Alloy designs of light-weight high and medium entropy alloys (LW-HEA and LW-MEA, respectively) are discussed in relation to solving the problem of the empirical alloy parameter, ΔH_mix, and the difficulties of the fabrication process. The ingots of newly-designed Al–Mg–Li–Ca LW-MEAs were fabricated by the conventional casting process via crucible melting without using a vacuum furnace, and casting under air atmosphere. The ΔH_mix parameter is the average value of the mixing enthalpy of ΔH_i-j between the two components, i-j, in the multicomponent alloys, and the dispersion of ΔH_i-j cannot be evaluated. The parameter δ(ΔH_mix) was suggested for the evaluation of the dispersion of ΔH_i-j. The Al–Mg–Li–Ca LW-MEAs were designed based on the empirical alloy parameters, including δ(ΔH_mix). The alloy ingots of equiatomic AlMgLiCa, non-equiatomic Al₂MgLiCa, and AlMgLiCa_0.3 were successfully obtained by the conventional casting process. The solidification microstructure of the ingots in the Al₂MgLiCa LW-MEA was investigated, with particular focus on the position dependences of the chemical composition, constituent phases, solidification microstructure, and hardness. The present study clarified that LW-HEAs and LW-MEAs containing Al, Mg, Li, and Ca can be obtained by the conventional casting process under air atmosphere, without specific expensive casting equipment.

 

This Paper was Originally Published in Japanese in J. JFS 91 (2019) 717–729. Minor corrections in abstract, main text, figure and table captions, and references were performed with translation from Japanese to English and proof reading by native speakers. Some references written in Japanese was replaced by the related references written in English: Ref. 11) was changed from “the 24th Committee on casting of Japan Society for the Promotion of Science (JSPS), Subcommittee on casting process, the 20th meeting document, No. 22 (2018) 1–5.” to “Mater. Des. 184 (2019) 108172”. Ref. 41) was changed from “J. Soc. Mater. Sci., Japan 68 (2019) 205–211.” to “Mater. Trans. 61 (2020) 311–317”.

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In this study, the multi-pass high-pressure sliding (MP-HPS) process was applied for grain refinement of Al–3Mg–0.2Sc (mass%) rods with an upsized dimension of 16 mm in diameter. To achieve a homogeneous microstructure throughout the cross-section, the rod sample was rotated with 60° around the longitudinal axis (MP-HPS-R) for three times. A microstructure with an average grain size of 280 nm was developed around the center of the cross-section through the MP-HPS-R process. Superplasticity with total elongations of more than 400% was achieved in the 9 mm diameter range on the cross-section of the MP-HPS-R-processed rod.

 

This Paper was Originally Published in Japanese in J. JILM 70 (2020) 63–65.

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In regard to spot welding of manufactured products like automotive parts, it is common practice to join three steel sheets having different grades and thicknesses and subjected to different kinds of surface processing. In such a case, it is more difficult to set the welding conditions than in the case of joining two sheets of the same grade and thickness. The present study aimed to determine by investigation the optimum welding conditions for spot-welded joints consisting of three stacked steel sheets of the sort supposed to be applied in fabricating actual auto components. Furthermore, the effect of zinc plating (i.e., hot-dip galvanizing) on the outer sheet on spot-welding characteristics—which is becoming a serious manufacturing problem—was investigated. According to the results of these investigations, as for a three-layer spot weld, the fused part of the welded joint (i.e., “nugget”) is distorted to the high-tensile side. This distortion causes a difference in the resistance of the sheets, and the nugget is originated on the high-tensile side of the joint (which has higher resistance) and grows from that point of origin. Although two fracture modes are possible, namely, fracture in the base material or fracture in the weld nugget, fracture diameter and tensile shear strength have a proportional relationship in both cases regardless of the welding conditions. In the case that a zinc-plated (i.e., hot-dip zinc galvanized) steel sheet is used in the sheet stack, tensile shear strength falls and becomes more variable at low welding current. Accordingly, it is necessary to control the welding conditions so as to reduce that tensile-shear-strength variability.

