Creep studies of coarse-grained AZ91D magnesium castings

Creep properties of specimens taken from the core of AZ91D magnesium alloy (9% Al–1% Zn) ingots were investigated in the temperature range 120–180°C and stress range 40–115 MPa. Creep tests were performed under constant and varying loads, solution treated creep specimens were also tested. Observed creep rates are about three orders of magnitude lower than those of pure magnesium and elongations to fracture are at least twice those of pure magnesium. No steady-state stage is observed, however a minimum creep rate is reached after approximately two thirds of the creep life of the specimens. The stress exponent is ≈11 and is independent of temperature and stress. The activation energy decreases with increasing temperature, from 220 to 94 kJ mol−1. Fractography studies show that the fracture is intergranular.