Recent studies show significant advances in improving the mechanical properties of magnesium and its alloys. While many papers deal with different alloy compositions, it is apparent that grain size plays a key role in the mechanical behavior of these materials. The ability to produce samples with very fine grain sizes leads to observations of high strength and/or high elongations. There are recent reports of exceptional elongations of over 100% in pure magnesium and a few alloys. These recent findings are critically reviewed in the present study. The experimental data from over 300 papers are collected, and trends between flow stress, elongation, strain rate sensitivity, and grain size are identified. The role of alloy content is examined. The data clearly shows a transition in the flow stress vs. grain size relationship which is attributed to a change in deformation mechanism from twinning controlled in coarse grained to slip controlled in fine and ultrafine grained samples. The slip controlled deformation agrees with the model of grain boundary sliding, which has shown good agreement with multiple metallic materials. It is shown that the elongations display a maximum in the grain size range in which there is a transition in the deformation mechanism. Three strategies are described for achieving high strength, high ductility, and good strength-ductility combination.

Multi-principal element alloys (MPEAs) are in the forefront of materials science since their compositions can be found in the undiscovered central parts of phase diagrams. These materials contain 3–6 elements with similar fractions, i.e., the constituents do not play the classical solvent and solute roles in the alloys. This class of materials includes high entropy alloys (HEAs). Novel MPEA compositions often have unique and superior properties compared to conventional materials. It has been shown that the features of MPEAs can be further improved by nanostructuring using severe plastic deformation (SPD) techniques. In this study, the evolution of the microstructure in MPEAs during SPD-processing (defect formation, grain refinement and phase transformation) is overviewed on the basis of the literature. The corresponding changes of the mechanical and physical properties, such as the strength, corrosion resistance and hydrogen diffusivity are discussed. In addition, the potential applications of SPD-processed MPEAs are presented.

In tensile testing, polycrystalline materials generally fail at relatively low total elongations but under some limited conditions it is possible to achieve exceptionally high elongations of up to and exceeding 400%. This superplastic condition is important scientifically but also it has important uses through the industrial development of superplastic forming operations. This overview traces the historical development of this superplastic effect and it provides a summary of the main characteristics of the superplastic flow process. Conventional thermomechanical processing is not able to produce exceptionally small grain sizes within the submicrometer or nanometer range but this limitation was effectively overcome through processing using severe plastic deformation (SPD). The advantages of SPD processing are discussed and examples are presented. Finally it is demonstrated that the experimental data may be easily and effectively displayed through the construction of deformation mechanism maps based on combinations of stress, grain size and temperature.

Due to their unique properties, titanium (Ti) and Ti-alloys are particularly suitable for biomedical devices. Ti has a high specific strength and low Young’s modulus (reducing stress shielding), high corrosion resistance, and superior biocompatibility. However, Ti’s moderately low Young’s modulus (100–110 GPa) is still considerably higher than to bones (5–30 GPa). The β-Ti phase, whose elastic modulus is closer to the bone, can be kept by increasing the contents of non-toxic β stabilizing elements. Besides stress shielding and corrosion resistance, adjusted bioactivity (bone-bonding ability) is another primary prerequisite for implants that can be improved by ultrafine-grained (UFG) microstructures and surface modification (anodization and acid+alkaline treatment). UFG by HPT also enhances wear resistance and mechanical properties. Representative alloys (Ti–6Al–7Nb (TAN), Ti–13–Nb–13Zr (TNZ), and Ti–35Nb–7Zr–5Ta (TNZT)) and cp-Ti, were presented in this overview. Samples started with different phases and morphologies. Deformation by HPT induced phase transformation in the alloys, which depended on the amounts of α or β stabilizers, the strain rate, applied loads, and starting phases and α morphologies. Grain sizes were reduced to about 120 nm. Mechanical properties depended mainly on the number of grain boundaries and their nature and different phases, sizes, and strengths. Young’s modulus diminished when the β was increased. Polished surfaces and cp-Ti presented similar corrosion resistance, improved by surface treatments, which reached maximum protection in anodized samples processed by HPT. After bioactivity tests, different growth rates for various processing conditions and alloys were observed, the highest for the TNZ alloy, and improved after HPT processing.

