Exceptional precipitation strengthening via novel β′-phase structures mediated by stacking faults in a Mg-Gd-Er-Ag-Zr alloy

The Mg-8Gd-3Er-0.5Ag-0.5Zr (wt.%) alloy fabricated by hot-extrusion + stamping exhibits an exceptional aging hardening response, with the yield strength being improved by >200 MPa. Traditional extrusion microstructure was observed in the as-extruded sample, with the elongated non-recrystallized grains showing typical (1010) fiber texture. After stamping, almost all non-recrystallized grains were twinned mainly following the (1012) twin orientation. This significantly changed precipitate morphologies formed during the following aging, from the well-known granular β′ precipitate into net-work β′ + β′_H structure, with the granular β′ precipitates being connected by chain-like β′_H precipitates. Additionally, a novel fault was found in the β′ precipitates, whose formation is highly related to the metastable I₂-type stacking fault. This new precipitation structure means the effective interparticle spacing being approximately zero, and the faults in the β′ precipitates can not only enhance strength of precipitates but also could efficiently impede dislocation motion, thus resulted in positive contribution on alloy’s yield strength. This work provides new insights in developing high-strength Mg alloys by modifying precipitation structure along with the inner faults in precipitation.