Thermal expansion, by which a material’s volume increases when heated, is a universal phenomenon deriving from the thermal vibration of the atoms that constitute solids. Since the discovery of low thermal expansion in Invar alloys at the end of the 19th century, the “abnormality” of thermal expansion has given birth to new sciences and technologies. This article describes studies of thermal expansion anomalies from low thermal expansion of Invar alloys to the giant negative thermal expansion (giant NTE) materials recently discovered. Moreover, prospects for future research are presented. One turning point is the large isotropic NTE of ZrW₂O₈ discovered in 1996. Starting with manganese nitride in 2005, various materials have been found to have negative linear expansion coefficients that are many times larger than those of conventional NTE materials, although some operating-temperature constraints exist. These achievements have overturned the conventional wisdom which holds that negative coefficients of linear expansion do not engender large values. Additionally, the achievements established the concept of “giant” or “colossal” NTE. This article emphasizes recent important findings of enhanced NTE caused by material microstructural effects that are peculiar to ceramic bodies, and “hybrid” NTE by which multiple mechanisms work simultaneously. Additionally, this article introduces an attempt to produce fine particles of NTE material and use them as a thermal expansion compensator, especially for thermal-expansion control of resin.

This Paper was Originally Published in J. Jpn. Soc. Powder Powder Metallurgy 70 (2023) 361–370.

Valence electron concentration (VEC) analysis of high-entropy alloys (HEAs) has been extended to a scheme using a two-parameter plot by introducing the period of the periodic table as the second variable. The extension aimed to include HEAs with a hexagonal close-packed (hcp) structure comprehensively in the framework of VEC analysis, along with other HEAs with body- and face-centered cubic (bcc and fcc) structures. A statistical analysis using a VEC–period chart was performed for 261 single-phase exact- and near-equiatomic HEAs with bcc, hcp, or fcc structure acquired from the literature. Supplemental experiments were conducted in the present study for an hcp-Co₃₅Cr₂₀Fe₁₅Ru₂₀V₁₀ alloy (Ru₂₀ alloy) predicted by Thermo-Calc software and the TCHEA6 database. The experiments revealed that the water-quenched Ru₂₀ alloy annealed at 1600 K for 1 h exhibited a single hcp structure. The formation of the hcp-Ru₂₀ HEA was attributed to the inclusion of a minimum amount of 20 at%Ru, as an hcp forming element, from a 4d-transition metal (TM). The Ru₂₀ alloy with (VEC, Period) = (7.65, 3.2) was located immediately below the conventional fcc-HEAs plotted along the segment of (VEC, Period) = (8–9.5, 3). The VEC-period chart revealed a strict boundary of the hcp/fcc regions in the VEC–period chart between the Ru₂₀ alloy and fcc-HEAs. It is practically difficult to prepare hcp-HEAs comprising 3d-TM only through conventional solidification from a melt and subsequent annealing, as required.

The advancement of electronic devices and their capabilities has driven the demand for specific material property combinations, such as mechanical strength and electrical conductivity, in materials pertinent to device fabrication. To this end, the microstructure and properties of Cu–4.2 at% Ti alloy sheets, produced through a multi-step process involving over-aging and severe cold rolling, were investigated. The microstructure of the over-aged alloy prior to cold rolling consisted of cellular components with laminated plates of terminal copper solid solution (Cu_ss) and β–Cu₄Ti. When the over-aged alloy was severely cold rolled for a 99% reduction in thickness, a hierarchical double-phase microstructure was formed parallel to the cold-rolling direction, with Cu_ss bands and two-phase bands containing small β–Cu₄Ti pieces stacked within Cu_ss phase. The strength of the over-aged alloy sheet increased steadily during increasing degrees of cold rolling, caused by a large volume fraction and fine dispersion of hard β–Cu₄Ti pieces and high dislocation density in the Cu_ss matrix. The electrical conductivity decreased in the later stages of cold rolling; however, the conductivity was higher than that of the alloy sheet prepared by peak aging and cold rolling. Eventually, the balance between strength and electrical conductivity of this Cu–Ti alloy was significantly improved by over-aging and severe cold rolling compared to conventional peak-aging and cold rolling processes.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 62 (2023) 68–72. Figures 4 and 5 were slightly modified.

