The study of the fundamentals of the α → ω and β → ω phase transformations induced by high-pressure torsion (HPT) in Ti–Nb-based alloys is presented in the current work. Prior to HPT, three alloys with 5, 10, and 20 wt% of Nb were annealed in the temperature range of 700–540°C in order to obtain the (α + β)-phase state with a different amount of the β-phase. The samples were annealed for a long time in order to reach equilibrium Nb content in the α-solid solution. Scanning electron microscope (SEM), transmission electron microscopy, and X-ray diffraction techniques were used for the characterization of the microstructure evolution and phase transformations. HPT results in a strong grain refinement of the microstructure, a partial transformation of the α-phase into the ω-phase, and a complete β → ω phase transformation. Two kinds of the ω-phase with different chemical compositions were observed after HPT. The first one was formed from the β-phase, enriched in Nb, and the second one from the almost Nb-pure α-phase. It was found that the α → ω phase transformation depends on the Nb content in the initial α-Ti phase. The less the amount of Nb in the α-phase, the more the amount of the α-phase is transformed into the ω-phase.

High-entropy composites (HECs) were subjected to severe straining by high-pressure torsion (HPT) to evaluate their influence on the evolution of microstructure and deformation behavior. Severe straining leads to a homogeneously strained microstructure and inhomogeneous micro-shear bands in these HECs. Nb addition in HECs varies the microstructure from single phase to eutectic, and the Vickers microhardness in HPT HECs increases to 7.45 GPa. Nb addition up to x = 0.80 in as-cast HECs improves the strength of these materials at the expense of its plasticity. Nevertheless, severe straining provides a better combination of strength and ductility without sacrificing its plasticity. Such improvement in properties is attributed to the evolved microstructural features, formation of “transformation-shear bands (T-SBs)” and “deformation-shear bands (D-SBs)” at severe straining. This assures the homogeneous deformation by shear banding and suggests that shear banding is the dominant deformation mechanism when the lamellar spacing becomes saturated upon severe straining.

This report summarizes a recent study demonstrating simple and rapid synthesis of a new Al–Mg alloy system and ultimately synthesizing a metal matrix nanocomposite, which was achieved by processing stacked disks of the two dissimilar metals by conventional high-pressure torsion (HPT) processing. The synthesized Al–Mg alloy system exhibits exceptionally high hardness through rapid diffusion bonding and simultaneous nucleation of intermetallic phases with increased numbers of HPT turns through 20, and improved plasticity was demonstrated by increasing strain rate sensitivity in the alloy system after post-deformation annealing. An additional experiment demonstrated that the alternate stacking of high numbers of dissimilar metal disks may produce a faster metal mixture during HPT. Metal combinations of Al–Cu, Al–Fe, and Al–Ti were processed by the same HPT procedure from separate pure metals to examine the feasibility of the processing technique. The microstructural analysis confirmed the capability of HPT for the formation of heterostructures across the disk diameters in these processed alloy systems. The HPT processing demonstrates a considerable potential for the joining and bonding of dissimilar metals at room temperature and the expeditious fabrication of a wide range of new metal systems.

The Al–Mg–Sc alloys have become important materials in research conducted on superplasticity in aluminum-based alloys. Many results are now available and this provides an opportunity to examine the consistency of these data and also to make a direct comparison with the predicted rate of flow in conventional superplasticity. Accordingly, all available data were tabulated with divisions according to whether the samples were prepared without processing using severe plastic deformation (SPD) techniques or they were processed using the SPD procedures of equal-channel angular pressing or high-pressure torsion or they were obtained from friction stir processing. It is shown that all results are mutually consistent, the measured superplastic strain rates have no clear dependence on the precise chemical compositions of the alloys, and there is a general agreement, to within less than one order of magnitude of strain rate, with the theoretical prediction for superplastic flow in conventional materials.

