We report on high-pressure torsion (HPT) processing of Si and related semiconducting materials, and discuss their phase transformations and electrical, thermal, and optical properties. In-situ synchrotron x-ray diffraction revealed that the metastable bc8-structure Si-III and r8-structure Si-XII in the HPT-processed Si samples gradually disappeared and hexagonal-diamond Si-IV appeared during annealing up to 473 K. The formation of Si-III/XII in the samples processed at a nominal pressure of 6 GPa indicated the strain-induced phase transformation from diamond-cubic Si-I to a high-pressure tetragonal Si-II phase during HPT processing, and a following phase transformation from Si-II to Si-III/XII upon decompression. The resistivity decreased with increasing the number of anvil rotations due to the formation of semimetallic Si-III. The thermal conductivity of Si was reduced to ~3 Wm⁻¹K⁻¹ after HPT processing. A weak and broad photoluminescence peak associated with Si-I nanograins appeared in the visible light region after annealing. Metastable bc8-Si_0.5Ge_0.5 with a semimetallic property was formed by HPT processing of a traveling-liquidus-zone-grown Si_0.5Ge_0.5 crystal. These results indicate that the application of HPT processing to Si and related semiconductors paves the way to novel devices utilizing nanograins and metastable phases.

Mater. Trans. 64（2023）1346-1352に掲載

This paper presents an overview of the recent development of incremental feeding high-pressure sliding (IF-HPS) process for grain refinement of metallic sheets with enlarged areas. The IF-HPS process is a method of severe plastic deformation (SPD) under high pressure without increasing the machine capacity. The IF-HPS process combines an incremental feeding technique with the high-pressure sliding (HPS) process so that a severely deformed area can be extended. Development of the IF-HPS process includes the use of flat-type anvils instead of groove-type anvils, which makes it easier to enlarge the SPD-processed areas. The development is also described in terms of the sliding mode and the feeding pattern, where the former is determined by the sliding distance and the numbers of the reciprocation of the sliding and the latter by the feeding distance and the feeding direction. The application of the IF-HPS process is made to metallic materials such as a Ni-based superalloy (Inconel 718), a Ti-6Al-7Nb alloy (F1295) and commercially available Al alloys (A1050, A3105, A5052 and A5182). It is shown that the grain refinement is successfully achieved so that superplastic elongation more than 400% is attained in the Ni- and Ti-based alloys, and the room-temperature tensile strength is well enhanced in the Al alloys. It is then demonstrated that the IF-HPS process is promising to extend the SPD-processed area without increasing the machine capacity. Furthermore, a new approach is suggested for material design, such as the hybrid materials composed of conventional and fine-grained materials and functionally graded materials.

Mater. Trans. 64（2023）1364−1375に掲載済

It has been reported that severely cold worked Fe-24.6Ni-5.8Al-0.4C (mass%) had a yield strength of 2 GPa and a fracture elongation of 20 %, in which huge amount of Lüders-type deformation was observed. In the present article, we summarize the reports for high-strength Fe-Ni-Al-C, Fe-Mn, Fe-Cr-Ni and Fe-Ni-Mn base steels with the Lüders-type deformation so far, and provide our latest data on the effects of alloying elements and the cold-rolling reduction on the microstructure and mechanical properties of cold-rolled Fe-Ni-Al-C alloys. Previous reports imply that the phase stability of γ phase affects the size of Lüders elongation, while the strategies to control the microstructure to achieve high strength and high ductility are currently unknown. Our latest study also shows that the γ-phase stability affects the Lüders strain. In addition, it is confirmed that severe cold rolling by 80 % enables the prolonged Lüders strain as much as 25 % in nominal strain. This prolonged Lüders strain is achieved by multiple propagation of Lüders-type bands.

