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Impurity atoms have a significant effect on the strength of metals processed by severe plastic deformation (SPD), but their strengthening mechanism is still under argument. To gain an insight into the strengthening mechanism, iron samples with different purity levels such as 99.96% (IF steel), 99.94% (Armco steel), 99.88% and 97.78% and with different initial states (bulk, powder and ball-milled) were processed by high-pressure torsion (HPT). The steady-state hardness and tensile strength for the materials with the micrometer and submicrometer grain sizes reasonably followed the Hall-Petch relationships reported earlier for pure iron and mild steels. However, the nanograined materials followed an inverse Hall-Petch relationship. It was shown that the occurrence of softening by the inverse Hall-Petch effect can be significantly avoided by stabilizing the grain boundaries using carbon atoms. These findings indicate that the extra hardening by impurity atoms is mainly due to the grain-boundary strengthening mechanism.

Despite superior properties of ultrafine-grained (UFG) materials processed by severe plastic deformation (SPD), their thermal stability is a concern because of the supersaturated fractions of lattice defects. In this study, the microstructural stability of various UFG materials (2 alloys and 15 pure metals) after SPD processing through the high-pressure torsion (HPT) were investigated at room temperature for up to 10 years. While most of the metals with high melting temperatures remained stable, a softening by self-annealing occurred in pure silver, gold and copper (with moderate melting temperatures), and an unusual hardening occurred in pure magnesium, Al-Zn alloy and Mg-Li alloy (with low melting temperatures). These softening/hardening behaviors by grain coarsening were attributed to the contribution of grain boundaries to dislocation activity or grain-boundary sliding, respectively. It was shown that the self-annealing was accelerated by increasing the processing pressure and strain and by decreasing the processing temperature and stacking fault energy, due to the enhancement of stored energy and/or atomic mobility.

High-entropy alloys (HEAs), a novel type of materials with high configurational entropy, have aroused a huge interest due to a promising range of functional properties including biocompatibility. In this study, the high-pressure torsion (HPT) method was implemented as a mechanical alloying route to synthesize biocompatible nanostructured HEAs with the bcc structure. An equiatomic quinary TiNbZrTaHf HEA was successfully synthesized via the HPT method and its characteristics were compared with the binary TiNb, ternary TiNbZr and quaternary TiNbZrTa alloys to have an insight into the effect of configurational entropy on microstructure and mechanical properties of these biomaterials. The grain size decreased, the strain-rate sensitivity reduced, and the hardness increased with increasing the number of principal elements from 2 to 3, but these variations became less significant with further increase in the configurational entropy. Small nanograins, solid solution hardening, dislocation activity together with high entropy effect in the HEA led to a high hardness of 564 Hv and a moderate elastic modulus of 79 GPa which are promising mechanical properties for biomedical applications.

An age-hardenable A2024 alloy is processed by high-pressure torsion (HPT) for grain refinement and further aged for fine precipitation. The HPT is conducted under an applied pressure of 6 GPa for 0.75, 1 and 5 turns with a rotation speed of 1 rpm at room temperature and this results in a significant grain size reduction to a grain size of ~ 240 ± 80 nm. The hardness sharply increases with imposing strain at an early stage but level off after 5 turns. Further aging at temperatures of 373 K and 423 K leads to extra hardening above the elevated hardness of the HPT-processed condition. Components contributing to the strengthening were evaluated in terms of grain refinement and fine precipitation including the contributions from dislocation accumulation and solid solution. In this study, a conclusion is reached such that simultaneous strengthening due to grain refinement and fine precipitation is achieved by application of HPT processing and subsequent aging.

