Grain size and temperature effect on the tensile behavior and deformation mechanisms of non-equiatomic Fe41Mn25Ni24Co8Cr2 high entropy alloy

Abstract

The effect of the grain size on the tensile properties and deformation mechanisms of a nonequiatomic Fe41Mn25Ni24Co8Cr2 high-entropy alloy was studied in the temperature range between 298 and 1173 K by preparing the samples with three different grain sizes through severe plastic deformation and subsequent annealing: ultrafine (sub)grain size (<= 0.5 mu m), 8.1 mu m and 590.2 mu m. In the temperature between 298 and 773 K, the material with the large grain size of 590.2 mu m exhibited the largest tensile ductility (57%-82%) due to its high strain hardening associated with mechanical twinning, but it exhibited the lowest strength due to its large grain size. The material with the ultrafine (sub)grain size exhibited the lowest tensile ductility (3%-7%) due to a greatly reduced strain hardening ability after severe plastic deformation, but it exhibited the highest strength due to the dislocation strengthening and grain refinement strengthening. At tensile testing at temperatures above 973 K, recrystallization occurred in the material with the ultrafine (sub)grains during the sample heating and holding stage, leading to the formation of fine and equiaxed grains with the sizes of 6.8-13.5 mu m. The deformation behavior of the Fe41Mn25Ni24Co8Cr2 with different grain sizes in the high temperature range between 973 and 1173 K, where pseudosteady-state flow was attained in the stress-strain curves, could be explained by considering the simultaneous contribution of grain boundary sliding and dislocation-climb creep to total plastic flow. The activation energies for plastic flow for the materials with different grain sizes were similar as similar to 199 kJ/mol. In predicting the deformation mechanism, it was important to consider the change in grain size by rapid grain growth or recrystallization during the sample heating and holding stage because grain boundary sliding is a grain-size-dependent deformation mechanism. The sample with the ultrafine (sub)grains exhibited the large tensile elongations of 30%-85% due to its high strain rate sensitivity, m (0.1-0.5) at temperatures of 973-1173 K. The material with the large grain size of 590.2 mu m exhibited the very small elongations of 0.2%-8% due to its small m values (0.1-0.2) and occurrence of brittle intergranular fracture at the early stage of plastic deformation. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.