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In our work, MoS₂/graphene/MOF hybrid materials were synthesized via a two-step process consisting of the plasma-assisted electrochemical preparation of MoS₂/graphene and the wet formation of Cu-based metal-organic framework (MOF). The hybrid materials were characterized with Field Emission Scanning Electron Microscopy, High-Resolution Transmission Electron Microscopy, X-ray Diffraction and Raman Spectroscopy. Furthermore, some initial results on their electrochemical properties for hydrogen evolution and oxygen evolution reactions were also presented, showing the potential to be a bi-functional catalyst.

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DNA has proven to be a promising material in fabrication and construction of complex structures with precise controlled nanoscale features. Most of the systems involving DNA are functioned in an aqueous solution where DNA molecules are strongly negatively charged. Therefore, understanding electrostatics of DNA system is essential for better understanding and design of DNA as a biomaterial and as a biological structure. In this work, the mean-field Poisson-Boltzmann equation for the distribution of mobile ions in a two-dimensional hexagonal lattice of DNA cylinders is solved numerically using finite element method. The weak formulation of the Poisson-Boltzmann equation is derived. The equation is then solved numerically using FreeFem++ scripting language. Our results show that the excess counterions of DNA dominate over the bulk ion concentration for the physiological salt concentration considered, and they condense on the DNA surface leading to very high charge density. The results also demonstrate the strong influence of the entropic confinement of the ions when the distance between neighboring DNA is smaller than 10 nm. This effect cannot be ignored in this case, and should be taken into account in any electrostatic investigation of DNA system.

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We propose a new algorithm for numerical evaluation of x-ray absorption spectra based on the Green's function formalism in this paper. The method stands on the reduced-shifted conjugate-gradient (RSCG) method, which enables us to obtain the Green's functions with different energy-shifts simultaneously. The seed switching technique is combined into the RSCG method to improve the numerical accuracy and robustness of the algorithm, which we refer to the RSCG-SS method. The RSCG-SS method is applied for calculations of x-ray absorption spectra at the L_2,3-edges of transition metals in simple oxides as a benchmark. The theoretical spectra obtained by the new algorithms are identical to those obtained by the full-diagonalization method following the Fermi's golden rule. The RSCG-SS method can also be applied for the calculation of spectra with a sizable Hamiltonian matrix (∼300,000) with reasonable computational costs.

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In order to investigate the effectiveness of ultrasonic shot peening treatment (USP) as repairing method for SCS6 material with surficial fatigue crack for hydraulic turbine runner, plane bending fatigue tests were carried out for USP treated SCS6 containing a surface fatigue crack with 1 mm in length and the fatigue crack propagation after USP was observed by a plastic replica method. As a result, the fatigue crack propagation life of SCS6 containing a surface fatigue crack was dramatically improved by USP treatment. Furthermore, the initial effective stress intensity factor ranges were calculated in the USP treated and untreated SCS6 containing a surface fatigue crack, respectively. According to the calculation, it was clear that the surficial fatigue crack could be harmless under the condition that the calculated initial effective stress intensity factor range considering the stress opening a fatigue crack, which was acquired by the unloading elastic compliance method, was less than the threshold of effective stress intensity factor range. Therefore, USP treatment is effective for repairing method of SCS6 containing surficial fatigue crack for hydraulic turbine runner.

 

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 68 (2019) 897–903.

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Al–1.5 mass%Mn was chosen as the base alloy, and 1.0 and 3.3Si were added to the base alloys, keeping the same values in the ΔMk of s-orbital energy level as those of Al–1.5Mn–0.8 and 2.4Mg alloys with superior tensile properties for as-cast applications. The Si addition or increment in the base alloy showed strengthened tensile behavior of the 0.2% proof stress (σ_0.2) of 67 MPa and ultimate tensile strength (σ_UTS) of 160 MPa, although there was reduced in fracture strain (ε_f) to 9%. The increase and decrease in flow stress and strain, respectively, resulted from the increment in degree of solid solution strengthening by the increase of ΔMk of the alloys. There was a good linear relationship between the nanoindentation hardness or rate of elastic deformation work in the α-Al phase and σ_0.2 of the alloys. The dislocation density of Al–1.5Mn–xSi alloys increased linearly as the ΔMk-magnitude increased, compared with that of the base alloy. The behavior in the flow stress variation qualitatively agreed with that of dislocation density. There was a linear relationship between the lattice constant and Mk_α in Al–1.5Mn–Si/Mg alloys. As the Mk_α changed, the σ_0.2 of the alloys also increased and its increment rate was similar in both Si and Mg addition alloys. It may be considered that the trend of change in the lattice constant, σ_0.2, work hardening amount and dislocation density was predominantly consistent with that in the Mk_α or ΔMk_α showing the indication of solid solution hardening level of the α-Al phase, and the effect of difference of third elements such as Si and Mg on their mechanical properties could be ignored in tensile examination procedures in this study.

