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

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 Mg17Sr2 and alpha-Mn phases, and, when heat treated (HT), the precipitation of rod-like alpha-Mn phases from the supersaturated Mg matrix and polygonal alpha-Mn phases from the supersaturated interdendritic Mg17Sr2 phase is observed. No coarsening of the interdendritic Mg17Sr2 occurs, while some coarsening is seen in the primary alpha-Mn phases present in the as-cast alloys when the Mn level is similar to 2 wt.%. An orientation relationship has been determined between alpha-Mn precipitated in the interdendritic regions during ageing and the interdendritic Mg17Sr2. The alpha-Mn that precipitates from the Mg17Sr2 intermetallic has a morphology which is different from the intradendritic alpha-Mn precipitating from the alpha-Mg matrix during ageing. Creep tests were conducted at temperatures between 150 and 200 degrees C and stresses of 40-50 MPa on Mg-(3-5)wt.%Sr-(0-2)wt.%Mn. The final creep strain (epsilon(final)) values decreased with increasing Sr, and mostly with increasing Mn in the composition range studied. (c) 2012 Elsevier B.V. All rights reserved.