The role of α-Mn precipitation on the creep mechanisms of Mg-Sr-Mn

The creep mechanisms of cast Mg-5Sr-2Mn (JM52-wt%) and Mg-5Sr-1.25Mn (JM51-wt%) alloys are investigated via creep tests conducted at a temperature range of 150-225 degrees C using 50 MPa stress (typical automotive powertrain conditions) and in-depth TEM analysis. It is determined that, in both alloys, dislocational processes are responsible for creep deformation. At the low temperature range, pipe diffusion is found to be the creep rate determining mechanism for the two alloys studied. A transition in mechanism occurs in both alloys at high temperatures. With increasing temperature, higher activation energy mechanisms (dislocation climb in JM52 and activated cross-slip in JM51) become rate controlling. JM51 possesses superior creep resistance up to 200 degrees C. The inferior creep resistance of JM52 is due to the higher coarsening rate of the alpha-Mn phase dynamically precipitated from the Mg17Sr2 intermetallic phase. The creep strengthening of the alloys is caused by the dynamic precipitation of alpha-Mn phases on dislocations at the intradendritic regions during creep. (c) 2012 Elsevier B.V. All rights reserved.