In a thermoelastic martensitic transformation, there is a “self-accommodation” in which the microstructure itself relieves strain induced by the transformation. In this study, the crystal structure and self-accommodation microstructure of martensite in Ti₃₀Ni₅₀Zr₂₀ alloy were investigated using X-ray diffraction, scanning electron microscopy with electron backscatter diffraction, and transmission electron microscopy. In addition, the deformation microstructure was investigated by observing the samples after tensile tests. The crystal structure of the martensite in the Ti₃₀Ni₅₀Zr₂₀ alloy was determined to be the B19′ monoclinic structure. In this alloy, plate- and polygonal-like variants with a width of a few microns were observed, and pairs of habit-plane variants (HPVs) forming {011}_B19′ twins were observed at the interface. The self-accommodation has a mosaic-like morphology because of the combination of these pairs of HPVs. The (001)_B19′ compound twins were formed as internal defects. These twins are considered to be lattice-invariant shear (LIS) of martensite in this alloy. No plateau region was observed in the tensile test. In the deformed specimen, the self-accommodation was collapsed by the movement of the HPV interface and the introduction of (100)_B19′ compound twins was observed as an internal defect. This defect is considered to be not LIS but a deformation twin. The lack of a plateau region in the stress–displacement curve was attributed to these deformed microstructures.

Microstructure and its evolution of solute-enriched stacking faults (SESFs) in kink-deformed Mg–Zn–Y alloys have been investigated by scanning transmission electron microscopy (STEM). Mass-thickness contrast in annular dark-field (ADF) STEM images and diffraction contrast in weak-beam dark-field (WBDF) STEM images visualized locations of solute-enriched regions and dislocations, respectively. Most of the SESFs are located at or near kink boundaries, and a variety of arrangements and morphology of the SESFs are followed by dislocations with the c component of α-Mg matrix and small-angle lattice rotations along the c-axis as well as the a-b axes. As notable microstructural features of the SESFs, rectangular-shaped and step-shaped arrangements of the SESFs, observed respectively before and after a heat treatment at 573 K for 168 h, were observed in detail, and their formation processes were discussed.

{001}〈101〉 texture evolution in Ti–37 mol%Nb (Nb–46.5 mass%Ti) alloy was determined under plane strain compression at the temperatures of 800°C, 950°C, and 1100°C, in which preferential dynamic grain growth (PDGG) took place. At lower temperature and higher strain rate such as 800°C–10⁻²/s, almost no grain growth occurred in the transverse direction (TD), and the α-fiber + near {111}〈110〉 in the γ-fiber texture was developed, which was a stable orientation as deformation texture. At higher temperature and lower strain rate such as 1100°C–10⁻³/s, the grain growth along the TD remarkably appeared by grain boundary bulging, and an extremely high pole density of the texture near the α-fiber, especially the rotated cube {001}〈101〉, evolved. A planar dislocation structure with pile-ups appeared and individual dislocations were uniformly distributed in the grains. The rotated cube texture fulfills the conditions of the deformation stability and the low Taylor factor in accordance with the PDGG mechanism. The essential aspect of the mechanism is the preferential growth of grains with a stable orientation for deformation and a low Taylor factor in the given deformation mode.

We have enhanced a basic single-variable dislocation evolution modelling framework to encompass the acoustic emission (AE) characteristics observed in metals and alloys during the homogeneous strain hardening stage up to the point of macroscopic necking instability under tensile stress. To validate our proposed modelling approach, we conducted comprehensive experiments using pure Ag, Cu, Al, and Ni, representing fcc metals with stacking fault energies increasing in the listed order of materials. Our model successfully captures both previously established and newly discovered patterns in the evolution of the AE spectral density. A key focus of this research is to demonstrate the ability to directly derive the critical parameter that governs the evolution of dislocations and, consequently, overall strain hardening, namely the rate of dynamic dislocation recovery, directly from AE measurements. As a prime result, we can predict the conditions leading to necking, specifically the necking strain, well in advance solely from AE data.

