The Creep of Dispersion-Strengthened Ni–Co Alloys

Abstract

The creep behaviour of three Ni–Co–2.5% A1₂O₃ alloys containing 0, 30) and 67% Co has been studied at temperatures in the region of 770 and 1000 K. It is shown that in both ranges the steady-state creep rate depends upon stress to the power four. At the lower temperature the activation energy is composition-dependent, values of 85, 184, and 121 kJ/mol being recorded for the 0, 30, and 67% Co alloys, respectively. At the higher temperature the activation energy corresponds with that for self-diffusion and, like the pre-exponential term, is essentially independent of composition except through small variations in diffusivity. The low-temperature creep is explained in terms of a thermally activated cross-slip and pipe-diffusion mechanism that enables dislocations to by-pass particles. At the higher temperature it is suggested that bulk diffusion is the rate-controlling process and that the absence of any marked composition-dependence is due to the jog distribution being determined mainly by dislocation/particle interactions.