Evidence for superplasticity in a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

A CoCrFeNiMn high-entropy alloy was processed by high-pressure torsion to produce a grain size of similar to 10 nm and then tested in tension at elevated temperatures from 773 to 1073 K using strain rates in the range from 1.0x10(-3) to 1.0x10(-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.