Inverse Thermal Softening Behavior in a High‐Entropy Alloy Under Various Strain Rates

ABSTRACT Although face‐centered cubic high‐entropy alloys (HEAs) have received extensive attention and in‐depth research, most existing studies focus on single temperature or strain rate conditions, lacking systematic investigations into deformation mechanisms under extreme thermomechanical coupling environments. In this work, a CoCrFeNiTi 0.2 HEA containing ultrafine L1 2 precipitates and η phases was fabricated, and compression tests were performed over a wide range of temperatures (293–1273 K) and strain rates (0.001–5000 s −1 ), followed by systematic characterization of the corresponding microstructural evolution. The results demonstrate an inverse thermal softening behavior at various strain rates. Anomalous double‐humped stress peaks, separately induced by third‐type strain aging and in situ coarsening of L1 2 precipitates during deformation, were observed in the flow stress–temperature curves, leading to the inverse thermal softening behavior. The stress peak associated with third‐type strain aging appears at various strain rates and shifts toward higher temperatures with increasing strain rate. Ti atoms serve as solute atoms, forming solute atmospheres and effectively pinning mobile dislocations. In contrast, the other stress peak induced by in situ coarsening of L1 2 precipitates is nearly insensitive to strain rate. The dependence of multiple deformation mechanisms—including dislocation slip, twinning, stacking fault‐mediated plasticity, and dynamic recrystallization—on temperature and strain rate was thoroughly investigated. Finally, a schematic diagram of deformation mechanisms covering the entire investigated temperature and strain rate range was constructed. This study elucidates the deformation mechanisms of the CoCrFeNiTi 0.2 HEA in extreme environments, offering an important theoretical basis and experimental support for the design and application of high‐performance HEAs in aerospace, weaponry, and other fields.