MODEL FOR GRAIN-BOUNDARY SLIDING AND ITS RELEVANCE TO OPTIMAL STRUCTURAL SUPERPLASTICITY .1. THEORY

An assessment of the experimental findings leads to the conclusion tha t optimal structural superplasticity results from grain/interphase bou ndary sliding-diffusion coupled flow. An analysis of the boundary slid ing process is presented first. By suggesting that both regions I and IIa (lower stress range of region II) of superplastic flow result from sliding-diffusion coupled flow, and treating mesoscopic (cooperative) boundary sliding as the rate controlling mechanism for optimal superp lasticity, the stress, temperature, and grain size dependences of the strain rate of deformation are predicted. The above equation is then r elated to the stress exponent n (the inverse of the strain rate sensit ivity index m). An analysis for determining the true activation energy for the rate controlling process is presented. Expressions for the di stribution of internal stresses arising from sliding and the boundary viscosity are derived and the presence of an initial unsteady region, predicted in an earlier analysis, is shown to be a natural consequence of this approach. (C) 1996 The Institute of Materials.