Mg-Li based alloys, distinguished by their low density among structural metallic materials, emerge as pivotal candidates in ushering in the era of next-generation lightweight metals. Their notable drawbacks encompass poor room-temperature strength and creep resistance, prompting diverse efforts to enhance these aspects through microstructure control techniques, including heat treatment. The amelioration of mechanical properties in Mg-Li based alloys holds significant promise for advancing various industries, spanning aerospace, automotive, and biomaterials sectors. This review article provides an overview of endeavors aimed at improving the mechanical properties of Mg-Li based alloys, with a specific focus on alloying, heat treatment, and severe plastic deformation as strategies for microstructure control.

This Paper was Originally Published in Japanese in J. JILM 73 (2023) 297–306. The title was changed due to the addition of “Review -”.

Light metals and alloys based on magnesium, aluminum, and titanium are of significance in daily life and industrial applications due to their low density and superior mechanical and functional properties. The formation of nanostructures and ultrafine grains can further improve the properties of these materials. High-pressure torsion (HPT), as a severe plastic deformation (SPD) method, is one of the most effective processes for nanostructuring these materials. Various modifications of HPT such as conventional HPT with discs, HPT with rings, and continuous HPT with strips and wires are currently applied to light metals and their alloys, composites, intermetallics, and metallic glasses. The HPT processing of these materials is effective for grain refinement, hardening through the Hall–Petch mechanism, lattice defect generation, phase transformations, and solid-state reactions through fast diffusion with reasonable time/thermal stability. This article after discussing these fundamental issues, reviews some mechanical and functional properties of nanostructured lightweight materials such as tensile, compression, and bending properties, superplasticity including room-temperature superplasticity, wear resistance, electrical conductivity, superconductivity, biocompatibility, hydrogen production, and hydrogen storage.

Nanoindentation-induced deformation in Zr-based bulk metallic glasses in distinct structural states was studied over a broad temperature range, both below and above the glass transition temperature. These findings emphasize the occurrence of a predominant deformation mechanism, identified as a percolation or diffusion process, triggered by exceeding a chemical and topological short-range order-insensitive energy barrier.

The mechanism of curving that occurs during the quenching process of Japanese swords has not been clearly explained. Experiments on this phenomenon were conducted using Japanese sword type specimens made of the same steel and processes as Japanese swords, and model Japanese sword type specimens made of carbon steel (S55C) and austenitic stainless steel (SUS304) by machining. Applying the simulated strains-based approach to heat treatment simulation results for these experiments found that positive plastic strain and transformation strain on the cutting-edge side are main contributors to curving generation in Japanese sword.

This Paper was Originally Published in Japanese in NETSUSHORI 63 (2023) 76–82.

The nanoindentation test is a widely adopted technique for characterizing the mechanical properties of materials. In this study, a dislocation density-based and a phenomenological crystal plasticity hardening model are employed to investigate the evolution of plastic anisotropy and pile-up of a single-crystal aluminum specimen with varying crystallographic orientations during nano-indentation. Utilizing crystal plasticity finite element (CPFE) simulations, we delve into the influence of crystal orientations on key factors such as depth-load curves, stress distributions, shear strains across different slip systems, and dislocation density evolution. Our analysis highlights the plasticity anisotropy inherent in the material, elucidated through the evolving shear strain exhibited by activated slip systems. Furthermore, we gain insights into the pile-up phenomenon by examining the evolution of shear strains within slip systems and the associated dislocation density, employing various modeling approaches. The height of pile-up evolution is determined by the localized cumulative shear strains and evolution of dislocation density.

Surface relief formation processes of the high cyclic-loaded coarse-grained aluminum polycrystals with point defect clusters were investigated. Until the loading of 1 × 10⁵ cycles with the stress ratio of −1 and the maximum stress of 8.0 MPa, the coarse ribbon-like primary persistent slip markings (PSMs) consisting of extrusions and intrusions had been formed, and the average extrusion height of the PSMs had reached 2.0 µm. This value was much higher than that of the ordinary aluminum single crystal. The high mobile dislocation density accompanied by the dislocation channeling effect inside the persistent slip bands (PSBs) were considered to produce the high extrusions. Until the loadings to 2.4 × 10⁵ cycles with the stress ratio of −1 and the maximum stress of 8.0 MPa, activities of the primary PSBs had been weakened or terminated, and instead, the secondary slips had been activated and deformed the shapes of the preexisting primary PSMs. And the deep brittle-like cracks along the grain boundaries (GBs) were observed. The accumulation of the dislocations and the vacancies into the GBs were considered to be the trigger for the energy reduction of the GBs as the interfaces and the brittle-like cracks formation.