Ultrafine grained (UFG) and nanostructured steels have gained attention in the last years because of the possibility of improving both strength and ductility, but also because of their potential for improving several properties in metal applications which allows replacing some conventional steels. The refinement of the microstructure obtained through Severe Plastic Deformation (SPD) has allowed for the improvement of mechanical properties and the performing of several studies related to corrosion control, mitigation, and protection. In this review, the corrosion behavior of ultra-fine grained (UFG) steels is presented regarding the existing literature and the microstructural changes produced through different SPD processes. A focus is placed on the importance of the processes for grain refinement and microstructural changes and, therefore, on the corrosion behavior.

The aim of the article is to discuss the characteristics of structural transformations occurring under ultra-high plastic deformation in polymeric materials of various types: semi-crystalline and amorphous polymers, polymer blends and composites, polymer powders, and their influence on properties. Methods of obtaining ultra-high plastic deformations such as the equal channel angular extrusion (ECAE) process and its modified version - the equal channel multi-angular extrusion (ECMAE) process - are considered.

Severe plastic deformation drives bulk materials far away from equilibrium and thus opens up a new opportunity to explore hitherto uncharted regions of structure-property correlations with respect to grain size, strain and defect density under extreme conditions. This allows addressing long-standing issues in materials research, such as the validity of “effective temperature” concept and provides a set-screw for exploring the possible levels of defect design and property tuning by deformation processing. As a pre-requisite, basic issues concerning the interaction of defects of different dimensionality during deformation and their interrelation with fluxes of solutes or with segregation fields, resulting in chemo-mechanical coupling effects need to be analyzed quantitatively. In the following review, we address issues related to internal interfaces in severely deformed metals and alloys by highlighting recent observations.

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 W m⁻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.

The deformation behavior of nearly fully pearlitic, bainitic and martensitic steels during severe plastic deformation is summarized in this paper. Despite their significantly different yield stresses and their microstructures, their hardening behavior during SPD is similar. Due to the enormous hardening capacity the SPD deformation is limited by the strength of the tool materials. The microstructure at the obtainable limit of strain are quite similar, which is a nanocrystalline structure in the order of 10 nm, dependent on the obtainable strain. The nanograins are partially supersaturated with carbon and the grain boundaries are stabilized by carbon. Another characteristic feature is the anisotropy in grain shape which results in an anisotropy of strength, ductility and fracture toughness. The results are important for the development of ultra-strong materials and essential for this type of steels which are frequently used for application where the behavior under rolling contact and sliding contact is important.

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.

The development of hydrogen energy will help to reduce the use of nonrenewable energy sources and achieve global carbon neutrality. The aluminum-water reaction is an important method of producing hydrogen because aluminum has abundant reserves, a high yield, and no pollution. However, the dense passive oxide film on the surface of aluminum, on the other hand, often obstructs this reaction, which is the primary issue limiting the development of aluminum-based hydrolytic materials. Mechanochemical activation by processing severely plastic deformed aluminum-based materials is one effective approach and has been developed in recent years. This article reviews recent progress of hydrogen production from hydrolysis of severely plastic deformed aluminum-based materials. The kinetic model of aluminum-water reaction, aging protection of the materials, catalytic mechanism and stable rate control for the hydrolysis of aluminum-based materials are reviewed. Furthermore, some existing problems as well as some suggestions for future research on hydrogen production from aluminum-based materials are also discussed.

Solid-state hydrogen storage in various metal hydrides is among the most promising and clean way of storing energy, however, some problems, such as sluggish kinetics and high dehydrogenation temperature should be dealt with. In the present paper the advances of severe plastic deformation on the hydrogenation performance of metal hydrides will be reviewed. Techniques, like high-pressure torsion, equal-channel angular pressing, cold rolling, fast forging and surface modification have been widely applied to induce lattice defects, nanocrystallization and the formation of abundant grain boundaries in bulk samples and they have the potential to up-scale material production. These plastically deformed materials exhibit not only better H-sorption properties than their undeformed counterparts, but they possess better cycling performance, especially when catalysts are mixed with the host alloy promoting potential future applications.

Publications on obtaining bulk nanostructured materials with special properties by various modes of severe plastic deformation, using nickel as an example, are briefly reviewed. Particular attention is paid to the state of grain boundaries, commonly referred to as nonequilibrium, or deformation-modified boundaries, revealed by electron microscopy, including scanning tunneling, Mossbauer spectroscopy, and diffusion studies. It is shown that contribution of specific state of grain boundaries to additional strengthening is often overestimated. In particular, in Ni processed by high pressure torsion, nonequilibrium grain boundaries are formed, which have increased energy and are ultrafast diffusion paths, but they contribute relatively little to the total strengthening.

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.

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.