In this study, 6061-T6 aluminum alloys were plated with electroless Ni–P with different phosphorus content and three-point bending and rotating bending fatigue tests. The effects of hydrogen due to plating on mechanical properties were investigated. It was shown that the ductility decreased immediately after low-phosphorus type and high-phosphorus type Ni–P plating by three-point bending test because a hydrogen was introduced into the alloys. The fatigue strength of the low-phosphorus type Ni–P plated specimen was higher than that of the un-treated 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. 6061-T6 aluminum alloy have been reported to exhibit no hydrogen embrittlement in low strain rate tensile tests under wet condition. However, it was found that hydrogen embrittlement occurred when 6061-T6 aluminum alloy was plated with the high-phosphorus type electroless Ni–P and fatigue-tested on rotating bending machine.

This Paper was Originally Published in Japanese in J. Japan. Inst. Met. Mater. 87 (2023) 114–119.

Lattice structures, characterized by their exceptional strength-to-weight ratios and energy absorption capabilities, have paved the way for pioneering designs in additive manufacturing (AM). To fully harness the potential of AM, robust inverse design methodologies are essential. In this study, a novel FEM-LSTM based lattice structure inverse design framework was proposed for horseshoe lattice structures characterized by Length (L), Radius (R), and Angle (A) to establish the structure-performance response. Using finite element analysis, a substantial dataset with distinct geometries and mechanical responses was meticulously furnished for training. Delving deeper into modeling, we developed an autoencoder framework anchored in long short-term memory (LSTM) networks, designed to adeptly decode the temporal intricacies of stress-strain attributes and seamlessly encode sequence characteristics. Compared to traditional GPR models and DNN models, the proposed model’s predictability increased by 9% and 7%, respectively, which is attributable to the exceptional capability of LSTM structure in handling time-series data. Our model, being versatile, can seamlessly integrate multiple stress-strain inputs, rendering precise geometric parameters that resonate with tailored design specifications. Such a streamlined approach effectively supplants the conventionally tedious iterative forward design and exhaustive simulation phases. In summation, the model emerges as a swift conduit for bespoke inverse design pertaining to lattice structures. And the paradigm of discerning time-series correlations through LSTM autoencoders holds vast potential across diverse time-dependent properties inherent to materials science.

In this study, we investigated the effects of the corrosion inhibitor benzotriazole (BTA), with the chemical formula C₆H₅N₃, on the oxidation behavior of copper through microstructure analysis. Our results demonstrate that BTA coating on copper decreased its oxidation rate and altered the morphological characteristics of the resulting copper oxide. The biocidal ability of copper was found to be unaffected by the presence of BTA. Our findings suggest that the stability of the interface between copper and its oxide, Cu₂O, is primarily determined by atmospheric pressure rather than the presence of BTA during the oxidation process. This study provides valuable insights into the role of BTA in the oxidation behavior of copper, which can have implications on various applications.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 62 (2023) 242–246. Figures 1, 3, 5, and 7, as well as all of Figure captions were marginally modified.

This study has developed a surface modified titanium carbide (TiC)–titanium (Ti) composite for application in dry press working. This material was synthesized through mechanical alloying using Ti and graphite powder, followed by spark plasma sintering, and subsequently surface-modified via oxidation/carburization treatment. The oxidation/carburization behavior was investigated to determine the treatment conditions that formed a hard coating layer, and its wear resistance was evaluated. The activation energy for the isothermal oxidation of this composite material was 271 kJ/mol, with the rate primarily determined by the O and Ti diffusion in titanium dioxide (TiO₂). In contrast, the activation energy for the growth of the carburized layer growth was 261 kJ/mol, with the rate determined by the carbon diffusion in TiC. The oxide film of the TiC–Ti composite material, oxidized by holding at 1073 K for 300 s, exhibited porous rutile TiO₂ with a thickness of approximately 1.7 µm and hardness of 11 GPa. The carburized layer of the TiC–Ti composite material, carburized at 1073 K and held for 2400 s, had a thickness of approximately 3 µm and hardness of 20 GPa. During a wear test utilizing a stainless-steel pin as the mating material, stainless steel adhered to the surface of the unmodified sample. The oxidized sample experienced wear due to the peeling of the oxide film. However, the carbonized sample neither adhered to nor wore the mating material, suggesting that the carbonization treatment helps to improve the wear characteristics. This study offers valuable insights into the fabrication of TiC-based cermets as mold materials for realizing dry press working.

