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 elas tic domains is applied to a transformation with a polytwin product phase wh ich consists of two or more different domains (twins) forming plane-paralle l alternations. The volume fractions of the phases and different twin compo nents in the product phase are obtained as functions of applied stress (str ain) and temperature. It is shown that during stress- or strain-induced tra nsformation a twin fraction in martensite plates changes and a product phas e becomes incompatible with an initial phase. This fact leads to the intrin sic instability of a transformational deformation, which appears as a negat ive Young's modulus under strain-controlled deformation and a thermodynamic hysteresis under stress-controlled deformation. The deformation is irrever sible 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 presente d theory. (C) 2001 Elsevier Science Ltd. All rights reserved.