REBUTTAL TO IN DEFENSE OF DIFFUSIONAL CREEP

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