Symmetry, texture and the recoverable strain of shape-memory polycrystals

Shape-memory behavior is the ability of certain materials to recover, on heating, apparently plastic deformation sustained below a critical temperature. Some materials have good shape-memory behavior as single crystals but little or none as polycrystals, while others display good shape-memory behavior even as polycrystals. In this paper, we propose a theoretical explanation for this difference: we show that the recoverable strain in a polycrystal depends on the texture of the polycrystal, the transformation strain of the underlying martensitic transformation and especially critically on the change of symmetry during the underlying transformation. Roughly, we find that the greater the change in symmetry during transformation, the greater the recoverable strain. We include an extensive survey of the experimental literature and show that our results agree with these observations. We make recommendations for improved shape-memory effect in polycrystals.