Processing map for controlling microstructure and unraveling various deformation mechanisms during hot working of CoCrFeMnNi high entropy alloy

Abstract

In the current study, the hot deformation characteristics and workability of a CoCrFeMnNi high entropy alloy was characterized using processing maps developed on the basis of dynamic materials model in the temperature range 1023–1423 K and strain rate range 10−3 – 10s−1. The processing map delineated various deterministic domains including those of cracking processes and unstable flow, thus enabling identification of a ‘safe’ processing window for the hot working of the alloy. Accordingly, a deterministic domain in the temperature and strain rate ranges of 1223–1373 K and 10−2 – 5 × 10−1s−1, respectively, was identified to be the domain of dynamic recrystallization (DRX) with a peak efficiency of the order of ~34% at 1293 K and 3 × 10−2s−1 and these were considered to be the optimum parameters for hot deformation. The DRX grain size was dependent on the deformation temperature and strain rate, increasing with the increase in temperature and decrease in strain rate, whereas DRX volume increased with the strain rate. At still higher temperatures (1403–1423 K) and lower strain rates (10−3 – 3 × 10−3s−1), there was a sharp decrease in efficiency values from 27% to 5% thus forming a trough and the microstructure was characterized with coarse grains. In the instability regime, grain boundary cracking/sliding and localized shear bands manifested at temperatures <1223 K and strain rates <10−2s−1. The increase in strain rate resulted in an intense adiabatic shear banding along with formation of voids. At 10s−1 and temperatures >1398 K, microstructural reconstitution occurred in the shear bands leading to the formation of fine grains, presumably as a consequence of continuous recrystallization.