin-walled metal tubes, because of their low cost, high stiffness and strength combined with a relatively low density, have been extensively applied in transportation vehicle industry as energy absorbers. The traditional methods applied to the metal tube fabrication, however, are time-consuming and materially-inefficient processes. In the present study, taken AlSi10Mg alloy as the model material, the hexagon thin-walled tubes were prepared by Selective laser melting (SLM). The effect of post heat treatment on the energy absorption characteristics of hexagon thin-walled tubes is systematically studied. The results revealed that the subsequent heat treatment significantly improved the energy absorption properties of the hexagonal thin-walled tube, especially the solution heat treatment. Moreover, the energy absorption properties of the AlSi10Mg hexagonal tube obtained in the present work is higher than most of recently reported thin-walled AlSi10Mg tubes fabricated by SLM.

  • Synthesis of Highly Activated Magnesium by Niobium and Tantalum Gel Oxide Catalyst

    Pankaj Kumar Singh, Hiroyuki Gi, Keita Shinzato, Kiyofumi Katagiri, Hiroki Miyaoka, Takayuki Ichikawa, Yoshitsugu Kojima

    pp. 284-289

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    DOI:10.2320/matertrans.MT-M2020249

    Abstract

    As catalysts to improve the kinetics of the reaction between Mg and H2, Nb2O5, Ta2O5, and Nb2O5–Ta2O5 mixture gels with and without heat treatment were synthesized using simple sol-gel methods. Furthermore, MgH2 was ball milled to superficially disperse 1 mol% of each oxide for 2 h, which was ten times shorter than that of previous works. All the oxides show catalytic effects on using easier and simpler synthesis processes. The catalysis of Nb oxides is better than that of Ta oxides and mixtures. The as-synthesized Nb2O5 gel without heat treatment is the best catalyst and improves hydrogenation kinetics at room temperature. The Nb2O5 gel with higher catalysis is further reduced compared to the heat-treated one because the gel oxide is more unstable owing to the lesser network between Nb and O atoms due to the existence of –OH groups. By using gel oxides, highly activated Mg can be synthesized under milder conditions compared with previous cases.

  • Surface Microstructure and Its Growth Behavior by Nitrocarburizing with Lithium Added Molten Salt in Fe–0.4 mass%C Alloy

    Haruna Ishizuka, Youichi Watanabe, Masao Takeyama

    pp. 290-295

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    DOI:10.2320/matertrans.H-M2020865

    Abstract

    The surface microstructure and growth behavior of an Fe–0.4 mass%C alloy formed by lithium-added salt-bath nitrocarburizing were investigated. The Fe–0.4 mass%C alloy was prepared by arc melting and nitrocarburized by a salt-bath containing Li+, Na+, K+, CNO−, and CN− at 823 K from 0.1 h up to 10 h. A compound layer forms on the surface at the beginning of nitrocarburizing, and then an oxide layer forms on the compound layer after nitrocarburizing for 1.0 h. Afterwards, the thickness of both layers increase. A grain boundary oxide forms at the interfaces of columnar crystals in the compound layer. The oxide consists of LixFe1−xO with an NaCl-type structure, and the growth of the oxide layer is controlled by the outward diffusion of iron in the oxide layer. Meanwhile, the compound layer consists mainly of an ε-Fe2(N,C)1−y phase and a slight γ′-Fe4(N,C)1+z phase near the substrate. This Paper was Originally Published in Japanese in Jpn. Soc. Heat Treatment 59 (2019) 215–221.
  • © 2021 The Japan Institute of Metals and Materials

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