Microstructures and mechanical properties of compositionally complex Co-free FeNiMnCr₁₈ FCC solid solution alloy

Recently, a structurally-simple but compositionally-complex FeNiCoMnCr high entropy alloy was found to have excellent mechanical properties (e.g., high strength and ductility). To understand the potential of using high entropy alloys as structural materials for advanced nuclear reactor and power plants, it is necessary to have a thorough understanding of their structural stability and mechanical properties degradation under neutron irradiation. This requires us to develop a similar model alloy without Co because material with Co will make post-neutron-irradiation testing difficult due to the production of the ⁶⁰Co radioisotope. To achieve this goal, a FCC-structured single-phase alloy with a composition of FeNiMnCr₁₈ was successfully developed. This near-equiatomic FeNiMnCr₁₈ alloy has good malleability and its microstructure can be controlled by thermomechanical processing. By rolling and annealing, the as-cast elongated-grained-microstructure is replaced by homogeneous equiaxed grains. The mechanical properties (e.g., strength and ductility) of the FeNiMnCr₁₈ alloy are comparable to those of the equiatomic FeNiCoMnCr high entropy alloy. Both strength and ductility increase with decreasing deformation temperature, with the largest difference occurring between 293 and 77K. Extensive twin-bands which are bundles of numerous individual twins are observed when it is tensile-fractured at 77K. No twin bands are detected by EBSD for materials deformed at 293K and higher. The unusual temperature-dependencies of UTS and uniform elongation could be caused by the development of the dense twin substructure, twin-dislocation interactions and the interactions between primary and secondary twinning systems which result in a microstructure refinement and hence cause enhanced strain hardening and postponed necking.