A new theory is developed to explain superplastic flow in two-phase materials. It is postulated that boundary-dislocations, piled up in dislocation-Interphase-Boundaries (IPBs) climb away into disordered regions of the IPB. Sliding then occurs at an IPB as dislocations glide toward the head of the pile up to replace those which have climbed into disordered regions of the boundary. An energy barrier which would otherwise render sliding virtually impossible on dislocation-IPBs can, it is shown, be largely eliminated if the dislocations glide in pairs. The disorder (actually an antiphase domain boundary) which is created by the passage of the leading dislocation is then repaired by passage of its successor. The threshold stress for superplastic flow is provisionally identified with the stress which pins IPB dislocations to boundary ledges. The activation energy is theoretically that for IPB diffusion. Good agreement is obtained between the theoretical equation for superplastic flow and the results of published experiments.

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