Assessing strength of phases in a quadruplex high entropy alloy via high-throughput nanoindentation to clarify origins of strain hardening

This paper describes the main findings from an experimental investigation into local and overall strength and fracture behavior of a microstructurally flexible, quadruplex, high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at.%). The alloy consists of metastable face-centered cubic austenite (gamma), stable hexagonal epsilon martensite (e), stable body-centered cubic ferrite (alpha), and stable tetragonal sigma (sigma) phases. The overall behavior of the alloy in compression features a great deal of plasticity and strain hardening before fracture. While the contents of diffusion created alpha and sigma phases remain constant during deformation, the fraction of e increases at the expanse of gamma due to the diffusionless strain induced gamma -> e phase transformation. High-throughput nanoindentation mapping is used to assess the mechanical hardness of individual phases contributing to the plasticity and hardening of the alloy. Increasing the fraction of the dislocated e phase during deformation due to the transformation is found to act as a secondary source of hardening because gamma and e exhibit similar hardness at a given strain level. While these two phases exhibit moderate hardening during plasticity, significant softening is observed in sigma owing to the phase fragmentation. While the phase transformation mechanism facilitates accommodation of the plasticity, the primary source of strain hardening in the alloy is the refinement of the structure during the transformation inducing a dynamic Hall-Petch-type barrier effect. Results pertaining to the evolution of microstructure and local behavior of the alloy under compression are presented and discussed clarifying the origins of strain hardening. While good under compression, the alloy poorly behaves under tension. Fracture surfaces after tension feature brittle micromechanisms of fracture. Such behavior is attributed to the presence of the brittle sigma phase.