Effect of microstructure on creep behaviour of cast Mg97Y2Zn1 (at.%) alloy

The creep resistance of the Mg97Y2Zn1 (at.%) alloy in the as-cast condition and after a isochronal thermal treatment up to 560 °C is evaluated from 200 °C to 350 °C. The as-cast alloy shows a microstructure characterized by magnesium dendrites and a Long-Period Stacking Ordered Structure in the interdendritic regions. The heat treated alloy shows a fully lamellar structure within magnesium grains. In both cases, the stress dependence of the creep rate presents two different regions. For low temperature and/or high strain rates, the creep behaviour shows a high stress exponent (n = 11) and high activation energy. The alloy behaves as a metal matrix composite where the magnesium matrix transfers part of its load to the LPSO-phase. Moreover, the lamellar structure within the magnesium grains in the thermal treated alloy results in an additional barrier against creep deformation. At high temperature and/or low strain rates, creep is controlled by non-basal dislocation slip. The cast alloy showed higher creep resistance than the heat treated alloy at low stresses since the diffusion of Zn and Y atoms nucleate at Shockley partial dislocations inhibiting their movements.