A CoCrFeNiMn high-entropy alloy (HEA) with an addition of 2 at.% Ti was processed by high-pressure torsion to produce a grain size of ~30 nm and then tested in tension at elevated temperatures from 873 to 1073 K using strain rates 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 830% at a temperature of 973 K. It is shown that the Ti addition contributes to the formation of precipitates and, combined with the sluggish diffusion in the HEA, grain growth is inhibited to provide a reasonable stability in the fine-grained structure at elevated temperatures. By comparison with the conventional CoCrFeNiMn HEA, the results demonstrate that the addition of a minor amount of Ti produces a smaller grain size, a higher volume fraction of precipitates and a significant improvement in the superplastic properties.

A CoCrFeNiMnTi 0.1 high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) followed by post-deformation annealing (PDA) at 200-900 °C. Microstructural evaluations revealed that the initial and HPT-processed microstructures consisted of a single fcc phase and there was no evidence for decomposition during severe plastic deformation. However, PDA at temperatures below 900 °C promoted the formation of a multi-phase microstructure containing new precipitates and significant grain coarsening occurred after PDA at >800 °C due to a dissolution of the precipitates. PDA at 800 °C for 60 min led to very good mechanical properties with an ultimate tensile strength (UTS) and elongation to failure of >1000 MPa and ~40%, respectively. The results demonstrate that the minor addition of Ti to the CoCrFeNiMn alloy has no direct effect on the strengthening mechanisms but nevertheless this addition significantly increases the thermal stability of the precipitates and these precipitates are effective in minimizing grain coarsening. Therefore, the Ti addition plays an important role in strengthening the HEA.

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