Mater. Trans. 64（2023） 1410-1418 に掲載．Table 1, Fig.6を修正．

Studies have shown that the corrosion resistance of stainless steels in passive environments is enhanced by grain refinement into the order of submicron or nanoscale via various methods, including severe plastic deformation (SPD). This beneficial effect has been attributed to the enhanced protective nature of the passive film due to a greater Cr enrichment in the film. Two independent mechanisms for the greater Cr enrichment in passive films have been proposed: enhanced selective dissolution of Fe and faster Cr diffusion. Both mechanisms originate from high density grain boundaries. However, recent studies have used high-resolution scanning transmission or in-situ atomic force microscopy to visualize the near atomic-scale passivation process and suggest that the increased protectiveness of passive films caused by the Cr enrichment is limited to a zone in the vicinity of grain boundaries. This finding suggests that both these mechanisms, facilitated by grain refinement, might be capable of the homogeneous passive film formation over the entire surface if the grain size is extremely small (＜100 nm), which most classical SPD methods, represented here by equal channel angular pressing, cannot achieve. Therefore, for the formation of a uniform and homogeneous passive film inside all the grains, the role of factors other than that of grain size might be involved. A fresh review of the literature on the corrosion behavior of ultrafine grained (UFG) stainless steels with grain size smaller than 1 μm and nanocrystalline ones smaller than 100 nm, generated by classical SPD, surface SPD, and other physical methods, was undertaken in light of the uniformity of the passive film. The possible role of high internal stress and residual dislocations, which are common constituents of UFG materials obtained by SPD, on the formation of the protective passive film was discussed.

Mater. Trans. 64（2023）1419-1428に掲載．Fig.5のキャプションを修正．

This article reviews the microstructural evolution in ultrafine-grained and nanotwinned austenitic stainless steels that have been subjected to hydrogen embrittlement (HE) and fatigue cracking. It provides guidelines for the development of high-strength austenitic steels without sacrificing HE and fatigue performance. The author focuses on the hydrogen-induced ductility loss and short fatigue crack growth associated with deformation-induced martensitic transformation, using micro-tension and micro-fatigue testing technologies. In type 304 metastable austenitic stainless steel, the microstructure produced by high-pressure torsion depends strongly on the processing temperature. Nanocrystalline austenite with enhanced strength and moderate ductility can be obtained at a processing temperature of ~423–573 K, whereas dual-phase microstructures comprising austenite and martensite are formed by processing at room temperature. Introducing ultrafine grains and nanotwin bundles mitigates the hydrogen-induced ductility loss in metastable austenitic steel by controlling the dynamic martensitic transformation. The microstructure refinement also contributes to enhanced resistance to short fatigue crack growth by changing the route of the damage accumulation process via phase transformation and detwinning.

Mater. Trans. 64（2023）1474-1488に掲載．Fig.2のCaptionを修正．

The electronic structure of the band gap determines the amount of light and its wavelength that can be absorbed by a semiconductor. Most photocatalysts are semiconductor materials, therefore, the state-of-art band gap engineering plays an important role in the efficiency of the photocatalytic reactions. Metal oxides are the most abundant semiconductors in the Earth’s crust, most of which possess large band gaps. In order for oxides to be able to absorb solar energy, the band gap must be reduced. In this review, band gap of high-pressure phases of some well-known metal oxides like TiO₂, ZnO, and Y₂O₃ are studied, which are known to be unstable at ambient pressure while having the advantage of narrow band gaps. High-pressure torsion (HPT) is introduced as an effective method for stabilization of high-pressure phases, and these phases show good activity under visible light for water splitting hydrogen or oxygen production, and/or CO₂ reduction reactions. High-entropy oxides and oxynitrides are another group of materials that will be introduced for effective photocatalytic properties, synthesized by the HPT method.

