Constitutive equations in creep of Mg-Al alloys

The creep response of commercial Mg-Al alloys for die-casting was described by taking into account the distinctive microstructure consisting of α-Mg primary grains and a divorced eutectic formed by supersaturated α-Mg and β-Mg17Al12 typical of these materials. The α-Mg grains were assimilated to soft zones in a composite reinforced by hard zones rich in precipitates, the latter being the grain boundary regions of supersaturated α-Mg and β-Mg17Al12. Constitutive equations correlating minimum creep rate, applied stress and temperature were derived for dilute Mg-Al solid solutions and used to calculate the forward stress acting in soft and hard regions, respectively, on the assumption that both creep with a similar strain rate. The role of the particles was then expressed by a threshold stress; as a consequence, Si addition resulted in a substantial increase in the threshold stress in the hard zones, a mechanism that explains the reduction in minimum creep rate observed in the high-temperature low-stress regime in Mg-Al-Si compared with Mg-Al-Zn alloys.