Effect of Zener–Hollomon parameter on hot deformation behavior of CoCrFeMnNiC0.5 high entropy alloy

Abstract

Hot compressive deformation mechanism of the carbon-contained face-centered cubic CoCrFeMnNi high-entropy alloy (HEA) was investigated in the temperature range between 700 °C and 1000 °C and in the strain rate range between 0.001 and 1 s−1. The constitutive equation of CoCrFeMnNiC0.5 (at.%) alloy was obtained, which can predicate the flow stresses accurately. The apparent activation energy (Q) was calculated as 362 kJ/mol, suggesting that the carbon addition cause the work hardening during hot deformation. The deformation mechanisms at various Zener–Hollomon parameter values have been discussed. The Zener-Hollomon parameter (lnZ) of CoCrFeMnNiC0.5 alloy deformed at 700 °C increases with the increasing of the strain and strain rate. At low Zener-Hollomon condition (lnZ≤40), density dislocation walls (DDWs) caused the obvious work hardening by producing a long-range back stress. With the lnZ increasing, the addition of carbon in solid solution effectively reduces the dislocation cross-slip, and causes the transition from well-development DDWs to microbands (MBs). MBs can be regarded as another imported deformation mode to provide additional work-hardening source. At high Zener-Hollomon condition (lnZ>46), the pronounced effect of MBs results in slight increasing of the flow stress after steady flow. The Zener-Hollomon parameter of CoCrFeMnNiC0.5 HEA deformed at ε = 0.8 decreases with the temperature increasing and strain rate decreasing. Discontinuous dynamic recrystallization (DDRX) is the dominant microstructural evolution mechanism and leads the flow softening at low lnZ condition (lnZ≤40). The DDRX nucleation is attributed to the microbands and pinning effect on the dislocation movement induced by M23C6 carbides, which can lead to local grain boundary expansion.