Heat treatment effects on microstructure, deformation mechanisms and mechanical response in additive friction stir deposited Mg-Gd-Y-Zr alloy

• AFSD Mg-Gd-Y-Zr alloy exhibits heterogeneous onion-ring microstructure. • The onion-ring microstructure showed alternating fine and coarse grains. • T6 treatment modified the heterogeneous grain structures. • Both yield strength and ductility were enhanced after AFSD and T6. • The interplay of basal slip and twins accounted for strength-ductility synergy. Additive friction stir deposition (AFSD), as a solid-state-additive manufacturing technique with a high deposition rate, provides an innovative route for fabricating high-performance magnesium (Mg) alloys while avoiding solidification defects. In this study, the effects of T6 heat treatment on the microstructure, mechanical properties, and deformation mechanisms of an AFSD Mg-9Gd-3Y-0.5Zr (wt.%) alloy were systematically investigated. The AFSD alloy exhibited a heterogeneous onion-ring microstructure composed of alternating fine grains (∼3.9 µm) enriched with nanoscale cuboid Mg 24 (Gd, Y) 5 precipitates and coarse grains (∼10.6 µm) containing limited precipitates, which originated from the non-uniform precipitate distribution in the feedstock. The T6 heat treatment eliminated dislocation structures, introduced abundant nano-Mg 7 (Gd, Y) (β′) precipitates and coarsened the fine and coarse grains to ∼7.3 and ∼67.4 µm, respectively. Consequently, the AFSD and AFSD-T6 alloys achieved superior strength-ductility combinations compared to the feedstock, with yield strength/ultimate tensile strength/elongation of 293.5 MPa/330.3 MPa/8.1% and 366.8 MPa/374.3 MPa/5.1%, respectively. The heterogeneous grains induced pronounced hetero-deformation-induced hardening in both conditions, while basal 〈a〉 slip activity was progressively enhanced during deformation. However, extensive twinning, particularly double twinning in the coarse grains of the AFSD-T6 alloy, led to reduced ductility. Overall, this work demonstrates that coupling AFSD with tailored heat treatment enables effective microstructural heterogeneity engineering, offering a robust strategy for developing Mg-Gd-Y-Zr alloys with outstanding mechanical performance. Journal of Magnesium and Alloys, submitted for publication