Achieving ultrahigh strength in pre-ageing-extruded Mg-Gd-Y-Zn-Mn alloys via ageing treatment

Abstract

Ultrahigh-strength Mg-9.2Gd-4.4Y-1.0Zn-0.8Mn (wt%) alloys were produced through isothermal ageing at 200 °C applied to pre-ageing-extruded bars. The remarkable thermal stability exhibited by the β and LPSO phases ensures that the recrystallization state of α-Mg grains and the morphology and distribution of secondary phases in peak-aged alloys resemble those observed in the as-extruded alloys at the micron scale. The microstructure of peak-aged alloys is characterized by bimodal α-Mg grains with a pronounced fibrous texture, with the major strengthening phases consisting of prismatic β′ and basal γ′ phases precipitated during ageing. Through solid-solution and pre-ageing treatments, the extruded alloy at peak age demonstrated the highest tensile strength, with an ultimate tensile strength (UTS) of 555 MPa, a tensile yield strength (TYS) of 488 MPa, and an elongation of 5.8%. The considerable enhancement in tensile strength with satisfactory ductility in peak-aged alloys can be attributed to dense number distribution co-precipitation, which increases both basal and non-basal critical resolved shear stresses (CRSS) and inhibits microcrack nucleation and propagation. These findings have significant implications for the advancement of high-strength Mg-Gd-Y-Zn alloys with improved mechanical properties and performance in practical applications.