Grain growth kinetics, phase evolution during annealing and strain rate sensitivity of TRIP-assisted, dual-phase, non-equiatomic high entropy alloy

Abstract

Transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) belong to a recently developed class of HEAs. TRIP-DP-HEAs have been reported to exhibit superior strength-ductility combination at room temperature as well as at high temperatures. Small amounts of carbon additions to TRIP-DP-HEAs are known to further improve the strength as a result of interstitial solid solution and nanoprecipitate strengthening effects. Current investigation compares the grain growth kinetics in a metastable, dual-phase (FCC and HCP), Fe49.5Mn30Co10Cr10C0.5, carbon-containing high entropy alloy (C-DP-HEA) with that in the corresponding carbon-free variant, Fe50Mn30Co10Cr10 HEA (DP-HEA). The alloys after being subjected to cold rolling, were annealed at 900 °C, 1000 °C and 1100 °C for 3, 30, 60 and 120 min, followed by water quenching. Slower grain growth kinetics was observed in C-DP-HEA due to Zener pinning by carbide particles. The activation energy for grain growth in C-DP-HEA was determined to be 339.71 kJ/mol, which is larger than that of DP-HEA (282.68 kJ/mol). The microstructures of the C-DP-HEA samples, after annealing at 900 °C for different durations of times and quenching, were investigated. The HCP phase fraction in the microstructure initially decreased with increase in annealing time/grain size and later increased with further increase in annealing time/grain size. This is explained based on the counteracting effect of grain boundary density and back stresses acting in samples having increased grain size. Strain rate sensitivity (SRS) parameter, ‘m’, was evaluated for both C-DP-HEA and DP-HEA. The strain rate jump tests were conducted at two quasi-static strain rates, 1 × 10−3 s−1 and 5 × 10−3 s−1, and at room temperature as well as at higher temperatures, 900 °C, 1000 °C and 1100 °C. The ‘m’ value was found to depend on the grain size, test temperature, and the presence of carbon in the alloy.