Friction stir gradient alloying: A high-throughput method to explore the influence of V in enabling HCP to BCC transformation in a γ-FCC dominated high entropy alloy

The compositional possibilities in high entropy alloys (HEAs) is very vast and effective strategies are needed to establish potential alloy chemistries. In this study, friction stir gradient alloying (FSGA), a recently-introduced high-throughput (HT) technique incorporating compositional and microstructural gradients, was used to explore the possibility of introducing a bcc transformation domain in gamma-fcc dominated TRIP HEA Fe38.5Mn20Cr15Co20Si5Cu1.5 (at.%) by vanadium addition. FSGA has accelerated the process of attaining the composition-microstructure library and the results suggest that vanadium addition of similar to 1 at.% results in nucleation of alpha-bcc. This is the first observation of epsilon-hcp to alpha-bcc transformation in a TRIP HEA and supports the Olson-Cohen model of martensitic transformation (gamma-fcc -> epsilon-hcp -> alpha-bcc). epsilon-hcp is nucleated with a pyramidal orientation by the formation of planar faults on the {111} close-packed planes of gamma-fcc, and alpha-bcc is nucleated in the vicinity of epsilon-hcp along the {111} gamma-fcc trace. There are indications for the formation of basal orientated epsilon-hcp from intersecting epsilon-hcp planes orientated for the pyramidal slip. Thus, the study shows the nucleation and growth of the alpha-bcc phase attributed to the dual effects of chemistry and strain and potential co-existence of all the three phases. (C) 2020 Elsevier Ltd. All rights reserved.