Improving the Mechanical Properties of Mg-Gd-Y-Ag-Zr Alloy via Pre-Strain and Two-Stage Ageing

In this study, we investigated the effects of single-stage ageing (SSA), two-stage ageing (TSA), 2% pre-strain + single-stage ageing (P2%SSA) and 2% pre-strain + two-stage ageing (P2%TSA) on the mechanical properties of as-extruded Mg-8Gd-3Y-0.5Ag-0.5Zr alloy (E alloy). Compared with the SSA treatment, the TSA treatment increased the number density of beta' phase. The P2%SSA and P2%TSA treatments generated the gamma' phase and chain-like precipitates in addition to the beta' phase. The contributions of these ageing treatments to the alloy strengthening can be ranked as P2%TSA > P2%SSA > TSA > SSA, because the increments in the tensile yield strength were estimated to be 199 MPa > 148 MPa > 144 MPa > 110 MPa. Different from the traditional strengthening of beta' phase in the E + SSA and E + TSA alloys, the composite precipitates comprising the beta' phase, gamma' phase and chain-like precipitates in the E + P2%SSA and E + P2%TSA alloys provided better combined strengthening effect. The beta' phase was still dominated in the strengthening effect of the composite precipitates. Owing to the higher number density of beta' phase in the composite precipitates, the E + P2%TSA alloy exhibited the better mechanical performance as compared with the E + P2%SSA alloy. Finally, the E + P2%TSA alloy had the ultimate tensile strength of 452 MPa, the tensile yield strength of 401 MPa and elongation to failure of 3.3%.