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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 (g), stable hexagonal epsilon martensite (ε), stable body-centered cubic ferrite (a), and stable tetragonal sigma (σ) 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 a and σ phases remain constant during deformation, the fraction of ε increases at the expanse of g due to the diffusionless strain induced γ→ε 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 ε phase during deformation due to the transformation is found to act as a secondary source of hardening because g and ε exhibit similar hardness at a given strain level. While these two phases exhibit moderate hardening during plasticity, significant softening is observed in σ 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 σ phase. 

In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage.