Analysis of the creep behaviour of a thixoformed AZ91 magnesium alloy

Experiments were conducted to determine the creep response of a thixoformed Mg-Al-Zn AZ91 alloy in the temperature range between 120 and 180 °C. The initial microstructure of the alloy consisted of large globular dendrites of α phase (solid solution of Al in Mg) surrounded by a divorced eutectic of β phase (Mg₁₇Al₁₂) and particles of α phase rich in Al. The minimum creep rate (ε_m) dependence on applied stress σ and temperature T was analyzed in detail. The high stresses applied in this study resulted in power law breakdown, leading to the use of the equation ε_m∝[sin h(α₀σ)]ⁿ exp(-Q/RT) instead of the conventional power law. The stress exponent n was close to 5 with α₀ = 0.024 MPa^-1. The activation energy for creep Q was close to 140 kJ mol^-1. These results, and the comparison with creep data obtained by testing the same material after solution treatment, led the authors to conclude that in the considered temperature regime, creep of the thixoformed AZ91 is controlled by high temperature climb of dislocations.