Phenomenological and physical modelling of high homologous temperature deformation
Phenomenological and physical modelling of high homologous temperature deformation
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Date
2016-04-01
Authors
Sriharsha, Sripathi
Journal Title
Journal ISSN
Volume Title
Publisher
University of Hyderabad
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