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Die-casting is widely applied to the production of automotive components because of its high productivity for manufacturing complex-shaped castings. However, since molten metal is injected into a cavity at high speed and it solidified rapidly under high pressure in this process, many casting defects are liable to occur. Although the most frequent internal defect is gas entrapment, shrinkage porosity also forms in the thick-wall portions of die castings. In order to avoid shrinkage porosity, there is a need to feed adequate molten metal to compensate the shrinkage volume during solidification. Therefore, it is desirable to better understand the feeding behaviors of molten metal under high pressure and rapid cooling conditions in the die casting process. In this study, the permeabilities of Al–Si alloys during the solidification process under die-casting conditions were determined by measuring the pressure transmission of the molten metal from the plunger to the mold cavity so as to obtain the feeding resistance coefficients. The formation of shrinkage porosities in die-castings was proven to be predictable by numerical simulation using the obtained feeding resistance coefficients.

 

This Paper was Originally Published in Japanese in J. JFS 91 (2019) 529–533.

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The effects of ultrasonic nanocrystal surface modification (UNSM) treatments on the microstructure, mechanical, and corrosion behavior of LZ91 Magnesium–Lithium (Mg–Li) alloy were systematically investigated. The results show that UNSM treatments have greatly positive effects on the mechanical and corrosion behavior improvements. As compared to the bare sample (BS), the near-surface grains were refined and the surface roughness (R_a) of the sample was reduced from 0.514 µm to 0.082 µm; and the maximum hardness and tensile yield strength remarkable increased by 70% and 65.4% after UNSM treatments, respectively. In addition, the corrosion behavior of LZ91 Mg–Li alloy was also improved after UNSM treatments. The mechanical and corrosion behavior improvements can be attributed to the introduced thick hardening layer with low roughness and smaller grains by UNSM treatments.

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As an emerging technology, additive manufacturing (also known as 3D printing) is developing rapidly in various fields, which is regarded as one of the important symbols of the third industrial revolution. Here, we introduce its development status, materials and application fields, then analyze its development trends and prospects, which can provide some predictable analysis of the future development direction of additive manufacturing.

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It is well known that severe plastic deformation (SPD) produces ultrafine-grained structures in bulk metallic materials. The SPD process becomes more versatile when it is performed under high pressure as high-pressure torsion (HPT) and high-pressure sliding (HPS). Not only the grain size is more refined but also the process is applicable to hard-to-deform materials such as intermetallics, semiconductors and ceramics, leading to enhancement of functional properties as well as structural properties. The major drawback is that the sample size is small so that the applicability is limited to a laboratory scale and it is an important subject to increase the sample dimensions. This paper presents an overview describing efforts devoted thus far to deal with this upscaling issue.

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The mold filling simulation in the casting CAE is used to investigate the filling pattern of molten metal and to predict the casting defect. However, the prediction method of defects caused by unsuitable flow has not been established because the verification of analysis accuracy is insufficient. Especially, the air entrapment during mold filling is very important problem to obtain the sound casting. In this study, the direct observation of mold filling behavior using a water model equipment is carried out. The movement of free surface and the volume change of water are measured from observed filling behavior. The air entrapment is analyzed by quantification method using image processing. It is clear the location of thickness direction and entrainment timing of air varied with the experimental conditions. Further, the mold filling simulation is done using the casting CAE software TopCAST, which has the two-phase flow analysis MARS method. As a result of the analysis, the mold filling behavior of water and the volume of air entrapped almost agreed with experimental result. Although the calculated results have almost the same tendency with experiment, it has some unsuitable results. Therefore, it is necessary to make simulation closer to the real phenomenon by the validation of calculation conditions, and by introducing new algorithm of air movement.

