Power-law and exponential creep in class M materials: discrepancies in experimental observations and implications for creep modeling

This paper discusses our current understanding of the processes thought to be dominant in the exponential creep regime as well as the implications for creep modeling relating to both power-law and exponential creep regions. The significance and implications of creep controlled by vacancy diffusion along dislocation cores are discussed. It is pointed out that creep substructures, other than subgrains, have been reported in the literature, and a bifurcation diagram is presented to demonstrate how this evolution can occur from an initially homogeneous dislocation substructure. The use of nonlinear dislocation dynamics in creep modeling is advocated to rationalize the observed diversity in the creep substructures. It is demonstrated that the dislocation substructure evolution models can be coupled with a viscoplastic model through the volume fractions of the ‘hard’ and ‘soft’ phases. This coupling is shown to lead to the stress-subgrain size relationship in a simple and a natural way.