Effect of manganese on the creep behavior of magnesium and the role of alpha-Mn precipitation during creep

Abstract

Mg-Mn binary alloys (Mn <2 wt%) exhibit superior creep resistance over pure Mg, even though the only stable precipitate (alpha-Mn) has no significant effect on the room-temperature (RT) mechanical properties of Mg. To determine the strengthening and deformation mechanisms of Mg-Mn alloys under creep conditions, the activation energies of creep deformation (Q(c)) were calculated from compression creep curves (sigma: 15 MPa, 100-225 degrees C) and microstructure analysis was conducted on as-cast (AC), heat treated (HT) and creep tested (CT) samples via transmission electron microscopy (TEM). Creep tests (150 h) indicated that Mn enhances the creep resistance of Mg over a wide temperature range (100-225 degrees C). The creep enhancement in Mg-Mn alloy was attributed to the dynamic precipitation of alpha-Mn on the (0 0 0 1) planes of Mg during creep, which hinders dislocation motion. The HT samples exhibited growth of thin alpha-Mn rods elongated mainly parallel but also perpendicular to basal planes of Mg matrix. Two different orientation relationships were found between alpha-Mn rods and alpha-Mg matrix. Activation energy (Q(c)) calculations for pure Mg indicated pipe diffusion for the low temperature range (100-150 degrees C) and cross-slip for the high temperature range (150-225 degrees C). The Mg-1.5Mn alloy showed dislocation climb and cross-slip for the low temperature (100-175 degrees C) and the high temperature range (175-225 degrees C), respectively. (C) 2011 Elsevier B.V. All rights reserved.