Serrated flow behavior mediated via nano‐twinning and phase transformation in FeCoCrNiMo0.2 high‐entropy alloy at cryogenic temperatures

Abstract The serrated flow behavior, known as the Portevin–Le Chatelier (PLC) effect, is commonly observed during high‐temperature deformation. In this study, we report a serrated flow behavior in FeCoCrNiMo0.2 high‐entropy alloy (HEA), which is mediated by nano‐twinning and phase transformation at cryogenic temperatures. During uniaxial tensile deformation at 77 K, the alloy exhibited the formation of high‐density deformation nano‐twinning, cross‐twinning, stacking faults (SFs) and Lomer–Cottrell locks (L‐C locks). Additionally, the lower stacking fault energy (SFE) at low temperatures promotes the formation of the 9R phase. The high‐density twin boundaries effectively hinder dislocation movement, leading to the instability of plastic deformation and promoting the serrated flow behavior. Furthermore, the rapid and unstable transformation of the 9R phase contributes to the pronounced serrated flow behavior. Nano‐twinning, SFs, cross‐twinning, L‐C locks and 9R phase collectively induce a dynamic Hall–Petch effect, enhancing the strength‐ductility synergy and strain‐hardening ability of deformed alloy at 77 K. Our work provides valuable insights into the mechanism of tensile deformation at cryogenic temperatures in single‐phase FCC HEA.