Al–Sn binary alloys are fabricated by powder consolidation using high-pressure torsion (HPT). The HPT-processed samples are immersed in pure water and hydrogen generation behavior is investigated with respect to the imposed strain through the HPT processing at a selected temperature in the range of 297–333 K. Microstructures of HPT-processed alloys are analyzed by x-ray diffraction, transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and electron back scattered diffraction (EBSD) analysis. Results show that it is important to add more than 60 wt% of Sn to activate hydrogen generation from the Al–Sn alloys in pure water. TEM and EBSD images reveal significant grain refinement while EPMA results exhibit homogenous distribution of elements achieved by HPT. The grain refinement and distribution of elements attained by HPT processing influence greatly the hydrogen generation rate and yield of the alloys. An Al–80 wt% Sn alloy with an average grain size of ∼270 nm exhibits the highest hydrogen yield and generation rate in pure water at 333 K.

Disks of commercial Al-1050 and ZK60A alloys were stacked together and then processed by conventional high-pressure torsion (HPT) through 1 and 5 turns at room temperature to investigate the synthesis of an Al–Mg alloy system. Measurements of microhardness and observations of the microstructures and local compositions after processing through 5 turns revealed the formation of an ultrafine multi-layered structure in the central region of the disk but with an intermetallic β-Al3Mg2 phase in the form of nano-layers in the nanostructured Al matrix near the edge of the disk. The activation energy for diffusion bonding of the Al and Mg phases was estimated and it is shown that this value is low and consistent with surface diffusion due to the very high density of vacancy-type defects introduced by HPT processing. The results demonstrate a significant potential for making use of HPT processing in the preparation of new alloy systems.

A CoCrFeNiMn high-entropy alloy (HEA), in the form of a face-centered cubic (fcc) solid solution, was processed by high-pressure torsion (HPT) to produce a nanocrystalline (nc) HEA. Significant grain refinement was achieved from the very early stage of HPT through 1/4 turn and an nc structure with an average grain size of ∼40 nm was successfully attained after 2 turns. The feasibility of significant microstructural changes was attributed to the occurrence of accelerated atomic diffusivity under the torsional stress during HPT. Nanoindentation experiments showed that the hardness increased significantly in the nc HEA during HPT processing and this was associated with additional grain refinement. The estimated values of the strain-rate sensitivity were maintained reasonably constant from the as-cast condition to the nc alloy after HPT through 2 turns, thereby demonstrating a preservation of plasticity in the HEA. In addition, a calculation of the activation volume suggested that the grain boundaries play an important role in the plastic deformation of the nc HEA where the flow mechanism is consistent with other nc metals. Transmission electron microscopy showed that, unlike conventional fcc nc metals, the nc HEA exhibits excellent microstructural stability under severe stress conditions.

The paradox of strength and ductility is now well established and denotes the difficulty of simultaneously achieving both high strength and high ductility. This paradox was critically examined using a cast Al–7%Si alloy processed by high-pressure torsion (HPT) for up to 10 turns at a temperature of either 298 or 445 K. This processing reduces the grain size to a minimum of ∼0.4 μm and also decreases the average size of the Si particles. The results show that samples processed to high numbers of HPT turns exhibit both high strength and high ductility when tested at relatively low strain rates and the strain rate sensitivity under these conditions is ∼0.14 which suggests that flow occurs by some limited grain boundary sliding and crystallographic slip. The results are also displayed on the traditional diagram for strength and ductility and they demonstrate the potential for achieving high strength and high ductility by increasing the number of turns in HPT.

To provide insight into the influence of the length scale on the kinetics of phase evolution during severe plastic deformation, we studied the microstructure evolution of cryomilled Al and Ti mixture, which is further subjected to high-pressure torsion (HPT). The cryomilled microstructure consisted of elemental Al and Ti, and the subsequent HPT deformation at ambient temperature led to the solid state formation of Al-rich intermetallics. X-ray diffraction peaks originating from TiAl2 and TiAl3 were observed after one revolution of HPT, suggesting a shear strain-assisted formation of the intermetallics. A high resolution transmission electron microscope confirmed the formation of TiAl2 following HPT for one revolution. Further HPT straining led to microstructure refinement and a mixing of the Ti and Al, as well as of any phases formed initially. The solid state formation of the intermetallics and the overall evolution of the microstructure are discussed based on the generation of a high density of lattice defects that evolve under the strain conditions present during HPT.