Ultrasonic waves are effective in examining the contact state between the die and the material during sheet metal forming. In this study, ultrasonic measurements were performed on wrinkles and contaminant in press forming. A model specimen simulating wrinkles was used in the measurement of wrinkles. The contaminant was placed on a flat specimen and sandwiched between dies in the ultrasonic measurement of contaminant. The reflection intensity of 2.25 MHz ultrasonic waves irradiated to the convex part of the wrinkle decreased as the number of wrinkles with a wrinkle wavelength of 4 mm increased. However, the ultrasonic reflection intensity at the convex part of the wrinkles was not related to the number of wrinkles in 5 MHz ultrasonic waves. The contaminant changed the reflection characteristics of ultrasonic waves. The frequency characteristics of the reflected wave changed markedly at a specific frequency because of wrinkling and contamination. 2.25 MHz and 5 MHz ultrasonic waves were able to detect contaminants of the size occurring at the production site and wrinkles with a wrinkle height of 0.01 mm not only from the ultrasonic reflection intensity but also from the frequency characteristics.

This Paper was Originally Published in Japanese in J. JSTP 63 (2022) 79–85.

In this paper, we report the structural and electrical properties of TiZrN thin films deposited by dc-magnetron sputtering under a working pressure of 0.67 Pa at room temperature. The phase structure, crystallinity, morphology, electrical properties, and hydrophilicity of TiZrN films were found to be strongly dependent on the nitrogen gas ratio of N₂/(Ar+N₂). In vitro Baby hamster kidney (BHK) cell adhesion on the TiZrN films was also tested for primary biocompatibility evaluation. The sputtered films exhibit a single-phase structure of (Ti, Zr)N_x without the presence of secondary phases such as TiN_x or ZrN_x. The lowest surface resistivity and resistivity values are observed in the films deposited at a 10% nitrogen ratio, with values of 12 Ω/□ and 1.79 × 10⁻⁵ Ω.m, respectively. TiZrN films deposited at different nitrogen ratios between 3–30% show hydrophilic properties, with contact angles ranging from 60 to 83 degrees. In vitro BHK cell test indicated that TiZrN films possess excellent biocompatibility. With the interesting performances indicated, TiZrN films have great potential for electrode applications and biomedical implants.

The effects of process parameters on room-temperature (RT) deep drawability were investigated for AZ31B magnesium alloy sheets, in which the formation of basal texture was suppressed by high-temperature rolling. When the blank holder force was set to the minimum value to prevent wrinkling, high RT drawability was obtained. In particular, a significantly high limiting draw ratio (LDR) of 1.83 was obtained by the optimization of the process parameters of the servo-press machine. At higher punch speeds, the drawability tended to be deteriorated; however, a high LDR of 1.72 was obtained even at a high punch speed of 50 mm/s by the optimization of the process parameters of the servo-press machine. Furthermore, when the punch speed in the initial stage of deep drawing was reduced, it was possible to perform deep drawing without fracture even if the punch speed in the later stages was increased. The results of the study of the effect of lubricants indicated that relatively high RT drawability was obtained when molybdenum disulfide was utilized. It was also suggested to be important to select a lubricant that reduces punch load and does not delaminate during deep drawing.

This Paper was Originally Published in Japanese in J. JSTP 64 (2023) 13–18.

Accumulation behavior of transformation-induced dislocation upon thermal cycling of Ti–30Ni–20Cu which satisfies the triplet condition (TC) was compared to that of Ti–39Ni–11Pd which is known as low hysteresis shape memory alloy designed by the condition that the middle eigenvalue of lattice deformation is 1 (CC1). The dislocation density was determined by the Williamson-Hall method. Although the 0.2% proof stress of the Ti–30Ni–20Cu alloy was about half that of the Ti–39Ni–11Pd alloy, the dislocation accumulation behavior was almost the same in both alloys. The formation of transformation-induced dislocation is effectively suppressed by satisfying TC, more than equivalently to satisfying CC1 only. TC is suggested to be a new guideline to design durable shape memory alloy.