In this study, experimental investigations were conducted to create fatigue limit diagrams for NSSC180 ferritic stainless steel, which is used for automobile exhaust system parts, subjected to excess deformation, in order to clarify the relationship between the mean stress and fatigue limit. The fatigue tests were conducted under a fixed maximum stress, which allowed a fatigue design diagram to be obtained. The results indicated that the fatigue limit curve has a region where the effect of increasing the mean stress on the decrease in the fatigue limit is more moderate than that predicted by a modified Goodman line. Furthermore, it was found that a fatigue design based on static crack initiation is more appropriate than the modified Goodman line for higher values of mean stress.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 72 (2023) 535–541.

Intermetallic catalysts were investigated for ammonia decomposition, a key technology to extract hydrogen from an ammonia source as a hydrogen carrier. Since Mn, Cr, and V were active in pure 3d-transition metal catalysts, their IMCs with p-block metals were selected. The pure Mn, Cr, and V were nitrided during the reaction, while their IMCs resisted the nitridation. In the Cr- and V-based IMC catalysts, a larger composition of Cr and V resulted in a larger conversion. The Cr₃X and V₃X catalysts exhibited a high activity with any X elements. These results indicate that in IMC catalysts for ammonia decomposition, the activity is dominated by transition metals, and the resistivity against nitridation is improved by p-block metals.

In the hydrometallurgical process used for the recycling of platinum group metals (PGMs), a residue containing Cr₂O₃ and PGMs is generated. In this study, a pyrometallurgical process was applied, in which PGMs from the residue generated in the hydrometallurgical processes were concentrated in a molten Cu phase as a collector metal, and Cr₂O₃ was separated into a slag phase with SiO₂ and CaO as the flux. To reduce the loss of PGMs into the slag, the dissolution of PGMs into the slag must be reduced. Therefore, the distribution ratio of Rh, as a representative PGM, between the liquid SiO₂–CaO–Al₂O₃–CrO_x or the liquid SiO₂–CaO–CrO_x slag and molten Cu were measured at 1773 K under an oxygen partial pressure of p_O2 = 10^-10. The experimental results revealed that the distribution of Rh in the slag increased with increasing CrO_x concentration. At a constant Cr₂O₃ concentration in the slag, the solubility of Rh increased with increasing slag basicity, which is defined as B = (mass%CaO)/(mass%SiO₂). Furthermore, compared with the distributions of Rh and Pt between the slag system and molten Cu, Rh was more easily lost to the slag, and the dependence of Rh on basicity was greater than that of Pt.

Intermetallic compounds can be novel catalysts due to their unique structures consisting of multiple elements that occupy specific atomic sites. Heusler alloys (X₂YZ) are a ternary intermetallic group consisting of various sets of X, Y, and Z. This group is useful in investigating the catalytic roles of individual elements under the same crystal structure. In this study, we investigated the catalytic characteristics of 3d transition metals and the group 13,14 elements in Heusler alloys for hydrogenation of propyne (C₃H₄). A larger number of valence electrons for 3d transition metals seemed to result in higher activity for the hydrogenation reaction, as well as pure metal catalysts, the activity hierarchy of which was Ni > Co > Fe. For the group 13,14 elements, the alloys with Al and Si were slightly active, whereas the ones with Ga and Ge were active, in which the Ge alloys were highly selective for producing propylene (C₃H₆). All the Sn-containing alloys significantly caused side reactions producing C₄ and C₆ species. This indicates that Sn possesses the ability to crack and couple carbon chains.

