Phase hardness, load transfer, texture, and martensitic transformation induced dynamic grain refinement sources of hardening in three structurally transforming high entropy alloys

This paper describes the findings from an experimental investigation into local and overall strength of three microstructurally flexible high entropy alloys (HEAs), Fe₄₂Mn₂₈Co₁₀Cr₁₅Si₅ (Si-HEA), Fe_38.5Mn₂₀Co₂₀Cr₁₅Si₅Cu_1.5 (Cu-HEA) and Fe_37.5Mn₂₀Co₂₀Cr₁₅Si₅Cu_1.5V₁ (V-HEA) in at.%. The alloys are similar in composition and consist of metastable face-centered cubic austenite (γ) and stable hexagonal epsilon martensite (ε) phases. The Si-HEA also contains stable tetragonal sigma (σ) phase. The contents of diffusion-created phase, σ, remains constant during plastic deformation, while the fraction of diffusion less strain-induced γ → ε phase transformation increases. The evolution of phases, as well as the local and overall strength, during deformation of the alloys were characterized. High throughput nanoindentation mapping was employed to assess the mechanical hardness of individual phases with plastic strain. The γ and ε phases were found to exhibit moderate hardening with plasticity owing to dislocations, while the σ phase softened owing to the phase fragmentation. The primary source of the overall strain hardening in the alloys was found to be dynamic refinement of the structures and underlying barrier effect, while the increasing fraction of the dislocated ε phase and texture were found to act as secondary sources of hardening. The load transfer strengthening was found significant only in the Si-HEA because of the presence of the strong and brittle σ phase. The measured hardness per phase, load transfer, texture, and martensitic transformation induced dynamic grain refinement hardening mechanisms were assessed to explain similarities and differences in the hardening behavior of the three structurally transforming HEAs.