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 degrees C. The initial microstructure of the alloy consisted of large globular dendrites of alpha phase (solid solution of Al in Mg) surrounded by a divorced eutectic of beta phase (Mg17Al12) and particles of alpha phase rich in Al. The minimum creep rate ((epsilon) over dot (m)) dependence on applied stress sigma and temperature T was analysed in detail. The high stresses applied in this study resulted in power law breakdown, leading to the use of the equation (epsilon) over dot (m) proportional to [sinh(alpha(0)sigma)](n) exp(- Q/RT) instead of the conventional power law. The stress exponent n was close to 5 with alpha(0) = 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. (C) 2000 Elsevier Science S.A. All rights reserved.