Ultra-severe plastic deformation for room-temperature superplasticity and superfunctionality

. Ultra-severe plastic deformation (ulta-SPD) is a terminology used for the introduction of extremely large shear strains (over 1000) to material so that the thickness of sheared phases geometrically reaches the subnanometer level. Under such extreme shearing conditions, new nanostructured phases with unique properties are formed even from the immiscible systems. Various metallic alloys and ceramics were developed by this concept for different applications such as room-temperature superplasticity, room-temperature hydrogen storage, photocatalytic hydrogen production, photocatalytic carbon dioxide conversion, etc. This article reviews recent advances regarding ultra-SPD with a focus on low-temperature superplasticity, which was reported for the first time at room temperature in aluminum and magnesium alloys.


Introduction
Severe plastic deformation (SPD) is a popular technology to introduce large plastic strain in a workpiece [1,2]. There are various SPD methods such as high-pressure torsion (HPT), equalchannel angular pressing (ECAP) and accumulative roll-bonding (ARB) [3,4]. The main target to employ SPD is the generation of ultrafine grains (UFG) [5,6]. Formation of such UFG structures leads to strengthening and enhanced functional properties [7,8]. A review paper written by 49 experts in SPD referred to these SPD-processed UFG materials as superfunctional materials due to their excellent functional properties [9]. SPD has been used since ancient times, but its scientific principles were documented by Bridgman in the 1930s [10,11], Segal et al. in the 1970s [12], and Valiev et al. in the 1980s [13]. For the latest progress in the SPD field, the readers are referred to review papers published in a special issue of Materials Transactions [14].
Ultra-SPD is one of the new directions which is getting attention to synthesize new materials with promising properties [15]. This manuscript reviews recent advances in the application of ultra-SPD with a focus on room-temperature superplasticity, which could be realized for the first time in aluminum and magnesium alloys by ultra-SPD [16,17].

Main Findings by Ultra-Severe Plastic Deformation
Ultra-SPD is promising to synthesize a variety of materials including conventional binary/ternary alloys [15], intermetallic compounds [18], high-entropy alloys [18], hydrides [58], and highentropy ceramics [59]. Such materials show promising properties like high strength, roomtemperature superplasticity, hydrogen storage, biocompatibility, superconductivity, thermal stability, and photocatalysis, as reviewed in [60]. A summary of materials processed by ultra-SPD and their characteristics are given in Table 1  . These properties are briefly summarized below, but room-temperature superplasticity is discussed in the next section.
SPD-processed materials usually show low thermal stability due to a large fraction of defects [89][90][91][92][93], but the materials processed by ultra-SPD can have high thermal stability. For example, the application of ultra-SPD to produce supersaturated Al-Ca [69], Al-Fe [70], Al-Zr [75], and Al-La-Ce [76] alloys, leads not only to high thermal stability but also to age hardening.
Orthopedic materials should have biocompatibility, high strength and low elastic modulus. Tibased biomaterials suffer from low strength and high modulus [97], but SPD can improve their strength [98][99][100][101]. Ultra-SPD was successfully used to synthesize alloys with a good combination of high strength and low elastic modulus from the Ti-Zr-Hf-Nb-Ta biocompatible system [79,80].
Nanostructured Nb-Ti superconductors are commercially fabricated by several repetitions of wire drawing and long-time annealing [102][103][104][105]. Ultra-SPD followed by short annealing is a fast method to produce such superconductors with properties comparable with commercial ones [88].
The design and synthesis of magnesium alloys that can reversibly store hydrogen at room temperature is a long-term challenge [106,107]. Ultra-SPD not only contributed to the synthesis of various hydrogen-storing compounds such as Mg2X intermetallics (X: other elements) [61], immiscible Mg-Ti [62,68], Mg-Zr [63] and Mg-Hf [66] alloys, ternary Mg-V-Cr alloy [65] and high-entropy MgVTiCrFe alloy [67], but also it introduced Mg4Ni1pd as the first Mg-based material with reversible hydrogen storage performance at room temperature [66].
Ultra-SPD introduced the first high-entropy photocatalysts for hydrogen and oxygen production from water splitting as well as for CO2 conversion. The high-entropy oxides TiZrHfNbTaO11 [81,82] and TiZrNbTaWO12 [85] and the high-entropy oxynitride TiZrHfNbTaO6N3 [83,84] are the first high-entropy photocatalysts with high activity.

Room-Temperature Superplasticity
Superplasticity is defined as the capability of a material for large tensile deformations over 400%. Such elongations can be achieved at homologous temperatures over 0.5, where grain-boundary sliding is dominant. Superplasticity is quantified by the creep equation with a strain rate sensitivity of ~0.5 [108]. The equation suggests two solutions to achieve low-temperature superplasticity: (i) grain size reduction which was used to attain low-temperature superplasticity by SPD processing [109][110][111]; and (ii) acceleration of grain boundary diffusion by grain boundary engineering, which was used to attain room-temperature superplasticity in Mg-Li and Al-Zn by ultra-SPD [16,17].
The application of ultra-SPD (200 HPT turns) to a two-phase Mg-8wt.%Li alloy (Mg-rich α phase + Li-rich β) resulted in 440% elongation at room temperature (Fig. 2a) with a strain-rate sensitivity of 0.37 (Fig. 2b) [16]. However, the extrusion and SPD process for 5 HPT turns did not lead to such an elongation in agreement with earlier studies [112]. Superplastic deformation after ultra-SPD is promising because room temperature corresponds to a homologous temperature of 0.35 for the alloy. Room-temperature superplasticity occurs due to fast grain-boundary diffusion by increasing the fraction of α/β interphase boundaries and the segregation of lithium along the α/α grain boundaries, as shown in Fig. 2d using high-angle annular dark-field (HAADF) image and in Figs. 2d and 2e using energy-filtered transmission electron microscopy (EFTEM).
Ultra-SPD (200 HPT turns) was applied to an Al-30at%Zn alloy with two phases of Al-rich α and Zn-rich η [17]. The tensile behavior of the material was examined after ultra-SPD and after 100 days of aging at room temperature. As shown in Fig. 3a, the homogenized sample shows limited plasticity, but elongation is 480% after ultra-SPD and 330% after aging. It should be noted that this alloy did not show room-temperature superplasticity after SPD using 20 HPT turns [113]. The strain-rate sensitivity for ultra-SPD-processed samples reached a high value of 0.41, as shown in Fig. 3b. Superplasticity after ultra-SPD at room temperature (i.e. the homologous temperature of 0.36) is due to fast grain-boundary diffusion by increasing the fraction of α/η. interphase boundaries and the segregation of zinc along the α/α grain boundaries, as shown in Fig. 3c and 3d using HAADF image and in Figs. 3e and 3f using energy-dispersive X-ray spectroscopy (EDS).

Summary
The concept of ultra-SPD is effective to develop various superfunctional materials for energy, electrical and biomedical applications. One of the interesting results achieved by ultra-SPD is the development of first room-temperature-superplastic aluminum and magnesium alloys.