ELASTIC DOMAINS AND POLYDOMAIN PHASES IN SOLIDS

The discovery of polydomain phases has been one of the fundamental results of the experimental and theoretical studies of solid phase transformations. These phases consist of the alternation of domains: twins of a product phase or layers of different product phases. Elastic interactions between domains lead to the formation of the equilibrium polydomain structure corresponding to the Free energy minimum. Thus these domains have been named ''elastic domains''. The elastic domains form the polydomain phase whose average properties are dependent on its domain structure. Its basic element is a polytwin: a plane-parallel plate consisting of alternating plane-parallel domains or twins. The crystallographic orientation of the polytwin interfaces and its internal structure are equilibrium characteristics of the phase transition. Thus the parameters of the domain structure are the additional degrees of freedom of a polydomain phase. This fact considerably determines the properties of the polydomain phase, as well as the thermodynamics and kinetics of a phase transformation with its participant. This paper addresses the thermodynamic theory of elastic domains. The basic concepts and the equilibrium polydomain structures are considered using a tetragonal-orthorhombic phase transformation as an example.