Achieving strength-ductility synergy in Mg-1.1Gd-0.6Zn-0.3Mn alloy by regulating precipitation behavior via stress aging strategy

Regulating the precipitation behavior of Mg alloys to overcome the strength-ductility trade-off puzzle is a long-thought pursuit in the materials community. With this purpose, external stress has been recently applied during aging and shows immense potential in affecting atomic diffusion, and regulating the coherence of the phase boundaries. In this study, elastic tensile (TSA) and compressive stress aging (CSA) of Mg-1.1Gd-0.6Zn-0.3Mn alloy are carried out and the competition of precipitation between multiple precipitates occurs during stress aging. A significant quantity of β’ precipitates primarily distribute along grain boundaries in conventional peak aging alloy. Whereas high density of γ’ phases rather than β’ phase precipitate in both TSA and CSA alloys. The first-principle calculations reveal that the application of external stress introduces shear strain, which decreases unstable stacking fault energies, and thereby promoting the precipitation of γ’ phase and impeding the precipitation of β’ phase. Furthermore, the sequential transformation from γ’ phase to Long Period Stacking Ordered (LPSO) phase occurs in CSA sample, due to the release of elastic local strain at phase boundaries. After subjected to TSA treatment, the sample possesses an ultimate tensile strength of 356 MPa, a yield strength of 294 MPa, and a total elongation of ~14.3%. The excellent strength-ductility synergy of TSA sample is primarily contributed to the profuse γ’ precipitates hindering the motion of large number of pyramidal 〈c + a〉 dislocations during tensile deformation. This study offers new insights on regulating the precipitation behavior of Mg alloys containing multiple types of precipitates through the application of external stress, and extends the potential window for obtaining an excellent strength-ductility synergy in age-hardenable Mg alloys.