The microstructure and creep characteristics of cast Mg-3Si and Mg-3Si-1Gd alloys

The effect of 1 wt% Gd addition on the creep behavior of a cast Mg-3Si alloy was investigated by impression creep testing in the temperature range of 448-523 K under punching stresses in the range of 175-450 MPa for dwell times up to 4200 s. It was found that the creep strength of the Mg-3Si base alloy was improved by Gd addition. This was attributed to the modification of the primary Mg2Si particles, solid solution strengthening of Gd in the Mg matrix, and the formation of a new intermetallic phase with a composition close to Mg3Si2Gd2. Creep behavior of the binary Mg-3Si alloy can be divided into the low- and high-stress regimes, with the respective stress exponent values in the range of 5.1-6.0 and 8.8-11.5 and average activation energies of about 81 kJ mol−1 and 124.5 kJ mol−1. The creep behavior of the ternary Gd-containing alloy, however, was expressed by a single linear relationship over the whole stress and temperature ranges studied, with stress exponents in the range of 8.2-8.5 and an average activation energy of 111.1 kJ mol−1. The creep activation energies were all stress dependent, and thus in the binary alloy, stress-assisted climb of dislocations, controlled by pipe and lattice diffusion were the dominant creep mechanisms in the low- and high-stress regimes, respectively. In the ternary Gd-containing alloy, power law creep is valid and it can be suggested that the operative creep mechanism is dislocation climb controlled by lattice diffusion.