Nanoscale phase transition behavior of shape memory alloys - closed form solution of 1D effective modelling

We investigate the roles of grain size (I-g) and grain boundary thickness (l(b)) on the stress induced phase transition (PT) behaviors of nanocrystalline shape memory alloys (SMAs) by using a Core:shell type crystallite-amorphous composite model. A non-dimensionalized length scale (I) over bar (g)(= I-g/l(b)) is identified as the governing parameter which is indicative of the energy competition between the crystallite and the grain boundary. Closed form analytical solutions of a reduced effective 1D model with embedded microstructure length scales of l(g) and l(b) are presented in this paper. It is shown that, with (I) over bar (g) reduction, the energy of the elastic non-transformable grain boundary will gradually become dominant in the phase transition process, and eventually bring fundamental changes of the deformation behaviors: breakdown of two-phase coexistence and vanishing of superelastic hysteresis. The predictions are supported by experimental data of nanocrystalline NiTi SMAs. (C) 2017 Elsevier Ltd. All rights reserved.