Enhancing the creep resistance of a cast AZ91 alloy via minor Sc addition

Abstract

The influence of 0.2 wt% Sc addition on the microstructure and creep characteristics of a cast AZ91 magnesium alloy was studied by impression creep tests under the constant punch stresses ranging from 175 to 700 MPa in the temperature range 425–525 K. The findings indicated that creep rates decreased at all stress levels and temperatures after Sc addition. This was ascribed to the formation of the thermally stable Al3Sc and Mg5Al4Sc particles, reduction in the quantity of the unstable β-Mg17Al12 phase, and solid solution strengthening of Al in the Mg matrix. The creep stress exponent and activation energy values obtained from the studied alloys ranged from 5–6 and 95–117 kJ/mol, respectively. The detected drop in creep activation energy with increasing stress implies that two concurrent mechanisms of dislocation climb controlled by lattice and pipe-diffusion are competing with each other. The former one was the governing mechanism at low stress levels, while the latter one was the prevailing mechanism in the high stress regimes.