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In this study, we propose a simple approach to accelerate crack initiation in intergranular stress corrosion cracking (IGSCC) of Alloy 600 in a simulated pressurized water reactor (PWR) primary water environment. In addition, we perform three-dimensional crystallographic characterization of the crack initiation by electron backscatter diffraction (EBSD). Initiation of IGSCC of the alloy in the PWR primary environment was realized for a flat tensile specimen in a considerably short time through a slow strain rate test (SSRT). The accelerated initiation was attributed to the asperity of the alloy surface induced by mechano-chemical polishing with colloidal silica suspension before the SSRT was conducted. Following the SSRT, the specimen surfaces were covered with thick oxide films, which prevented EBSD measurements. The sputtering of thick oxide films enabled us to characterize cracks with EBSD, thus yielding important information on IGSCC. Finally, an approach for three-dimensional characterization of crack initiation is discussed.

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In order to significantly enhance the performance of graphene in metal matrix composites, and avoid the interface reaction via the common method, the powder semi-solid processing (PSSP) technique was successfully used to fabricate a graphene nanoplatelet (GNP)-reinforced Al2024 composite. The blended powders of Al2024 and GNPs were cold-compacted after ball milling and then hot compressed at a semi-solid temperature, and no further hardening was performed. The GNPs were observed to change during the ball milling process, and a uniform dispersion of GNPs in the aluminum (Al) matrix composite was achieved. The microstructure and mechanical properties of the composites were evaluated, and fracture surfaces of the composites were analyzed to investigate changes in performance. Results showed that GNPs have effective interface with the Al matrix, and that Al₄C₃ was absent. GNP/Al2024 composites showed significant improvement in strength and favorable ductility. Additionally, the supreme tensile strength of the GNP/Al2024 composite could increase to approximately 524 MPa with 1.0 wt% of GNPs, which is about 41.6% higher than that of the Al matrix composite. The effect of GNPs on the mechanical properties of the composites was discussed.

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Face-centered cubic Co-free Cu-containing solid solution concentrated alloys, Cu, CuNi, CuNiFe, Cu_0.3NiFeCr, Al_0.4CuFeCrNi₂ were prepared, and their microstructure, hardness, and tensile strengths were investigated in order to develop a new high entropy alloy with a high irradiation resistance, which is applicable for nuclear reactor components. All the as-cast alloys were identified as single-phase FCC alloys by X-ray diffraction analysis. While, the SEM observation indicated a new Cr-rich phase with Cu-rich phase in the annealed Cu_0.3NiFeCr alloy, which is probably due to low solubility of Cr and Cu in the alloy. After annealing at 1076°C for 120 hours, Cu_0.3NiFeCr alloy became a single-phase FCC. Mechanical property examinations indicated the highest Vickers hardness, the highest Tensile strength and the smallest elongation in the Al_0.4CuFeCrNi₂. The results indicate that the Al_0.4CuFeCrNi₂ alloy would have the potential to be a Co-free high-entropy alloy applicable to nuclear reactor components. In order to improve the elongation of Al_0.4CuFeCrNi₂, the detailed analysis of fracture surface and the optimization of annealing condition would be needed.

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Nitrogenation of Sm₂Fe₁₇ powder was performed under a zero field and a magnetic field of 5 T at 623, 673 and 743 K to clarify the magnetic field effect on nitrogenation. Applying a magnetic field of 5 T induced nitrogenation compared with zero-field nitrogenation, and almost fully nitride Sm₂Fe₁₇N_2.9 was obtained at 743 K. Mössbauer spectroscopy results suggested that a 5-T magnetic field promoted the phase transformation to the fully-nitride Sm₂Fe₁₇N₃ phase. The magnetic field effect was discussed based on the magnetic energy gain and magnetic properties of host Sm₂Fe₁₇ and fully nitride Sm₂Fe₁₇N₃.