Mater. Trans. 64（2023）1497-1503に掲載

In this study, a commercial hot-extruded AZ80Mg alloy was multi-directionally forged （MDFed） at room temperature by employing pass strains of Δε = 0.1. The effects of the combined processes of MDFing and ageing on the microstructural evolution and strengthening were precisely examined in advance. The coarse initial grains were gradually subdivided into ultrafine grains by multiple mechanical twinning and kinking. As observed, the multiple twinning effectively suppressed the evolution of the sharp basal texture and enabled MDFing at room temperature to high cumulative strains. Although the combined processes of MDFing and ageing tended to increase the hardness and yield stress compared to those fabricated using simple MDFing at lower cumulative strain regions, the mechanical properties were almost comparable and independent of the processes at regions of higher cumulative strain beyond ΣΔε = 2.0. Yield strength over 505 MPa, ultimate tensile strength of over 612 MPa and ductility of over 7% were constantly achieved in all the processes. Although certain selected processes were applied to bulk samples for fabricating the mechanical components, frequent cracking hindered the MDFing to high cumulative strain regions. This finding signified that adequate MDFing process is dependent on sample size. However, MDFing with smaller pass strains than Δε = 0.1 enabled MDFing to regions of high cumulative strain. Thus, bulk AZ80Mg alloy with well-balanced mechanical properties—yield strength of 420 MPa, ultimate tensile strength of 540 MPa, and ductility of 10%—could be successfully fabricated.

Mater. Trans. 64（2023）1504-1514に掲載．Figs. 4,6,7,8のCaptionを修正．

High-entropy alloys (HEAs) are a new class of material producing superior properties that have a potential for replacing many structural materials in industry. Single-phase solid solution HEAs with face-centered cubic crystal structure show significant ductility and toughness over a wide temperature range including at cryogenic temperatures. Nevertheless, the occurrence of decomposition at elevated temperatures is challenging for many applications. These materials reveal sluggish diffusion and therefore high thermal stability so that processing by severe plastic deformation gives increased kinetics of decomposition and leads to fine-multiphase microstructures which provide a potential for achieving superior superplastic elongations. The present review is designed to examine the available superplastic data for HEAs and thereby to compare the behavior of HEAs with conventional superplastic alloys.

Mater. Trans. 64 （2023） 1526-1536に掲載. Figs. 6, 12を修正．

Zinc oxide (ZnO) with a rocksalt crystal structure is attractive because of the bandgap which lies in the range of visible light absorption (1.2-2.6 eV). However, the rocksalt structure is not stable at ambient pressure and temperature according to an equilibrium phase diagram. Nevertheless, this study demonstrates, for the first time, that it is possible to realize a 100% fraction of the rocksalt structure at ambient pressure and temperature. ZnO powder is initially processed by severe plastic deformation under high pressure through a technique of high-pressure torsion (HPT). The HPT-processed ZnO is then examined using a high-pressure application system available at BL04B1 of SPring-8 and in situ X-ray diffraction (XRD) analysis is conducted under high pressures at elevated temperatures. It is shown that the initial presence of the rocksalt structure produced by the HPT process is effective to attain a 100% fraction of the rocksalt structure.

Mater. Trans. 64（2023）1585-1590 に掲載

This report is aimed at giving an overview of the significance of the novel and innovative microstructural and microscopic characterization techniques for bulk nanostructured metals processed by severe plastic deformation, specifically high-pressure torsion (HPT). In practice, the microstructural relaxation behavior upon heating of nanostructured 316L stainless steel and CoCrFeNi high-entropy alloy was characterized by in-situ heating neutron diffraction measurements; the heterogeneous phase distribution of an HPT-bonded hetero-nanostructured Al-Mg alloy was examined using synchrotron high-energy X-ray diffraction; and the microstructural evolution upon heating of a nanostructured CoCrFeNiMn high-entropy alloy was examined by laser-scanning confocal microscopy. These novel techniques are complementary to each other and any other in- or ex-situ testing methods, especially when nanocrystalline metals are transforming microstructurally and compositionally with temperature and time in a hierarchical manner. The outcomes of the studies emphasize the importance of the methodologies and the development of characterization techniques for further in-depth exploration in the research field of severe plastic deformation.

Mater. Trans. 64（2023）1683-1694に掲載. Fig.10のCaptionを修正.

