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.