Oxidation assisted creep of alloy 600

The influence of oxidation on the creep behaviour of Alloy 600 at 550 degrees C has been investigated by creep tests in air and in high Vacuum (10(-2) Pa). Thin strips (200 mu m) were chosen to emphasise the rob of the surface reaction phenomena. An important effect of the environmental conditions was observed, the steady-state creep rate being 14 times higher in air than in vacuum. Careful examination of the specimens showed that this effect could be attributed to an enhancement of the recovery processes, which are the limiting phenomena in the dislocation creep mechanism evidenced for the samples tested. Therefore a detailed study of the oxide scale and of oxidation kinetics in air and in vacuum was carried out. Results indicated that under Vacuum a thin, protective Cr2O3 layer grew on the surface of the samples, whereas the formation of a non-protective, nickel-rich oxide was evidenced in air. Thus, the oxidation mechanism was found to dramatically influence the creep properties. This effect was modelled in terms of an increase in the effective diffusion coefficient, and related to vacancy injection induced by the cationic, constrained growth of the non-protective oxide scale in air.