A theory of steady state structural superplasticity in different classes of materials: A materials-agnostic analysis

A mechanism in which grain/interphase boundary sliding (GBS) that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) is suggested to be the rate controlling process that leads to steady state structural superplastic flow in different classes of materials, viz., metals and alloys, ceramics, composites, intermetallics, nanostructured materials, bulk metallic glasses, geological materials and ice/ice-mixture, of grain sizes varying from a few micrometres down to a few nanometres. The accommodation processes of dislocation emission from sliding boundaries, highly localized diffusion in and in the vicinity of grain boundaries and/or grain rotations due to unbalanced shear stresses resulting from sliding along grain/interphase boundaries oriented differently to the applied stress axis are assumed to be faster than GBS and so they do not enter the strain rate equation. The actual accommodation process will depend on the nature and strength of boundary obstacles, chemical composition of the alloy and phases present. The equations derived are shown to be accurate by analysing the experimental data concerning many superplastic materials of different classes and grain size ranges. In the present state of development of the model one is able to predict without carrying out any new experiments the steady state structural superplastic flow in any material, including those whose superplastic response was not considered while developing the analysis and regardless of their class and grain size range, with the help of four mesoscopic scale constants, whose values are given in this paper, and the Frost-Ashby equations for estimating the shear modulus of any material at any temperature. The greater accuracy in prediction obtainable by this model is demonstrated by comparing its predictions with those of other (diffusion-based) mechanisms, which also predict a grain-size dependent strain rate.