Threshold creep behaviour of an aged Mg-Zn-Ca alloy

The creep characteristics of an aged Mg-4Zn-0.5Ca (wt%) were investigated using impression creep tests in the temperature range of 423-523 K and under punch stresses in the range of 225-750 MPa for dwell times up to 3600 s. The creep behaviour of the alloy was found to be associated with a threshold stress and well-described by the Garofalo hyperbolic sine equation modified for the threshold stress concept. The stress exponent of 5 and the activation energy for creep close to that of pipe diffusion in Mg, i.e. 97 kJ mol−1, implies that dislocation climb controlled by diffusion through dislocation cores is the dominant rate-controlling mechanism. However, fitting the Garofalo equation to the experimental data showed that power-law breakdown (PLB) occurred in the material at high stress levels, especially at low temperatures. The threshold stress, appeared at low stress levels and low temperatures, was attributed to the nano-precipitates formed in the microstructure during aging treatment. The threshold stress was temperature-dependent, decreasing with increasing testing temperature and disappearing at about 575 K.