Severe plastic deformation through the high-pressure torsion (HPT) method was applied to pure aluminum with a wide range of purity levels such as 99% (A1100), 99.5% (A1050), 99.99% (4NAl), 99.999% (5NAl) and 99.9999% (6NAl). The hardness of 6NAl decreased with straining and saturated to a level below the hardness level of the annealed sample. This softening behavior, which was similar to the behavior of metals with low melting temperatures such as indium, tin, lead and zinc, was not observed in 5NAl or less pure Al. It was found that the grain-size dependence of hardness became less significant with increasing the purity level, while the HPT-processed 6NAl followed an inverse Hall-Petch relationship. In 6NAl with large grain sizes, dislocations accumulated in the grains in the form of dislocation cells and enhanced the hardness, but when the grain size was small, the dislocations moved fast and disappeared in high-angle grain boundaries.

Titanium-niobium alloys with the bcc structure (β-Ti-Nb) are recently investigated for their potential use in biomedical applications; however, improvement of their mechanical properties (particularly hardness and elastic modulus) is still a challenge. In this study, nanostructured Ti-Nb alloys with different Nb contents were successfully synthesized by mechanical alloying of elemental powders via high-pressure torsion (HPT). The HPT process led to the homogenous mixture of the two elements and the formation of nanograined β phase. Examination of mechanical properties by nanoindentation and microhardness measurements revealed that all synthesized alloys exhibited high hardness and good plasticity; however, the best combination of low elastic modulus and high hardness was obtained for the sample with 25 at% Nb: E = 39 ± 11 GPa (close to the elasticity of human bone) and Hv = 3.7 ± 0.1 GPa (comparable to the hardest Ti-based biomaterials). The current results confirm the potential of HPT to synthesize nanograined Ti-Nb alloys for future biomedical applications.

Recent studies suggested that nanotwinned metallic materials can exhibit a good combination of high strength and high ductility. However, few studies examined the strength and ductility of nanotwinned alloys under hydrogen atmosphere. In this study, nanotwins are introduced in a TWIP (twinning-induced plasticity) steel by application of high-pressure torsion (HPT) followed by annealing. The nanotwinned austenitic TWIP steel exhibited a high tensile strength as ~1.4 GPa but without ductility after hydrogen charging. Unlike nanotwinned alloy, the HPT-processed sample, which experienced a phase transformation to a bimodal martensitic structure, exhibited both high strength (~1.6 GPa) and high uniform ductility (6%) after hydrogen charging. It was concluded that twin boundaries act mainly as crack initiation sites and propagation paths but not as effective barriers for dislocation accumulation to enhance the ductility of the TWIP steels under hydrogen atmosphere.

High-pressure torsion (HPT) method currently receives much attention as a severe plastic deformation (SPD) technique mainly because of the reports of Prof. Ruslan Z. Valiev and his co-workers in 1988. They reported the efficiency of the method in creating ultrafine-grained (UFG) structures with predominantly high-angle grain boundaries, which started the new age of nanoSPD materials with novel properties. The HPT method was first introduced by Prof. Percy W. Bridgman in 1935. Bridgman pioneered application of high torsional shearing stress combined with high hydrostatic pressure to many different kinds of materials such as pure elements, metallic materials, glasses, geological materials (rocks and minerals), biological materials, polymers and many different kinds of organic and inorganic compounds. This paper reviews the findings of Bridgman and his successors from 1935 to 1988 using the HPT method and summarizes their historical importance in recent advancement of materials, properties, phase transformations and HPT machine designs.

A Mg–Li alloy with 8 wt% Li was processed by severe plastic deformation (SPD) through the process of high-pressure torsion (HPT) to achieve ultrafine grains with an average grain size of ~500 nm. Tensile testing with an initial strain rate of 10 −3 s −1 showed that the alloy exhibited superplasticity at a temperature of 323 K or higher. Tensile testing in boiling water confirmed that the specimens were elongated to 350–480% at 373 K under the initial strain rates of 10 −3 s −1 to 1 0 −2 s −1 with a strain rate sensitivity of ~0.3. The current study suggests that not only superplastic forming but also superplastic hydroforming should be feasible after the grain refinement using the HPT method.

