Deformation mechanisms in h.c.p. metals at elevated temperatures-II. Creep behavior of a Mg-0.8% Al solid solution alloy

Abstract

Experiments were conducted to determine the creep behavior of a Mg-0.8% Al solid solution alloy over the temperature range from 473 to 810 K. The results show the occurrence of three different mechanisms, with the creep process dependent on the testing temperature and stress level. In the lower temperature range, up to ~600-750 K depending on the stress, the behavior divides into two regions: 1. (i) at the lower normalized stresses, the activation energy for creep is ~ 140 ± 10 kj mole-1. there is a very small or no measurable instantaneous strain, there is a very brief normal or inverted primary stage of creep, the stress exponent is ~ 3.0. there is extensive basal slip, and the substructure consists of a random distribution of dislocations 2. (ii) at the higher normalized stresses, the activation energy for creep is ~ 140 ± 10 kJ mole-1, there is a measurable instantaneous strain, there is an extensive normal primary stage, the stress exponent is ~6.0, and the substructure consists of well-defined subgrains. The data are consistent with viscous glide at the lower stresses (class A behavior) and a transition to dislocation climb at the higher stresses when the dislocations break away from their solute atmospheres. From measurements of the critical break-away stress, the solute-dislocation binding energy is estimated as ~0.14 ± 0.03 eV. In the higher temperature range, above ~ 600-750 K depending on the stress, there is a measurable instantaneous strain, a normal primary stage of creep, the activation energy is ~230 ± 15 kJ mole-1 and decreases with increasing stress, the stress exponent is ~4.0. and there is extensive non-basal slip. This behavior is consistent with the cross-slip of dislocations from the basal to the prismatic planes, and the constriction energy is estimated as ~ 160 ± 10 kJ mole-1. © 1982.