High-efficiency strengthening achieved by ultra small-sized and orientation-controlled precipitates in low-rare-earth Mg-Gd-Y-Zr alloys

High-rare-earth (HRE) magnesium alloys typically enhance strengthening by increasing rare earth (RE) content, but this strategy yields limited efficacy and high cost. In this study, a low-rare-earth (LRE) alloy, Mg-5.3Gd-4.1Y-0.13Zr (<10% wt.) was selected to explore a novel and efficiency strategy of fabricating high-strength LRE Mg alloy. A large cube-shaped compound is observed at grain boundaries, which gradually grows via absorbing dislocations and RE elements during aging period, resulting in a softening effect near the grain boundaries. Despite LRE and softening, the peak-aged(200°C,32h) ultimate tensile strength (UTS) reaches 521 MPa, which is higher than that of most Mg-(9∼15%) RE alloys, which achieves a significant aging strengthening efficiency, 12.5 times that of others. The main strengthening mechanisms originate from several aspects: first, the small-sized(5.7nm) and high-density β' precipitates(3.8%) lower the strengthening threshold and improve the efficiency of precipitation strengthening; second, precipitation-coupled twinning changes the orientation of some β' precipitates, making them orthogonal to each other, leading to a superior strengthening effect compared to single prismatic-plane precipitation; third, the precipitates formed during aging promote continuous dislocation multiplication, resulting in a high dislocation volume fraction and introducing substantial dislocation strengthening. This work paves the way for designing strengthening strategies for magnesium alloys with similar precipitation sequences.