Improvement in the erosion resistance of permanent molds to aluminum alloy melt is required. Since chemically and mechanically stable layer containing aluminum oxides can be formed on the cast irons by aluminum addition and heat treatment, the layer would improve the erosion resistance of the cast iron in the running melt. In this study, cast irons with different aluminum contents were fabricated, then they were heat-treated to form the oxide layer on the surface. The optimum heat treatment conditions to form the stable layer and the erosion behavior of the cast irons were clarified. Heat treatment in air was found to result in the formation of a layer that consists of the oxides, such as Fe₂O₃, Fe₃O₄ and Al₂O₃ on the cast irons. Based on the castability of the cast irons and morphology of the layer, it was concluded that the addition of 3% aluminum in the cast iron and heat treatment at 1173 K for 10 hours was the most suitable treatment. The heat treatment drastically improved the erosion resistance of the cast iron. The addition of magnesium to the melt temporarily decreased the time to erosion, but increased the time to erosion again when more than 0.75% magnesium.

This Paper was Originally Published in Japanese in J. JFS 94 (2022) 11–17. A typographical error is modified in Sec. 2.

The applicability of a cast-coating process for improving the oxidation resistance of cast Ni-based superalloys was evaluated. Specifically, metallic plates of Pr, Ir, and Re expected to improve oxidation resistance when they are enriched on the cast alloys were placed in a mold and cast coating using Ni-10at%Al alloy was performed in order to investigate the formation of the Pt, Ir, or Re-enriched layer on the casting surface. Then the microstructure of the Ni-based alloy/specimen interface was observed. To analyze the concentration profile in the interdiffusion region, solidification and diffusion simulations were performed. It was found that Pt easily dissolves into the molten Ni-based alloy, and Re cannot expected to modify cast metal surfaces due to its low solubility into the Ni-10at%Al alloy. On the other hand, Ir forms smooth interdiffusion layer, and numerical calculations predicted that Ir can maintain the modification ability even in a process time of 1 hour, which is equivalent to the casting time of Ni-based turbine blades.

This Paper was Originally Published in Japanese in J. JFS 94 (2022) 673–683. Some spelling errors were modified.

When low-carbon steel is embedded in graphite powder containing iron powder and subsequently heat-treated at 1273 K, carbon is diffused into the steel and a pearlite area near the steel surface is increased as compared with that before heating. Such a carburizing method was named “iron-powder (IP) carburizing”. It can be carried out in both an air atmosphere and a nitrogen flow. In this work, IP carburizing method using a mixture of iron, graphite and alumina powders was applied to pure iron in a nitrogen flow, and the carburization ability was compared with conventional solid carburizing method using a mixture of activated carbon and sodium carbonate. The amount of carbon diffused into the specimen by IP carburizing was smaller than that by conventional method. In IP carburizing, it is thought that the direct diffusion of carbon from the powder mixture was dominant, rather than the carbon diffusion through a gas phase like carbon monoxide and carbon dioxide. On the other hand, instead of graphite, activated carbon was used as a carbon source of IP carburizing and there was little effect on the carburized structure. This was also one of the features of IP carburizing, since the microstructure obtained by conventional solid carburizing was affected by the replacement of activated carbon by graphite.

A two-step aging process was investigated in which the first step aging was carried out in zinc acetate aqueous solution and the second step in deionized water, using layered zinc hydroxide, ZHC, as a precursor with chloride ions in the interlayer. When the first step of aging was performed using a zinc acetate solution, hexagonal plate-shaped ZnO seed crystals with and without chipped corners were obtained, and at the same time, chloride ions incorporated in the interlayer of ZHC were exchanged with acetate ions to form ZHA. When the obtained mixture of ZnO seed crystals and ZHA was aged again in ion-exchanged water, coin-shaped or hexagonal plate-shaped ZnO particles with a particle diameter of about 1 um were obtained in a single phase. When the amount of zinc acetate solution used in the first step aging was small, the ion exchange of chloride ions to acetate ions between the layers of ZHC was insufficient and so ZHC remained, thus resulting in columnar ZnO with small particle size along with plate-like ZnO during the second step aging.

Segregation behaviors on the prior austenite grain boundaries for the B-doped high C case hardening steel (Fe-0.82C-0.22Si-0.86Mn-0.02P-1.1Cr-0.21Mo-0.005B (mass%)) were evaluated using a three-dimensional atom probe (3DAP). Results revealed the intense segregation of C, Mo, Cr, and B on the grain boundaries. Findings also confirmed suppression of the segregation of P, known as a strong segregation element for steel. Thermodynamic analysis based on the parallel tangent law by McLean–Hillert was conducted to validate the segregation of each element. To evaluate the chemical potentials while taking interaction with multiple elements into account, the Calculation of Phase Diagram (CALPHAD) method was used, where liquid phase was adopted to estimate the Gibbs free energy of grain boundaries. The calculation results represent the segregation tendencies obtained from 3DAP. Detailed investigations of the interaction effects of C, B, and Mo on the other elements were also conducted. Results showed the suppression of P segregation by increasing the B content, therefore demonstrating the efficiency of the segregation prediction method which implemented CALPHAD for the B-added high C steels.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 87 (2023) 193–199. Figure 6 was slightly modified.

