Microstructural evolution and crystallographic control in cast Al–Zn–Mg–Cu alloys via TiB2 addition and optimized dual-stage heat treatment

This study investigates the synergistic effects of incorporating TiB 2 particles and implementing a dual-stage solution plus dual-stage aging treatment on the microstructure and mechanical properties of cast Al-9Zn-2.3Mg-1.5Cu-0.2Zr alloys. Alloys with 0 wt.% (Alloy 1), 1 wt.% (Alloy 2), and 2 wt.% TiB 2 (Alloy 3) were synthesized and characterized. TiB 2 particles, distributed primarily at grain boundary triple junctions, effectively transformed the as-cast grain morphology from dendritic to equiaxed and refined the grain size (e.g., 61.6 μm for heat-treated Alloy 3), acting as potent heterogeneous nucleation sites with negligible post-heat treatment growth. A tailored solid solution treatment, informed by DSC and JMatPro analysis to prevent incipient melting and maximize dissolution, achieved a homogeneous supersaturated solid solution. Subsequent dual-stage aging (120°C/24h + 170°C/4h) promoted a high density of nanoscale η(MgZn 2 ) precipitates (avg. dia. 8.1 nm) while eliminating detrimental grain boundary networks. HRTEM analysis revealed specific orientation relationships between Al 3 Zr/matrix and η-phase/matrix interfaces. The combined approach yielded exceptional property synergy in Alloy 3: a yield strength of 512.5 MPa, elongation of 4.68%, and peak hardness of 170.2 HV. This enhancement is attributed to the synergistic contributions of solution strengthening, TiB 2 -induced grain refinement, and optimized η-phase precipitation strengthening, demonstrating a viable route to achieving high performance in cast ultra-high-strength aluminum alloys without thermomechanical processing.