A CoCrFeNiMn high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) under 6.0 GPa pressure up to 10 turns at room temperature. It is shown that there is a gradual evolution in hardness with increasing numbers of turns but full homogeneity is not achieved even after 10 turns. Microhardness measurements reveal that the material reaches a saturation hardness value of ~4.41 GPa and in this condition the microstructure shows exceptional grain refinement with a grain size of ~10 nm. An ultimate strength value of ~1.75 GPa and an elongation to fracture of ~4% were obtained in a sample processed for 5 turns. The nanostructured HEA was subjected to post-deformation annealing (PDA) at 473-1173 K and it is shown that the hardness increases slightly to 773 K due to precipitation and then decreases up to 1173 K due to a combination of recrystallization, grain growth and a dissolution of the precipitates. The formation of brittle precipitates, especially σ-phase, at 873 and 973 K significantly reduces the ductility. Short-term annealing for 10 min at 1073 K prevents grain growth and leads to a combination of high strength and good ductility including an ultimate tensile strength of ~830 MPa and an elongation to failure of ~65%.

A CoCrFeNiMn high-entropy alloy was processed by high-pressure torsion to produce a grain size of ~10 nm and then tested in tension at elevated temperatures from 773 to 1073 K using strain rates in the range from 1.0 × 10 -3 to 1.0 × 10 -1 s -1 . The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of >600% at a testing temperature of 973 K. It is concluded that the formation of precipitates and the sluggish diffusion in the HEA inhibit grain growth and contribute to a reasonable stability of the fine-grained structure at elevated temperatures. The results show the activation energy for flow matches the anticipated value for grain boundary diffusion in nickel but the strain rate sensitivity is low due to the occurrence of some grain growth at these high testing temperatures.

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Experiments were conducted to investigate the processing parameters that may be used to optimize the properties of Ti-6Al-4V alloys. The alloy was initially subjected to two different heat treatments leading to the formation of martensitic αʹ and lamellar α+β microstructures and then both materials were processed by high-pressure torsion (HPT) for 10 turns at room temperature. This gave significant grain refinement to the nanometer range in both conditions and the occurrence of an allotropic hcp to fcc phase transformation in the martensitic alloy.These nanostructured alloys were subjected to post-deformation annealing (PDA) at temperatures in the range of 473 to 1023 K. The results show the hardness increases slightly to 773 K due to αʹ+fcc→α+β+fcc and α→α+β phase transformations in the martensitic αʹ and lamellar α+β alloys and then decreases up to 1023 K due to recrystallization and grain growth.An optimum property of a very high yield strength (~1120 MPa) and ultimate tensile strength (~1200 MPa), together with excellent ductility (elongation to failure of ~26%), was achieved in the Ti-6Al-4V martensitic alloy processed by a combination of HPT followed by PDA at 873 K for 60 min.

Two different initial microstructures, martensitic and lamellar, were developed in a Ti-6Al-4V alloy to examine their effect on the high temperature mechanical properties and superplasticity after high-pressure torsion (HPT). Significant grain refinement was achieved in both conditions with grain sizes after HPT processing of ~30 and ~40 nm, respectively. The nanocrystalline alloy in both conditions was subjected to mechanical testing at 923-1073 K with strain rates in the range from 10-3 to 10-1 s-1. The martensitic and lamellar alloys exhibited excellent ductility at these high temperatures including superplastic elongations at 973 K with maximum elongations of 815% and 690%, respectively. The fcc phase was stable at elevated temperatures in the martensitic alloy and the results suggests the fcc phase may contribute to the superior superplastic properties of the martensitic alloy.

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A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673 K and the results show that the strength increases gradually with increasing straining up to ~1 GPa with an elongation to failure of ~35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~100 nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673-1073 K for 60 min and it is shown that the hardness increases slightly due to precipitation to 773 K and then decreases to 1073 K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle σ-phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773 K produces a very high yield strength of ~1015 MPa and an ultimate strength of ~1080 MPa together with an excellent elongation to failure of ~30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.