Multi-scale interactions governing dynamic recrystallization: Coupled fragmentation of coarse T/η phases, DIP-induced MgZn2, and Al3(Sc,Zr) dispersoids in a high-alloy Al–Zn–Mg–Cu–Zr–Sc alloy

Understanding how multi-scale second phases interact during thermomechanical processing is essential for controlling dynamic recrystallization (DRX) in highly alloyed 7xxx aluminum systems. In this study, a high-alloy Al–Zn–Mg–Cu–Zr–Sc alloy was subjected to multi-directional forging (MDF) to elucidate the coupled roles of coarse T/η phases, deformation-induced MgZn 2 precipitates and nanoscale Al 3 (Sc,Zr) dispersoids in governing DRX. Comprehensive characterization using SEM/EDS, EBSD and DF-TEM/HAADF-STEM reveals that the evolution of coarse T/η phases follows a strain–temperature-dependent fragmentation →agglomeration → partial dissolution cycle. Under high cumulative strain, extensive fragmentation generates strong local strain incompatibilities and GND accumulation, which provide favorable sites for particle-stimulated nucleation (PSN). Simultaneously, deformation-induced precipitation (DIP) produces refined MgZn 2 (100–300 nm) at 410–430°C, further amplifying strain gradients and enhancing DRX nucleation. In parallel, thermally stable Al 3 (Sc,Zr) dispersoids (∼10–20 nm) exert strong Zener pinning, effectively restricting DRX grain growth. The synergistic action of these three particle populations establishes a multi-scale DRX regulation mechanism: coarse T/η fragments act as nucleation triggers, DIP-induced MgZn 2 precipitates serve as nucleation enhancers and nanoscale Al 3 (Sc,Zr) dispersoids stabilize subsequent grain growth. This hierarchy results in a narrow but optimal DRX processing window at 410–430 °C under high cumulative strain (7U6CS), where extensive and uniform DRX is achieved. At≥450°C, accelerated recovery and dissolution of deformation heterogeneities suppress DRX and lead to grain coarsening. These findings provide a mechanistic framework for understanding DRX behavior in high-alloy 7xxx systems and offer practical guidance for designing strain–temperature schedules that leverage multi-scale particle interactions to refine microstructures during industrial forging.