Qualitative model for creep of AZ91D magnesium alloy

Creep properties of specimens taken from the core of AZ91D magnesium alloy ingots (9 pet Al, 1 pet Zn) were examined in the temperature range of 120 d egreesC to 180 degreesC and stress range of 40 to 115 MPa. Solution-treated and aged creep specimens were also tested. The creep rates observed were a bout three orders of magnitude lower than those of pure magnesium, and elon gations to fracture were seen to be at least twice those of pure magnesium. A minimum creep rate was reached after approximately 2/3 of the creep life of the specimens (except for the aged specimens, in which the minimum cree p rate appears at the beginning of the test). A qualitative model for the c reep process is proposed on the basis of the creep tests and optical, scann ing electron, and transmission electron microscopy. This model proposes tha t dislocation motion on additional slip systems is the primary creep mechan ism and that cracking acts as a stress relief mechanism. Structural instabi lity dictates the amount of hardening and, hence, creep resistance.