Strain rate effect on twip/trip high entropy alloy

Abstract

High Entropy Alloys (HEA) present opportunities to develop new materials with outstanding mechanical properties. Through the careful selection of constituent elements along with optimized thermal processing for proper control of structure, grain size, and deformation mechanisms, many of the newly developed HEA systems exhibit superior strength and ductility levels across a wide range of temperatures, in particular at cryogenic deformation temperatures. Such a remarkable response has been attributed to the hardening capacity of HEA that is achieved through the activation of deformation twinning. More recent HEA compositions have considered phase transforming systems which have the potential for enhanced strengthening and therefore high strength and ductility levels. However, the strain rate sensitivity of such transforming HEA is not well understood and requires further investigation. In this study, the dynamic and quasi-static tensile properties of the non-equiatomic V10Cr10Fe45Co30Ni5 HEA were investigated temperatures ranging from 77K (196°C) to 573K (300°C).Depending on the deformation temperature, the considered HEA exhibits plasticity through either crystallographic slip, deformation twinning, or solid-state phase transformation. At 300°C, only slip mediated plasticity was observed for all the considered deformation rates. Deformation twinning was detected in samples deformed at room temperature (RT), while phase transformation, face-centered cubic to body-centered cubic, became more favorable at cryogenic deformation temperatures. At a deformation strain rate of 1.32e-3/sand 77K deformation temperature, the alloy reached an impressive tensile strength of around 1.2 GPa with a ductility exceeding 60%. Plastic deformation was accommodated in this case through phase transformation which consequently enabled superior strength and ductility. Both, the strain rate sensitivity (SRS) and strain hardening rates were shown to differ depending on the dominant deformation mechanism. For example, the SRS parameter m decreased as the deformation temperature dropped from RT (m = 0.05) to 77K (m = 0.017). Increasing the loading temperature to 300°C resulted in higher ductility, however at the expense of strength. Due to the absence of either twinning or transformation as hardening mechanisms, the HEA experienced a drop in strength reaching up to 23% at 300°C.