Microstructural evolution during creep of the Mg-Al-alloy AZ91hp

The light-weight pressure die-cast Mg-Al-base alloy AZ91hp used in automotive application in the as cast state was investigated with regard to evolution of the microstructure during annealing at 150 degrees C and creep at 135 degrees C and 210 MPa. Due to the fast cooling during die casting the microstructure is unstable, Annealing for 1 h at 150 degrees C leads to beginning of precipitation of coherent beta-particles. Creep for 34 min at 135 degrees C causes precipitation of beta on a large scale, but with strong local variation and formation of a subgranular dislocation structure with a spacing of free dislocations of about 4 b G/sigma and a subgrain size of about 15 b G/sigma, similar to the characteristic dislocation spacings in the steady state of deformation of cubic metals. These microstructural changes must be taken into account in the constitutive description of deformation of AZ91 at intermediate temperatures.