Controlling the conductance of miniaturized electrical components via spin-dependent tunneling is a challenging step for nano-scale implementation. In this study, we demonstrate the fabrication of lateral nanogranular films with oblate magnetic metal nanoparticles and achieve variable out-of-plane intergranular gap. Changes in insulating layer thickness from 0.4 to 2.1 nm resulted in a marked increase of 10,000-fold for both in-plane and out-of-plane electrical resistivities. A 4% enhancement in permittivity, namely the magnetodielectric effect, was obtained under an in-plane magnetic field of 10 kOe. The frequency at which the maximum magnetodielectric effect is found shifts from 15 kHz to 880 kHz depending on the out-of-plane resistivity. We demonstrated frequency control of the magnetodielectric effect via electrical resistivity by structural modulation of the lateral nanogranular system.

The effect of Laves phases, C36–(Mg,Al)₂Ca and C15–Al₂Ca, on high-temperature creep strength was quantitatively investigated for the Mg–5.0Al–1.5Ca alloy produced by die-casting. The homogenization treatment at 750 K for 1 h was carried out to divorce the interconnected skeleton of C36 phase, and the aging treatment at 523 K for 1–1000 h was performed to precipitate the C15 phase within the α-Mg grains. The creep tests to evaluate the creep strength were conducted at 447 K and 70 MPa. When the C36 skeleton is divorced, the minimum creep rate dramatically increases by a factor of 330. The coarsening of the C15 phase within the α-Mg grains increases the creep rate by a factor of 2.6. It was identified that the creep strength of the Mg–5.0Al–1.5Ca alloy is predominantly ascribed to the interconnected skeleton of C36 phase rather than the precipitation strengthening of C15 phase.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 88 (2024) 27–30.

In this work, corrosion behavior and the concurrent acoustic emission (AE) signals of AZ31 alloy under NaCl and Na₂SO₄ solution droplets were comparatively investigated in combination with in-situ optical microscopy observations. It was observed that after a short initial corrosion accompanied by the growth and rupture of H₂ bubbles, the later corrosion behavior of AZ31 alloy mainly developed into filiform corrosion under NaCl solution droplet and pitting corrosion under Na₂SO₄ solution droplet. AE signals were detected in both cases. In particular, AE parameters of amplitude and duration were found to well identify filiform and pitting corrosion. AE signals were mainly correlated with the observed evolution of H₂ bubbles of different shapes and positions, i.e., regularly round bubbles grew and ruptured at the filament head near the metal surface during filiform corrosion; whereas irregularly-shaped bubbles grew and ruptured at the pit mouth during pitting corrosion.

Due to its exceptional high temperature mechanical properties and low density, TiAl alloy has emerged as a promising structural material for high temperature applications. However, the inherent brittleness and susceptibility to cracking pose challenges during processing. In the additive manufacturing process of TiAl alloy, substrate preheating plays a crucial role in mitigating crack formation. This study focuses on the fabrication of crack-free Ti-48Al-2Cr-2Nb alloy via laser directed energy deposition (LDED), investigating the influence of preheating on sample cracks and microstructure. The results demonstrate that substrate preheating significantly affects the quality of formed samples. Without preheating, numerous cracks are observed in the samples; however, their severity gradually decreases with increasing preheating temperature. Notably, when the substrate was heated to 400°C, no cracks were detected in the samples. Moreover, higher preheating temperature lead to reduced grain length-diameter ratio and partial equiaxed crystal formation along with increased average grain size and α₂ phase proportion while decreasing average orientation difference slightly. The microhardness exhibited a subtle declining tendency. With an increase in the proportion of α₂ phase, the stress generated between different phases is reduced. Additionally, increasing the preheating temperature reduces dislocation density and releases stress, thereby inhibiting crack generation.