Study on dual-heterostructure in additive friction stir deposited Mg-Gd-Y alloys: Formation mechanism and mechanical response

• Dual-heterogeneous microstructure, featuring alternating fine grain (FG) bands embedded with dense nanoscale multiphase clusters and coarse grain (CG) bands containing sparse clusters, was expeditiously fabricated through AFSD. • The formation of dual-heterogeneous microstructure was driven by the evolution of rare-earth (RE) rich eutectic phases and transient supersaturated solid solutions of RE elements at grain boundaries. • In-situ formed nanoscale multiphase clusters exert exceptionally strong pinning effects at elevated temperatures. • Abundant heterogeneous FG/CG interface greatly activated non-basal slip systems. Constructing heterogeneous microstructures has been demonstrated as an effective strategy to overcome the strength-ductility trade-off in magnesium (Mg) alloys. Here, a dual-heterogeneous microstructure was fabricated in a Mg-6.49Gd-2.74Y-0.45Zr (wt.%) alloy via additive friction stir deposition (AFSD), featuring alternating fine grain (FG) bands embedded with dense nanoscale multiphase clusters and coarse grain (CG) bands containing sparse clusters. This unique architecture leads to simultaneous enhancement of strength and ductility. The AFSD alloy exhibits an elongation of 19.5 % and a yield strength of 262.2 MPa, which can be enhanced to 411.0 MPa following peak aging treatment. The formation mechanisms of heterogeneous microstructures and their influence on mechanical properties were systematically investigated. Fragmented rare earth (RE)-containing eutectic phases at grain boundaries induced recrystallization via particle-stimulated nucleation (PSN). Their subsequent complete and rapid dissolution led to the formation of supersaturated RE solid solutions, which promoted the precipitation of nanoscale multiphase clusters with pronounced pinning effects, ultimately leading to the growth of differential grains and the formation of dual-heterostructures. Furthermore, CG/FG interfaces were found to activate non-basal slip systems within adjacent grains, while the nanoscale multiphase clusters can effectively hindered dislocation motion. The synergic effect of these mechanisms contributed to the simultaneous enhancement of strength and ductility. This study provides fundamental insights for developing high-performance Mg-RE alloys.