Ti-6Al-7Nb alloys have been widely used in the medical field, particularly in artificial hip joints, spinal fixators, and dental implants, owing to their light weight, low toxicity, and superior corrosion resistance. Grain refinement through a severe plastic deformation process under high pressure, such as high-pressure torsion (HPT) or high-pressure sliding, is widely employed for strengthening metallic materials. This overview presents the recent advances in the effect of HPT on the mechanical properties of the Ti-6Al-7Nb alloy. This alloy was grain-refined through HPT under applied pressures of 2 and 6 GPa, and the results revealed that the alloy subjected to HPT processing at 6 GPa exhibited a higher strength. To inhibit the decrease in the total elongation of the alloy, the number of revolutions in the HPT process was set to moderate. The tensile properties achieved after HPT processing were found to be dependent on the initial microstructure before the HPT treatment. Furthermore, an alloy with a bimodal equiaxed and acicular structure was subjected to grain refinement via the HPT process. The results revealed that fragmentation of the acicular structure during HPT further increased the strength. Moreover, the HPT-processed Ti-6Al-7Nb alloy exhibited superplasticity. It was thus confirmed that grain refinement by HPT is an effective method for strengthening the Ti-6Al-7Nb alloy, which is advantageous for medical applications.

Mater. Trans. 64（2023）1784-1790に掲載．文献［40］を修正．

This study presents that A1050 commercial-purity aluminum increases the tensile strength and ductility using the processes of accumulative roll bonding (ARB) and high-pressure sliding (HPS). Both processes yield a similar tensile strength exceeding 240 MPa after processing by ARB for 10 cycles and by HPS for the sliding distance of 15 mm, respectively. The stress-strain behavior is evaluated through microstructure observations and measurements of strain hardening rates. Significant grain refinement with well-defined grain boundaries is responsible for the strength increase. The grain refinement also leads to an increase in strain hardening rate and thus an increase in the ductility.

Mater. Trans. 64 （2023） 1902–1911に掲載

A high-pressure torsion (HPT) processed Fe-21Cr-5Ni-2Mo (mass%) two-phase stainless steel was used to study the morphology and crystallographic features of austenite (γ) precipitated from ferrite (α) during aging in the (α + γ) two-phase region. The starting material was a gas-atomized powder with a completely ferritic structure. The HPT process was carried out to produce a fully dense compact under 6 GPa for 5 revolutions. The compact was given an equivalent strain of about 130. After the HPT process, the matrix ferrite formed a pancake–like nanograined structure with a strong texture, i.e. ND (Normal Direction) // {110} α. By annealing at 1173 K for 3.6 ks, an ultrafine (α + γ) microduplex structure with high-angle grain boundaries was formed. In addition, the strong texture formation of {110} α / {111} γ / ND plane was formed in the α and the γ grain duplex structure. The α and γ phases had average grain sizes of 2.1 μm and 1.6 μm, respectively. The area fraction of the γ phase was 37.2%, which exceeded that of a cold-pressed compact, 6.7%. Both ultrafine grain refinement and γ precipitation were accelerated by the HPT process. In other words, the application of the HPT process to the two-phase alloys enables the formation of the ultrafine microduplex structure. The Kurdjumov-Sachs (K-S) orientation relationship between α and γ phases is usually observed in the alloy, however, the K-S orientation relationship was not dominant except for the close packing plane parallel orientation relationship, {110} α / {111} γ, in the HPT-processed material.

Mater. Trans. 64（2023）1912-1919に掲載．和訳に際し，2章において不必要な熱処理条件を省略．Fig.11とテキスト間の面指数の不一致を修正．4，5章中，サンプルに加えた熱処理について詳細な温度条件の記述を追加．文献［25］を修正．

ASTM grade 1 titanium sheets with the strong TD-split type basal texture and grain sizes ranging from 9 to 152 µm were subjected to tensile tests at tensile speeds ranging from 0.1 to 100 mm/min to investigate the strain rate dependency, while considering anisotropy. The yield and flow stresses at each strain level increased proportionally with the logarithm of the strain rate. The slope of these proportionalities, namely, the strain rate sensitivity index of m decreased with increasing grain size and tensile angle to rolling direction, θ. However, in the high-strain region, the larger the grain size, the higher the index m. Meanwhile, uniform elongation decreased with an increase in strain rate except for a θ value of 90°, where uniform elongation was roughly constant. However, the highest uniform elongation was observed in relatively large grain-sized specimens when θ changed from 0° to 45°. There was an average reduction in local elongation with increasing strain rate and grain size; however, an inversed trend was observed with increasing grain size in specimens with a grain size of 9—36 μm when θ was 45°.