Deformation mechanisms in h.c.p. metals at elevated temperatures—I. Creep behavior of magnesium

Abstract

Experiments were conducted to determine the creep behavior of polycrystalline magnesium over the temperature range from 473 to 820 K. The results show the occurrence of three different mechanisms, with the creep process depending on the testing temperature and stress level. At lower temperatures, up to ~600–750 K, the activation energy for creep is equal to ~ 135 ± 10 kJ mole −1. there is normal primary and secondary creep, the stress exponent is close to ~ 5.2. and there is extensive basal slip. This behavior is consistent with control by dislocation climb, with a breakdown in power-law creep at normalized stresses above ~ 1.3 × 10 −1. At higher temperatures, above ~600–750 K. and at stresses above ~2.5 MPa, the activation energy for creep. Q. depends on stress, σ, through the relationship Q = (140 ± 10) + (295 σ) kJ mole −1 where σ is in MPa. Under these conditions, there is normal primary and secondary creep, the stress exponent is close to ~ 6.0. 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 in magnesium is estimated as ~ 135 ± 10 kJ mole −1. Finally, at high temperatures and stresses below ~ 2.5 MPa, the activation energy is equal to ~ 139 kJ mole −1. there is little or no primary creep, the stress exponent is close to 1.0. and there is an absence of visible surface slip lines. This behavior is attributed to a transition to lattice diffusion creep at these very low stress levels.