Impression creep behavior of the extruded Mg–4Zn–0.5Ca and Mg–4Zn–0.5Ca–2RE alloys

The creep characteristics of the extruded Mg–4Zn–0.5Ca and Mg–4Zn–0.5Ca–2RE (wt%) alloys were investigated using impression creep tests in the temperature range of 423–498 K. The creep behavior of both alloys was well described by the Garofalo hyperbolic sine relationship modified for the threshold stress concept. The stress exponent was 5 and the activation energies for creep were found to be close to that of the lattice diffusion in the α-Mg, i.e., 135 kJ mol–1. Lattice diffusion-controlled climb of dislocations was thus determined as the dominant rate-controlling mechanism. At lower temperatures and higher stresses, the power–law breakdown was observed. The appearance of the threshold stress was attributed to the finely dispersed second phase particles that are present in the microstructure. Threshold stresses decreased linearly with increasing temperature. The RE-containing alloy showed higher threshold stresses and better creep resistance, due to the effect of load transfer resulting from high volume fraction of the second phases and formation of the thermally stable (MgZn)12RE particles.