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The spin reorientation and Curie temperature of Mn_2−xFe_xSb_1−ySn_y (0 ≤ x, y ≤ 0.15) compound were investigated by magnetization measurements. Spin reorientation temperature increased from 255 K at x = 0 to 383 K at x = 0.15, whereas it almost unchanged by Sn substitution. Curie temperature decreased down to 518 K with both Fe and Sn substitution. Substitution of Fe stabilized the ferrimagnetic state with magnetic moment lying in c-plane. Substitution of Sn induced the antiferromagnetic phase at low temperature. It was found that the magnetic hysteresis derived from quasi first-order magnetic phase transition exhibited at x ≥ 0.10 or T ≥ 300 K.

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The low-dimensional III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) layered structures. We have performed the first-principles calculations on the structural, electronic, and mechanical properties of few-layer gallium nitride (GaN) nanosheet, formed from various bulk phases and stacking patterns, to investigate the effects of structural modification and sheet thickness on their structural, electronic, and mechanical properties. We observed that with the thickness increases, few-layer GaN nanosheets have suffered from the size-induced transition from indirect semiconductor to metallic as well as from the graphitic – planar honeycomb to the wurtzite buckled 2D form. Optimized geometries, binding energy, phonon spectra, electronic band structure, and elastic tensor calculation has ensured the dynamical and mechanical stability of the sheet. Our study indicates that there are two competing mechanisms that govern the polarity compensation with the sheet’s net dipole served as an important parameter for determining the sheet’s stable formation.

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(La_0.7Sr_0.3Mn_0.98Co_0.02O₃)_1−x(BaTiO₃)_x (x = 0.1; 0.2; 0.3) mixed perovskite were prepared by the solid state reaction method. It is shown that this compound exposes the multiferroic (magnetic and ferroelectric) behavior comparing with magnetic pristine La_0.7Sr_0.3Mn_0.98Co_0.02O₃. The increase of BaTiO₃ fraction from x = 0.1 to 0.3 leads to weakening (enhancing) of the ferromagnetic (ferroelectric) order. The metal-insulator transition (MIT) in the ferromagnetic La_0.7Sr_0.3Mn_0.98Co_0.02O₃ (x = 0) and the multiferroic x = 0.1, 0.2 samples were registered with reduction of the MIT temperature T_MI from 380 K of the x = 0 to 117 K of the x = 0.2 samples. Temperature dependence of resistivity and MIT of these samples are well described by the mixed conducting carrier model, which has crossover between the low temperature spin-scattering electron and the hopping small polarons conductions at T_MI.

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We have studied the magnetoresistance of an enhanced-biased spin valve device under high pressure. The magnetoresistance decreases by 0.0014 up to 2 GPa with increasing pressure, which is inferred to be due to slight deviation from an antiparallel-spin configuration of the free and pinned layers. In the pressure range between 2 and 2.75 GPa, the exchange bias field generated in the pinned layer decreases and the coercivity of the free layers clearly increases by ∼5 Oe, which is likely to be related to less hydrostatic pressure.

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Based on the Full potential linearized augmented plane wave (FLAPW) method, the electronic properties of incommensurate anti-ferromagnetic Sr₄V₂O₆Fe₂As₂ with wave vector q = (0, 0, 0.306) have been investigated. Comparing total energy between those of non-magnetic and incommensurate anti-ferromagnetic states (i-AF) we suggest that the i-AF is the stable configuration in this superconductor. The density of states data exhibit strong spin-polarized effect on the V site and possible hybridization between V-3d and Fe-3d orbitals. We found as many as six bands crossing the Fermi level, indicating strong inter-band scattering. The analysis of Fermi surfaces reveals a multi-sheet character, which is compiled of several hole-type cylinders around the center and electron-type sheets at the corners of the Brillouin zone.

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Thermal barrier coatings (TBCs) are necessary to protect nickel-based alloy blades of gas turbine against oxidation and thermal fatigue in high-temperature operating conditions. Ceramic materials, which are very good natural thermal insulator, attract the most interests from engineers and scientists. However, the brittleness of ceramics is a major obstacle for utilizing them as the TBCs, which are also required very good damage tolerance against physical impacts. The cracks appear on the blade surfaces during its operation can lead to the severe failure. In this research, a composite of Y₂Ti₂O₇ and Ni was developed as a self-crack healing material to overcome this problem. The crack-healing behavior is investigated by using Vickers indenter to create cracks on the composite surface intentionally, followed by annealing in an oxidizing environment. It is found that the main crack-healing mechanism is the filling of NiO, which was formed from the oxidation of the Ni fillers, into the cracks. Complete heal of cracks is achieved with 10 vol% Ni filler, which is confirmed by X-ray diffraction and scanning electron microscopy. Thermal conductivity and Weibull distribution for the strength of the composite were also investigated to find the appropriate volume fraction of Ni nanoparticles in this self-healing material.

