Superior elevated-temperature strength-ductility synergy in an additive-manufactured Mg-Gd-Y alloy: Dynamic precipitation evolution and deformation behavior under thermal-mechanical coupling

Significant strength loss at elevated temperatures has always been one of the great challenges for the extended application of magnesium (Mg) alloys. This work reports an exceptional increase in the elevated-temperature mechanical properties of magnesium rare-earth (Mg-RE) alloys fabricated by direct energy deposition using electric arc (DED-Arc). The tensile strength at 250 °C of the DED-Arc Mg-9Gd-3Y-0.5Zr (GW93K) alloy is almost 70 MPa higher than that of the cast counterpart, accompanied by desirable ductility. A quasi-in situ approach was employed to investigate the dynamic precipitation evolution of both DED-Arc and cast alloys under simple thermal effect and thermal-mechanical coupling for a comparative study. Molecular dynamics (MD) simulations were utilized to analyze RE diffusion in DED-Arc alloys. Compared to cast alloys, DED-Arc alloys exhibit nano-scale precipitates with significantly higher number density under thermal-mechanical coupling during tensile test. The residual dislocations from the DED-Arc process and uniformly distributed dislocations formed during tensile deformation impeded RE solute diffusion toward grain boundaries (GBs), thereby reducing grain boundary precipitation (GBP). The inhibited GBP and absence of precipitation-free zones (PFZs) reduce stress concentration at GBs, delaying crack initiation. The effects of microstructural features including grain size, texture, and precipitates on deformation modes were systematically compared. Consequently, DED-Arc Mg-RE alloys demonstrate superior strength-ductility synergy: ultimate tensile strength: 291.9 ± 7.3 MPa and elongation: 19.2 ± 1.0 % at 250 °C. This research could provide new insights into the application of Mg alloys in extreme conditions such as elevated temperatures.