An analysis of the transient stage in low stress viscous creep

Abstract

Transient creep data recorded during low stress viscous creep of α-Ti, α-Zr, β-Co and zircaloy-2 are presented. The data are analysed in terms of the effect of stress, temperature and grain size on transient time as well as transient strain. Transient time is seen to be independent of stress in all four materials. Transient time decreases with increasing temperature and the variation can be described by an Arrhenius relation. Upto a grain size of nearly 100 μm, the transient time increases with increasing grain size. In the case of α-Ti and β-Co, beyond 100 μm and under conditions that facilitate Harper-Dorn creep to dominate flow, transient time exhibits grain size independent behaviour. Transient strain, normalised with elastic strain, appears generally to be independent of stress, temperature and grain size. Transient viscous creep data presented here, excluding those pertaining to coarse grained α-Ti and β-Co, can be satisfactorily rationalised following the grain boundary diffusion based model proposed by Raj. In regard to materials with grain size coarser than 100 μm, the data for α-Ti were more comprehensive and have been explained by a model proposed here, which is based on mutual annihilation of preexisting dislocations by volume diffusion controlled climb.