Rebuttal to “In defense of diffusional creep”

Rebuttals to the recent criticisms of Burton and Reynolds and of Greenwood to our papers challenging the existence of diffusional creep are presented and responses by us to the following specific areas are discussed. (1) Denuded zones, universally to be considered direct evidence for diffusional creep, are seen only under conditions where a high stress exponent (as opposed to a value of unity) is observed, indicating that diffusion-controlled dislocation creep is the operative deformation process. (2) The creep rate of fine-grained superplastic materials is invariably orders of magnitude greater than predicted by diffusional creep; in these cases, the deformation process has been established to be grain-boundary sliding accommodated by slip (GBS) rather than diffusional creep. (3) The creep behavior of fine-grained copper measured by Burton and Greenwood can be described by a diffusion-controlled dislocation creep mechanism at high stresses, by GBS at intermediate stresses, and by Harper-Dorn creep at low stresses, with no clear evidence for diffusional creep.