Constitutive equations in creep of wrought Mg–Zn alloys

The creep behaviour between 100 and 150 °C of Mg–Zn–RE wrought extruded alloys was investigated in the present study by taking into account both solid solution and particle-strengthening mechanisms. Constitutive equations correlating minimum creep rate, applied stress and temperature were thus preliminarily derived for dilute Mg–Zn solid solutions; the role of secondary phase particles, that form when the solubility limit of the alloying elements is exceeded, was then expressed by introducing a threshold stress representing particle–dislocation interaction. The model was applied to both the ZEK200 and to a reference commercial alloy ZM21; in both cases the description of experimental data was excellent, and the variation of threshold stress with temperature was found to agree with the available information on the thermal stability of the different families of precipitates, i.e. Mg–Zn and Mg–RE particles in ZM21 and ZEK200 respectively.