Microstructure evolution from homogeneous as-cast state to annealed heterogeneous structure and mechanical properties of Al-Zn-Mg-Cu alloys with trace TiB2 particles

Materials with a final heterogeneous structure (HS) possess an excellent combination of strength and ductility. However, fine and homogeneous grains are desired in as-cast ingots to avoid defects. The evolution from an as-cast homogeneous microstructure to a pronounced HS owing to the trace addition of TiB₂ particles was studied in Al-Zn-Mg-Cu alloys with traditional thermomechanical treatment. It is revealed that the triple junctions and over 2 μm precipitates co-located with TiB₂ enhance the particle-stimulated nucleation of recrystallization. The dislocation density difference between the recrystallized and recovered grains is further enhanced by cold rolling. A pronounced HS with alternating soft and hard domains accompanied by multimodal precipitates is modulated, realizing a synergy of yield strength as 632.4 MPa and elongation as 8.8%. It is confirmed that the HS, rather than precipitates, is the primary source of geometrically necessary dislocations (GNDs), leading to the synergy of strength and ductility. Atomistic simulations on the deformation behavior of HS were used to elucidate the strain partition and role of GNDs on mechanical properties. Our results provide a convenient route to fabricate heterogeneous Al-Zn-Mg-Cu alloy by trace addition of ceramic particles in ingots casting instead of elaborated controlling of deformation processing.