Effect of a minor titanium addition on the superplastic properties of a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

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.