THRESHOLD CREEP-BEHAVIOR OF AN MG-ZN-CA-CE-LA ALLOY PROCESSED BY RAPID SOLIDIFICATION

The creep behaviour of an Mg-3Zn-Ca-1.5Ce-0.5La alloy processed by rap id solidification was investigated at temperatures ranging from 473 K to 523 K and applied stresses up to 82.6 MPa. It is shown that the min imum or steady-state creep strain rate is not attained up to a true cr eep strain of about 0.35. At least up to this creep strain, the true c reep strain rate decreases in time essentially linearly for strains hi gher than 0.03. Thus primarily the true creep strain rate epsilon(5), (the subscript 5 refers to a true creep strain of 0.05) is used to des cribe the creep behaviour of the alloy. In double logarithmic coordina tes, the true creep strain rate epsilon(5), depends on the applied str ess in a way that strongly suggests a true threshold stress. The thres hold stress sigma(TH) decreases with increasing temperature and the sa me holds for the true applied stress exponent n. The values of this st ress exponent are low, decreasing from 3 at 473 K to 2 at 523 K. At ap plied stresses ranging from about 10 MPa to about 85 MPa, the activati on enthalpy (true activation energy) of creep Delta H is significantly higher than the activation enthalpy of diffusion in the alloy matrix lattice Delta H-L. Analysis of the creep data suggests that the proces s (or dislocation mechanism) controlling the creep strain rate in the alloy is either dislocation slip in the basal planes dependent on cros s slip to non-basal planes or, more probably, dislocation glide in non -basal slip planes dependent on the nucleation of kink motion.