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 Mg₂₄(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-Mg₇(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.

Solute atoms and precipitates significantly influence the mechanical properties of Mg alloys. Previous studies have mainly focused on the segregation behaviors of Mg alloys after annealing. In this study, we investigated the segregation behaviors of an Mg-RE alloy under deformation. We found that the enrichment of solute atoms occurred in {1011} compressive twin boundaries under compression at 298 K without any annealing in an Mg-RE alloy by scanning transmission electron microscopy and energy-dispersive X-ray analysis. The segregated solutes and precipitates impeded the twin growth, partially contributing to the formation of small-sized {1011} compressive twins. This research indicates the twin boundaries can be strengthened by segregated solutes and precipitates formed under deformation at room temperature.