An age-hardenable Al–4 wt% Cu alloy is severely deformed using high-pressure torsion (HPT) to refine the microstructure to an average gain size of ~210 nm. High saturation hardness of 205 Hv and high tensile strength of 820 MPa are achieved after the HPT processing. It is shown that the strength of the HPT-processed alloy is further improved by natural aging at room temperature or by artificial aging at 353 K. A peak hardness followed by softening appears within a few days after natural aging and within a few minutes after aging at 353 K, suggesting the low thermal stability of the alloy. Quantitative evaluation of different strengthening mechanisms shows that the grain boundary hardening through the Hall–Petch relationship and the precipitation hardening through the Orowan relationship are dominant strengthening mechanisms.

Several metals were severely deformed at cryogenic temperature in liquid nitrogen and at room temperatures in air using high-pressure torsion (HPT). Extra grain refinement to the nanometer level and extra hardening were achieved after cryogenic-HPT in niobium, which has a high melting temperature. In copper, which has a moderate melting temperature, nanograins formed during cryogenic-HPT but self-annealing, i.e., abnormal softening and grain coarsening to the micrometer level, occurred within a few hours after the cryogenic-HPT. In low-melting-temperature metals such as zinc, magnesium and aluminum, cryogenic-HPT led to extra softening and/or formation of coarser grains because of enhanced static recrystallization. The effect of impurities on grain size, hardness–strain behavior and self-annealing was also studied after cryogenic-HPT.

An Al–30 mol% Zn supersaturated solid solution alloy was severely deformed using high-pressure torsion (HPT) at 300 K and subsequently annealed at 373–673 K. The hardness and tensile strength significantly decreased and the tensile ductility increased with straining by HPT and reached a steady-state level at large imposed strains. Despite this softening behavior, the lattice strain was increased, Zn-rich particles were precipitated and the initial coarse grains were refined significantly to a size of ~190 nm while being accompanied by decomposition to Al- and Zn-rich phases because of rapid atomic diffusion. The subsequent annealing led to a hardening, but microstructural observations showed that decrease in the lattice strain, increase in the grain size and reduction in the fraction of precipitates occurred by annealing. It was shown that the unusual softening/hardening behavior of the Al–Zn alloy was mainly due to the contribution of spinodal decomposition. The formation of nano-sized lamellae by spinodal decomposition resulted in increase in hardness after solution treatment and after post-HPT annealing, while this lamellar structure was destroyed by HPT, which resulted in softening. The softening was less significant when the hardness was evaluated at low homologous temperatures.

Experiments and finite element simulations were conducted to measure the temperature increase in processing disc samples by high-pressure torsion. Aluminum, copper, iron and molybdenum were selected as model materials. The temperature increases at the early stages of straining but saturates to steady-state levels at large strains. The increase of temperature is proportional to the hardness and rotation speed and is higher at higher imposed pressures and is somewhat higher at larger distances from the disc center.

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above ∼4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.

Pure Cu was subjected to severe plastic deformation through high-pressure torsion (HPT) using disc and ring samples. Vickers microhardness was measured across the diameter and it was shown that all hardness values fall well on a unique single curve regardless of the types of the HPT samples when they are plotted against the equivalent strain. The hardness increases with an increase in the equivalent strain at an early stage of straining but levels off and enters into a steady-state where the hardness remains unchanged with further straining. It was confirmed that the tensile strength also follows the same single function of the equivalent strain as the hardness. The elongation to failure as well as the uniform elongation also exhibits a single unique function of the equivalent strain. Transmission electron microscopy showed that a subgrain structure develops at an early stage of straining with individual grains containing dislocations. The subgrain size decreases while the misorientation angle increases and more dislocations are formed within the grains with further straining. In the steady-state range, some grains appear which are free from dislocations, suggesting that recrystallization occurs during or after the HPT process. The mechanism for the grain refinement was discussed in terms of dislocation mobility.