Effect of the deformation temperature and strain on the strain rate sensitivity of fcc medium-entropy alloys

Abstract

The primary objective of the present investigation is to elucidate the operative micromechanisms influencing the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. Room-temperature nanoindentation experiments at different loading rates were performed to study the evolution of the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. The (FeCrNi)99Si1 samples were subjected to plane strain deformation by rolling at 77 and 300 K to study the effect of temperature on the strain rate sensitivity, while the FeMnNi and (FeCrNi)99Si1 samples were subjected to simple shear deformation by high-pressure torsion at 300 K to examine the effect of strains. Contrary to the well-documented trend observed in fcc metals and alloys, where the strain rate sensitivity typically increases with decreasing grain size, the present study reveals a distinct behavior for the current alloys. Similarly, these alloys are characterized by extremely low activation volumes of a few tens of b3 compared to 100–1000 b3 for conventional fcc metals and alloys in the microcrystalline grain size regime. Unlike conventional fcc metals and alloys, there is an insignificant change in the activation volume of the current high-/medium-entropy alloy (H/MEA) with decreasing grain size from the microcrystalline to nanocrystalline regime. The unique evolution of strain rate sensitivity and activation volume in H/MEAs is explained in terms of the evolution of distinct dislocation structures as well as synergistic operation of additional mechanisms such as twinning, phase transformation from fcc to hcp phases, cluster strengthening, and short-range ordering due to the aperiodic energy landscape existing in MEAs.