Surface mechanical attrition treatment (SMAT) method has been widely acknowledged to obtain desired combination of strength and ductility in metals and alloys, and helpfully overcome the “trade-off” between the two mechanical properties. Within researches for more than 20-years, the favorable mechanical properties are related to the typical gradient structure (GS) in metals and alloys prepared by SMAT. In this overview, the principle and process parameters of SMAT are concisely presented. The strengthening mechanisms are interpreted by traditional dislocation-twin theory, hetero-deformation induced (HDI) theory and theory of shear band and strain delocalization. Besides, the strengthening mechanisms and methods to obtain excellent mechanical properties are discussed. Furthermore, both the latest progresses in design of GS materials prepared by SMAT and some other research interests are displayed.

Nanoscale microstructural analysis and evaluation of mechanical properties were conducted on severely cold-rolled aluminum alloys. In order to examine the effect of alloying elements on microstructure and mechanical properties, Al–Cu–Mg, Al–Mg–Si and Al–Zn–Mg–Cu alloy sheets were prepared for systematic investigation. The SAXS and SANS were used to analyze the nanoscale microstructures in solution-treated and rolled samples of Al–Cu–Mg and Al–Zn–Mg–Cu alloys, and the results quantitatively revealed that nanoscale clusters are formed regardless of with or without rolling. HR-TEM, HAADF-STEM, thermal analysis, hardness and electrical conductivity measurements also suggested that the clusters are formed in cold-rolled samples. Mechanical property evaluations showed that strength generally increased, and ductility decreased with increasing cold-rolling reduction. The strength tended to increase with increasing solute content regardless of the alloy system.

This Paper was Originally Published in Japanese in J. JILM 71 (2021) 562–568.

Processing by tube high-pressure shearing (t-HPS) is a relatively new technique now in the early phase of development within the family of severe plastic deformation (SPD) by comparison with other established techniques such as Equal-Channel Angular Pressing, High Pressure Torsion and Accumulative Roll Bonding. Nevertheless, the technique has demonstrated already its capability to restructure materials to featured microstructures that not reached by the other processing procedures and, in addition, it is efficient in refining the microstructure in a way that is consistent with the other SPD processes.

The initialization and development of the principle and processing technique of t-HPS is reviewed in this report together with the microstructure and property improvements of the processed materials. It is demonstrated that there is a unique capability of t-HPS in preparing distinctly featured microstructures and producing unprecedented properties of the processed material by comparison with other SPD processes. As a new SPD technique that is currently under development, several open topics are proposed which are significant for obtaining an understanding of the physical characteristics of t-HPS and for further exploiting the potential of t-HPS in establishing unprecedented properties through distinct and unusual microstructures.

Recovery of cold worked metals is associated with a loss of strength due to the reduction of the defect density. However, already in the 1960s analytical models and few experiments suggested that this may not generally be the case and even a hardening rather than a softening might occur. With the availability of severe plastic deformation and deposition techniques this anneal hardening phenomenon has been observed frequently. In this overview we summarize early findings on this topic before focusing on general observations and potential origins, with a special focus to show the similarities for nanostructures across different grain size scales (i.e., structures prepared by deposition and severe plastic deformation techniques, respectively). Comparison of different results indicate a grain size dependent hardening increment that could be additionally affected by segregation or the processing variables. Considering the agreement of the peak hardening temperatures with that for dislocation annihilation at grain boundaries, anneal hardening can be rationalized by the loss of intragranular defects and grain boundary relaxation. Grain boundary diffusivity hence plays a crucial role and particular solutes could amplify the hardening process even further. As anneal hardening already occurs for grain sizes at the micron scale, its effect on properties is even of technological relevance. Beneficial effects on the fatigue strength are evident, but the strain softening of anneal hardened specimens drastically shorten ductility. Nevertheless, some strategies to overcome this adverse effect seem promising to create metallic structures with exceptional combinations of strength and ductility.

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.

A demand for cheap and simple methods of processing of nanocrystalline materials attracted attention to metal forming techniques allowing introduction of large strains into metallic materials. Thanks to that a new scientific field, “Materials by severe plastic deformation”, had been born and became an important segment of material science. High pressure torsion (HPT) is the most effective SPD method in respect of the microstructure refinement and processing costs. Despite attractive mechanical and physical properties demonstrated by HPT-processed materials, this technique cannot be used in industry due to a limited size of processed samples. To overcome this problem, a plenty of SPD methods, allowing processing of large-scale billets, was developed. However, none of them has found any application in industry, except a few cases, which are rather exceptions that confirm the rule. This overview is an attempt to analyze the reasons why industrial players remain reluctant to use SPD in technological processes. For this purpose, the advantages and drawbacks of the High Pressure Torsion Extrusion method of SPD are regarded.

