Deformation of adaptive materials. Part III: Deformation of crystals with polytwin product phases

Deformation as a result of solid-solid phase transformations, particularly martensitic, is the subject of this paper. The thermodynamic theory of elastic domains is applied to a transformation with a polytwin product phase which consists of two or more different domains (twins) forming plane-parallel alternations. The volume fractions of the phases and different twin components in the product phase are obtained as functions of applied stress (strain) and temperature. It is shown that during stress- or strain-induced transformation a twin fraction in martensite plates changes and a product phase becomes incompatible with an initial phase. This fact leads to the intrinsic instability of a transformational deformation, which appears as a negative Young's modulus under strain-controlled deformation and a thermodynamic hysteresis under stress-controlled deformation. The deformation is irreversible also due to microstructure irreversibility of the transformation from a polytwin state to a single-domain state. Superelastic deformation due to martensitic transformations is discussed as an application of the presented theory. (C) 2001 Elsevier Science Ltd. All rights reserved.