A novel heat treatment combining cryogenic and aging treatment for enhanced mechanical properties of extruded Mg-8Gd-3Y-0.4Zr alloy

In this study, a novel heat treatment consisting of aging treatment and cryogenic treatment was conducted to process extruded Mg-8Gd-3Y-0.4Zr alloy. The microstructure characteristics were systematically investigated using scanning electron microscopy (SEM), Optical microstructure (OM), electron back-scatter diffraction (EBSD), X-Ray diffraction (XRD) and transmission electron microscope (TEM) to study their correlation with the mechanical properties. The results show that the combination of cryogenic treatment and aging treatment significantly enhances mechanical properties. The sample subjected to 1 h cryogenic treatment followed by aging treatment demonstrated the highest strength (435.5 MPa) among all tested specimens, exhibiting a strength improvement exceeding 100 MPa compared to that treated solely with 1 h cryogenic treatment. However, this was accompanied by a 12.8 % reduction in ductility. This phenomenon can be attributed to the aging process, which promotes the nucleation and growth of fine Mg₃(Gd,Y) and Mg₂₄(Gd,Y)₅ precipitates, thereby enhancing second-phase strengthening effects. Conversely, these precipitates may induce stress concentration, leading to decreased plasticity. In addition, the aging treatment following cryogenic treatment can alter grain orientation and reduce dislocation density. Samples subjected solely to cryogenic treatment exhibited more frequent activation of both prismatic <c> slip and pyramidal <c+a> slip systems, which significantly enhanced plasticity. Notably, prolonged cryogenic treatment duration may reduce strength.