Mechanistic transition into stress-activated dynamic recrystallization as the origin for catastrophic softening in precipitation-strengthened high-entropy alloys

High-entropy alloys, especially those with L1₂ precipitates, exhibit exceptional high-temperature strength and creep resistance largely owing to complex defect structures. Yet such performance is sustained only within a narrow temperature window, as evidenced by the abrupt softening of the Ni_33.3Co_33.3Cr_23.4Al₅Ti₅ HEA between 600 ◦C and 700 ◦C. The present study reveals that this catastrophic failure does not reflect a gradual thermal process but arises from a mechanistic transition near ~650 ◦C, quantified by kinetic analysis of stress-relaxation data. Below this threshold, deformation is controlled by dislocation climb with a high stress exponent of n≈5-6 and activation energy of 301.84 kJ mol^-1, a slow mechanism that preserves the strengthening defect network. Above this threshold, the operative mechanism shifts to stress-activated dynamic recrystallization (DRX), mediated by grain boundary diffusion with a collapsed stress exponent of n = 1.31 and a drastically reduced activation energy of 99.61 kJ mol^-1. Crucially, comparative analysis reveals that applied stress, rather than thermal energy alone, constitutes the primary driving force for the rapid grain boundary migration, triggering DRX and consuming the dislocation substructure. Our work thus uncovers a sharp, stress-activated performance cliff in L1₂-strengthened HEAs, suggesting that grain boundary engineering and tuning of DRX processes might overweigh precipitate coherency design for reliable elevated-temperature performance.