Increase Strength and ductility in high entropy alloy Fe49.5Mn30Co10Cr10C0.5 engineered by friction stir processing

Complex phase evolution has recently been observed in high-entropy alloys (HEAs) during thermomechanical processing, specifically through severe plastic deformation at elevated temperatures. This phase evolution, particularly following friction stir processing (FSP), exerts a significant impact on the deformation behavior and mechanical properties of the alloys. The primary objective of this study is to investigate the effect of regulated phase evolution on the mechanical properties of a two-phase interstitial high-entropy alloy (DP-iHEA) Fe49.5Mn30Co10Cr10C0.5, with a specific focus on transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). During the friction stir processing, a careful control of strain, strain rate, and temperature was employed to optimize the formation of a microstructure consisting of metastable face-centered cubic phase (γ-FCC) and hexagonal close-packed martensitic phase (ε-HCP). The processing conditions resulted in a significant uniform elongation, although some non-uniform ductility was observed after necking. Notably, a simultaneous reverse phase transformation from ε-HCP to γ-FCC and dynamic recrystallization occurred during the processing. Under dual-pass conditions, the grain size was refined to 4.2 μm, leading to the refinement of the dominant γ-FCC phase and an increase in grain boundary surface area. This refinement enhanced the material s deformation storage capability. The controlled transformation, induced by rebound stresses from grain boundaries, exhibited a delayed but stable transformation-induced plasticity (TRIP) effect. This effect resulted in an increase in ultimate tensile strength without compromising ductility, even at high yield strength levels. During room temperature tensile deformation increased with the increase of ε-HCP → γ-FCC reverse phase transformation during processing, even with the concurrent decrease in grain size. The combination of a very fine grain size and uniform strain distribution between the metastable γ-FCC and ε-HCP phases yielded a high work hardening rate over a wide range of plastic strain. This behavior can be attributed to the limited volume fraction distribution of the elongated ε-HCP phase along the γ-FCC grain boundaries. The processing conditions, including dual-pass effects, grain refinement, and phase evolution, collectively contributed to an exceptional ultimate tensile strength of 1.203 GPa, a yield strength of 825 MPa, and a 42% increase in elongation compared to previously reported high-entropy alloys processed using FSP or conventional thermomechanical methods.In conclusion, this study provides valuable insights into the relationship between phase evolution, microstructure, and mechanical properties in high-entropy alloys. The ability to tailor the phase evolution and microstructure of HEAs opens up new possibilities for designing advanced materials with superior mechanical properties for various applications, including aerospace, automotive, and structural engineering.