Phenomenological and physical modelling of high homologous temperature deformation

Abstract

The main focus of this study is to critically assess the relevance or otherwise of a mesoscopic grain boundary sliding controlled flow model, which has been proposed as the common basis for explaining superplastic deformation in different classes of materials. The rationale behind this approach is that, as superplasticity is observed to be a near-ubiquitous phenomenon, there could be an underlying physical phenomenon responsible for this. If this were the case, the phenomenology of the superplastic flow process should also be similar for different classes of materials, i.e. there should be a universal curve for superplastic flow in all systems if the experimental variables like stress, strain-rate, strain-rate sensitivity and temperature of deformation are correctly normalized. Starting from these premises, it has been shown that under isothermal conditions the    log logσ ε plots of superplastic materials of different classes and the variation of the strain-rate sensitivity with  log ε for materials of different classes have near-identical features. The viscosity and the free energy of activation of all the alloy systems at (nearly) the same homologous temperature also vary quite similarly. Thus, the universality in the mechanical response of superplastic alloys is demonstrated. Further, the mesoscopic-grain boundary sliding controlled flow model for superplastic deformation, initially proposed for micron-grained metallic materials, but later extended to include dispersion strengthened alloys, intermetallics, metals with a quasi-crystalline phase, ceramics and ceramic-composites was taken up for consideration. An algorithm was developed to analyze the experimental data in terms of this model, so that many systems could be analyzed successfully. It has been shown that the mesoscopic-grain boundary sliding model satisfactorily describes superplastic deformation in metals and alloys, dispersion strengthened alloysceramics, composites, intermetallics, nanostructured materials and a material containing a quasi-crystalline precipitates and of grain sizes ranging from a few micrometers to a few nanometers. Also, the same approach has been used to satisfactorily explain superplasticity in geological materials and ice. In the present state of its development, in the mesoscopic-grain boundary sliding controlled model, even though theoretical expressions exist, the values of the free energy of activation and the threshold stress needed for the onset of mesoscopic- grain boundary sliding are treated as fitting constants. By way of applying the ideas to an allied, relevant situation, the mesoscopic-grain boundary sliding controlled model was also used to satisfactorily account for the inverse/ reverse Hall- Petch effect observed in materials when the grain size is in the lower ranges of the nanometer scale. Future efforts could be towards a theoretical framework at a mesoscopic level, by estimating the threshold stress necessary for the onset of mesoscopic-grain boundary sliding a priori