Towards age-hardening ability enhancement and high strength in Mg–Gd–Ag alloy by balancing grain refinement and weakening of dynamic precipitation

The dilemma of choosing between superb grain boundary strengthening and excellent precipitation strengthening is commonly faced in the pursuit of high-strength age-hardenable Mg alloys. Here, a strategy for addressing this dilemma via the balance between grain refinement and weakening of dynamic precipitation is proposed and applied on a Mg–10.95Gd–2.06Ag (wt%) alloy treated by sequential processing of equal-channel angular pressing (ECAP), hot rolling and aging. The hot rolling leads to a noticeable dissolution of the dynamic precipitates intensively formed during prior ECAP, together with a tolerable grain growth. Meanwhile, numerous multi-twinning-induced lamellar bands (LBs) demarcated by coaxial grain boundaries (CGBs) subdivide the grains. For the first time, a quadruple twinning pattern for the formation of CGB with a misorientation of ~135° is raised. During post-rolling aging, besides the common β’ and γ” precipitates and the β nano-particles whose Gd/Ag ratio is much lower than that of dynamically precipitated β particles, the β”/β_T precipitates, CGB segregation, and stacking fault (SF)-associated segregation/γ’ precipitates within LBs, are also involved with the precipitation structure. Benefiting from this complex precipitation structure, a good age-hardening ability manifested by the aging-induced tensile yield strength (TYS) increment reaching 115 MPa is attained. Combining with the strengthening from grain boundaries, dislocations, solutes, and basal texture, a high TYS of 447 MPa is achieved.