Edington and co-workers have questioned the validity of the conclusions reached by Nicholson, on the basis of his marker experiments, regarding the mechanism or mechanisms responsible for superplastic flow. This note tries to show that Edington and co-workers have oversimplified the implications of Nicholson's findings. The opportunity is also taken to show that none of the presently available metallographic results concerning superplastic flow are inconsistent with a mechanism in which optimal superplastic deformation arises entirely from grain boundary flow processes.
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