Microstructural investigation and the creep behavior of Mg–Sr–Mn alloys

Abstract

Abstract The Mg–Sr–Mn ternary alloy system exhibits potential for good creep performance in powertrain applications. Consistent with thermodynamic calculations, the microstructure of as-cast (AC) alloys exhibit Mg 17 Sr 2 and α-Mn phases, and, when heat treated (HT), the precipitation of rod-like α-Mn phases from the supersaturated Mg matrix and polygonal α-Mn phases from the supersaturated interdendritic Mg 17 Sr 2 phase is observed. No coarsening of the interdendritic Mg 17 Sr 2 occurs, while some coarsening is seen in the primary α-Mn phases present in the as-cast alloys when the Mn level is ∼2 wt.%. An orientation relationship has been determined between α-Mn precipitated in the interdendritic regions during ageing and the interdendritic Mg 17 Sr 2 . The α-Mn that precipitates from the Mg 17 Sr 2 intermetallic has a morphology which is different from the intradendritic α-Mn precipitating from the α-Mg matrix during ageing. Creep tests were conducted at temperatures between 150 and 200 °C and stresses of 40–50 MPa on Mg–(3–5) wt.%Sr–(0–2) wt.%Mn. The final creep strain ( ɛ final ) values decreased with increasing Sr, and mostly with increasing Mn in the composition range studied.