THE INTER-RELATIONSHIP: EARTH SCIENCES — MATERIAL SCIENCES

Abstract

The development of concepts and models appertaining to the mechanisms of plastic deformation in solids is reviewed, with particular reference to thermally activated processes. Experience has shown that simple models of the plastic flow, based on a single intrinsic energy barrier, need to be refined, to allow for the heterogeneous internal-stress field, before they can describe adequately the flow of ductile materials above the “glass-transition” temperature; they cannot account for the ductile behaviour, observed down to temperatures approaching OK in some materials, e.g. face-centred cubic metals, in which a brittle-to-ductile transition is not observed. The complexity of the microstructure of most materials calls for models of plastic flow in which the dispersion of energy-barrier heights is an integral part of a description in terms of evolutionary stochastic processes. Techniques of computation and sophisticated experimental methods, particularly the various types of electron microscopy, are providing the technological basis for the formulation of theories in which the macro-deformation is described in terms of observed micro-processes. These conclusions are illustrated by reference to a tentative, stochastic, interpretation of the plastic flow of clay.