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.

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.

Developing electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of water splitting is very important for electrocatalytic hydrogen production. Very recently, emerging study have demonstrated that materials show some superior HER and OER performances of water splitting after processed by severe plastic deformation (SPD). In this work, recent advances on electrochemical hydrogen production performances of materials by SPD are summarized. Some basic principles of electrocatalytic water splitting are briefly described, and then the development of SPD technology and recent advances on SPDed materials with enhanced hydrogen production properties are comprehensively reviewed. Moreover, future direction on SPDed materials for hydrogen production is also prospected.

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.

Magnets are key materials for the electrification of mobility and also for the generation and transformation of electric energy. Research and development in recent decades lead to high performance magnets, which require a finely tuned microstructure to serve applications with ever increasing requirements. Besides optimizing already known materials and the search on novel material combinations, an increasing interest in unconventional processing techniques and the utilization of magnetic concepts is apparent. Severe plastic deformation (SPD), in particular by high-pressure torsion (HPT) is a versatile and suitable method to manufacture microstructures not attained so far, but entitling different magnetic coupling mechanisms fostering magnetic properties. In this work, we review recent achievements obtained by HPT on soft and hard magnetic materials, focusing on powder as starting materials. Furthermore, we give specific attention to the formation of magnetic composites and highlight the opportunities of powder starting materials for HPT to exploit magnetic interaction mechanisms.

High entropy alloys (HEAs) are a novel class of materials that have emerged as potential candidates for various industrial applications due to their excellent mechanical properties at cryogenic, ambient, elevated temperatures, and even under a hydrogen environment. The incorporation of nanocrystalline (nc) structure into HEAs has attracted significant attention for the further enhancement of their exceptional properties, as exceptional grain refinement usually results in enhanced strength without a large expense of ductility. High-pressure torsion (HPT) is often considered one of the most efficient methods for nanocrystallization, and this also holds true for HEAs. Recently, nanoindentation technique has been widely utilized to explore the relationship between HPT-induced grain refinement and mechanical behavior due to the inhomogeneous microstructure within the HPT disk. In this report, recent nanoindentation studies performed on HPT-processed HEAs are comprehensively reviewed with special emphasis on the nanomechanical behavior of nc HEAs.

The creep behaviour and properties of nanostructured materials are attributed to their operating deformation mechanisms, which could be different from those in their coarse-grained counterparts. Accordingly, in this review, recent progress on the creep behaviour of nanostructured materials will be described. The results of large sets of tensile creep tests on selected more complex metallic materials are analysed for evaluating the effect of different SPD processing methods on creep resistance at high temperatures. The resultant creep characteristics are compared with those attained in unprocessed conditions of the same materials. By contrast to the creep behaviour of UFG pure metals SPD processing of more complex materials mostly exhibit no essentially improved creep resistance. Evaluated stress dependences of the creep rate and the creep life suggest that creep deformation mechanisms in UFG materials are similar to those operating in coarse-grained materials. However, creep mechanisms in SPD processed materials are not clearly resolved and this is due to complexity of phenomenon and very small number of studies that have been carried out before now.

Severe plastic deformation (SPD) processes are excellent processing methods that can refine grains, introduce crystal defects and improve mechanical properties and functionality. Among them, high-pressure torsion (HPT) was used for the preparation of hydrogen storage materials in recent years owing to the fact that HPT can process powder materials and introduce greater plastic deformation. HPT technology is not only an effective method to prepare bulk samples from powders, but also a brilliant route to enhance the kinetics, activation, air resistivity and long-term stability of hydrogen storage materials. In addition, new alloys, intermetallics, composites, and hydrogen storage materials even in the immiscible Mg-based alloy systems can be successfully synthesized through the HPT process. In this review, different SPD methods and HPT technology applied for different hydrogen storage materials are carefully reviewed.

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.