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In this research, short bamboo fiber (BF) has been used as filler in the fabrication of the composite based on polyamide 11 (PA11) by melt-mixing method. In order to improve the compatibility between BF and PA11, maleic anhydride grafted ethylene vinyl acetate copolymer (EVAgMA) was introduced into the composite preparation process. The effect of EVAgMA contents on mechanical, thermal or weathering resistant properties of the composites was investigated. In the presence of the EVAgMA, the tensile strength, impact strength and flexural modulus of composites showed the improvement in comparison to without compatibilizer. Differential scanning calorimetry (DSC) analysis indicated that the melting temperature of composites did not depend on the EVAgMA contents, while its fusion heat (ΔHm) decreased with increasing EVAgMA contents. Dynamic mechanical thermal analysis (DMTA) evaluated the role of EVAgMA on the enhancement of composites’s storage modulus (G′) in the glassy plateau and the interaction between BF and PA11 matrix. After 840 hours of accelerated weathering test, the composites using EVAgMA were decomposed faster than that of neat PA11 and PA11/BF composite. This result was also agreed with the observation of scanning electronic microscopy (SEM) images and the change of carbonyl index (CI). However, the retention of tensile properties of PA11/EVAgMA/BF composites is higher than that of PA11/BF composite.

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First-principles calculations were performed to investigate the effect of Fe substitution on the structural transformation of (Mn,Fe)CoGe and Mn(Co,Fe)Ge. The activation energy (barrier) between orthorhombic and hexagonal structures was estimated from the total energy of each of several virtual structures between them.

Fe substitution reduces the activation energy, and movements of both of Co and Mn are closely related to the reduction. Moreover, the calculation result for the Fe substitution at the sites of both Mn and Co indicates that Fe atoms randomly occupy Mn and Co sites.

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Screening of surfaces that spontaneously reconstruct is important when investigating realistic surfaces. Evaluating the surface energy after macroscopic reconstruction to form pairs of facets is a very fast procedure to identify surfaces that spontaneously reconstruct. This paper discusses a method to identify orientations of pairs of facets that can form on an arbitrary crystal orientation, which can then be used to derive the surface energy after facet reconstruction. Another use of the algorithm is to find possible orientations of terrace surfaces when the vicinal surface orientation is known for a surface with step edges. The algorithm is expected to further accelerate high-throughput calculations of surface properties.

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Pressure effects on magnetic susceptibility χ and energy gap E_g⁰ of narrow-gap semiconductor FeSb₂ were investigated for the temperature range of 50–300 K and pressures up to 13 kbar. The estimated E_g⁰ for ambient pressure, E_g0(0), was 29 meV. By application of pressure, χ was suppressed, and E_g0(P) was estimated to be 52 meV for 13 kbar. The fourth-order expansion coefficient γ of the free energy in magnetization was positive and enhanced by applying pressures.

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The magnetization measurements were carried out for polycrystalline Au₄V in magnetic fields up to 130 kOe. The spontaneous magnetic moment p_s was determined to be 0.255 μ_B/V at 4 K by high field magnetization. The magnetization process was analyzed on the basis of Takahashi’s spin fluctuation theory for weak itinerant electron ferromagnets. The characteristic parameters of the spin fluctuation were estimated to be T_A = 1.3 × 10³ K and T₀ = 2.3 × 10³ K using the data for 110–130 kOe.

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Announcement Concerning Article Retraction
The following paper has been withdrawn from the database of Mater. Trans., because a description based on a misinterpretation of the experimental results was found by the authors in advance of publication after acceptance.
Mater.Trans. 52(2011) Advance view.
Improvement in Fatigue Strength of Biomedical β-Type Ti-Nb-Ta-Zr Alloy while Maintaining Low Young’s Modulus through Optimizing ω-Phase Precipitation

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