Phase transformations among the parent (B2), intermediate (I), and rhombohedral (R) phases were systematically investigated in Ti_50−xNi_47+xFe₃ (x = 0.0–1.0) alloys. In the Differential Scanning Calorimetry (DSC) curves of alloys with x = 0.40 to 0.80, broad peaks due to the B2/I phase transformation were detected at about 200 K. The B2/R transformation temperature and entropy change gradually decrease with increasing x, but drastically decline by the appearance of I phase at around x = 0.40. The existence of the R phase at low temperatures was confirmed by in situ XRD measurements for x = 0.00, 0.35, and 0.40. However, by in situ TEM observation, while the R phase exhibits an ordinary twin-boundary microstructure for x = 0.00 and 0.35, it has a fine cluster-like microstructure without twin boundaries for x = 0.40. Thus, the microstructure and phase stability of the R phase is significantly affected by the appearance of the I phase.

The temperature dependence of hardness in hypereutectoid steels with various microstructures was measured by using the small ball rebound hardness test, and the effect of carbon state on the high-temperature hardness was discussed. As-received and spheroidized cementite steels consisted of spheroidized cementite, graphite and ferrite matrix, and the volume fraction of graphite in the spheroidized cementite steel was larger than that in the as-received steel. Pearlite steel had ferrite and cementite lamellar structure without graphite. The characteristic hardening was detected above 700 K in all steels, suggesting that the solid solute carbon enhances the hardness even in hypereutectoid steels with high strength. The pearlite steel exhibited the highest hardness owing to the ferrite and cementite lamellar structure and no graphite. While the spheroidized cementite steel exhibited the lowest hardness, although the size of cementite was finer than that in the as-received steel. It was quantitatively demonstrated that the large volume fraction of graphite caused the low hardness in the spheroidized cementite steel.

Yield surfaces of A5052 aluminum alloy sheets with different tempers are modeled by the simplified identification method using the circumscribing polygon. In this method, a polygon circumscribing the equal plastic work contour is determined by uniaxial tensile, equal biaxial tensile and plane strain tensile tests. Anisotropic yield surfaces of A5052 aluminum alloy sheet are modeled by Yld2000-2d (Barlat et al., 2003) and Yld2004-18p (Barlat et al., 2005) yield functions. Both yield functions can express the inscribed curves of the polygons. The modeled yield surfaces of A5052-O agree with the stress points of the equal plastic work contours measured by the reliable bi-axial tensile tests. The proposed method to identify the yield function is effective for aluminum sheet metal. On the other hand, the two identified yield functions show different in-plane plastic anisotropy in the tension-compression combined stress state. To examine the suitability of identified models, the experiments and numerical analyses of the deep drawing tests are carried out. Comparing the experimental and analyzed results, the predicted ear height of drawn cup using the Yld2004-18p yield function agree with experimental results qualitatively. But the prediction of the ear using the Yld2000-2d cannot express the tendency of the experimental one.

This Paper was Originally Published in Japanese in KEIKINZOKU (Journal of JILM) 72 (2022) 403–410. Table A1 was slightly modified.

The macroscopic creep properties of negative electrodes in lithium-ion batteries and their estimation methods have been investigated based on the microscopic structure of the electrode. Tensile and creep tests were conducted on a negative electrode consisting of carbon powder and polyvinylidene fluoride (PVDF) binder. The stress-strain curve, the time history of the tensile strain, and the creep rupture time were measured in these tests and estimated using the simple model proposed in this study. The proposed model approximates the alignment of carbon particles as body-centered cubic (bcc) or face-centered cubic (fcc). The external load on the model was supported by a PVDF binder located between carbon particles. The test results showed that PVDF binder mechanical properties affect the macroscopic mechanical properties of the negative electrode, including the creep properties. The stress-strain curve and time history of the tensile strain were located between the upper and lower limits of the proposed model. The tensile strength and creep rupture time agree with the lower limit of the proposed model.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 71 (2022) 989–996.

The uniaxial compressive deformation characteristics of single-phase Nb₂Co₇ were investigated by an electron backscatter diffraction analysis focused on microstructural evolution and kink formation in order to determine whether this monoclinic intermetallic phase is a novel crystalline “mille-feuille” structured (MFS) material. During uniaxial compressive deformation, Nb₂Co₇ does not behave brittle but shows high plasticity. In some microstructure regions, kink-like structures are observed, showing no delamination. In kink-free regions, the frequency of boundaries with rotation angles of 60°, 120°, and 180°, which correspond to changes in the monoclinic stacking vector of Nb₂Co₇ layers between adjacent close-packed (CP) layers, increases significantly after the compression test. The interfaces in the kink-like structures are low-angle boundaries with rotation axes within basal (001) planes. The rotation angles and axes of interfaces in the kink-like structures take various values, suggesting that the origin of the kink-like structures is not twinning, but in fact, these structures are a result of deformation kinking. Deformation of single-phase Nb₂Co₇ is considered to occur due to dislocation glide on basal (001) planes and kink formation, which is regarded as playing an indispensable role in the plasticity of Nb₂Co₇ during compression. It can therefore be concluded that Nb₂Co₇ is a novel crystalline MFS material.

This study coated CrN films onto oxynitriding-treated Vanadis 23 high-speed steel using the DC magnetron sputtering process of the PVD technique. The experimental parameters include various deposition temperatures (275, 300, 325, and 350°C), a bias of −25 V, a power of 100 W, a gas flow rate of 45/30 (Ar/N₂) sccm, and a deposition time of 2.5 h. The research results show that Vanadis 23 high-speed steel formed an effective oxynitriding layer with a depth of about 50 µm after the oxynitriding treatment, and the surface hardness increased to 1100 HV_0.05. Furthermore, when the coatings were deposited at 325°C, the CrN coatings possessed an obvious columnar crystal structure, the highest hardness (13.4 GPa), and the highest elastic modulus (159.7 GPa). In addition, the CrN coating had the best wear properties (the lowest specific wear rates were 1.07 × 10⁻⁶ and 1.33 × 10⁻⁶ mm³·m⁻¹·N⁻¹ under the loads of 2 N and 4 N, respectively) and good corrosion resistance (corrosion current was 8.90 × 10⁻⁵ A·cm⁻², and polarization resistance was 822.11 Ω·cm² in a 3.5 mass% NaCl solution).

Porous metals, which include small pores inside metals, are promising materials due to their material and structural characteristics. Although they generally exhibit low strength because the pores behave as defects, porous metals are expected to achieve high specific strength due to their ultra-lightweight characteristic. This paper deals with a feasibility study on the fabrication of porous steels for developing unique metals with a high specific strength. Porous steels were fabricated via powder metallurgy-based space holder technique. Alloy tool steel, SKD11, and sodium chloride, NaCl, were used as a scaffold metal and spacer material, respectively. Mixed powders of SKD11 and NaCl were sintered via the spark plasma sintering technique. Each sintered compact was re-heated in an argon atmosphere to remove NaCl and densify the scaffold in the compact. Then, each compact was quenched and tempered. As a result, open-cell porous steels with porosities of 60% and 70% were successfully fabricated. The heat treatment refined the microstructure of the scaffold without changing the pore shape, porosity, etc., improving their strength property, irrespective of their porosity. Furthermore, the specific proof strength of heat-treated porous steels was comparable to that of dense pure aluminum.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 71 (2022) 969–975.

In this study, high-entropy alloys (HEAs) with Cu as the main constituent element were investigated, focusing on the distribution of Cu in the ingots. Based on the taxonomy of HEAs, those with Cu as the casting material were classified as (1) HEAs whose main constituent elements were 3d transition metals, such as Co, Cr, Fe, Mn, Ni, and Cu (3d-HEAs), and (2) high-entropy (HE) brasses based on the Cu–Zn alloy system and HE bronzes based on Cu–Sn and/or Cu–Al alloy systems. In the case of 3d-HEAs with Cu, the distribution of Cu in the ingots exhibited the following tendency: (1-1) segregation from the dendrite to the residual liquid, resulting in the formation of Cu-rich interdendritic regions in the ingots; (1-2) liquid-phase separation resulting in the formation of a Cu-rich liquid, which formed a macroscopically phase-separated structure; and (1-3) the dispersion of fine Cu precipitates embedded in the solid solution matrix.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 60 (2021) 167–175. Some figures were corrected for the revision from gray-contrast images to color-contrast images. Reference 50 and Fig. 4 were slightly modified.

An immiscible alloy with an amorphous phase, Al–Co–La–Bi, was designed by the combination of the empirical alloy parameters of the mixing entropy, the predicted ground state diagram constructed by Materials Project for the database of ab initio calculations, and thermodynamic calculations using FactSage software and FTlite database. The solidification microstructure of rapidly solidified melt-spun ribbons of Al–Co–La–Bi alloy was investigated focusing on the distribution of Bi and liquid phase separation behavior. Liquid-phase separation and the formation of an amorphous phase occurred simultaneously in the Al–Co–La–Bi alloy, resulting in the formation of Bi–La intermetallic globules embedded in the Al–Co–La-based amorphous matrix. STEM observation clarified the microstructure of oxygen-enriched globules with double shell layers in the Al–Co–La–Bi alloy.