Composition dependence and thermal hysteresis of martensitic transformations in Ni-Ti based shape memory alloys

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Martensite is formed by a massive phase transformation without solute redistribution and long range diffusion, but strongly dependent on the composition in many materials. In this project we address the effect of composition on the martensite phase formation in Ni-Ti shape memory alloys. We plan to develop a CALPHAD type Gibbs energy model to derive elastic constants of a binary alloy in dependence of its composition and temperature. The model data will be calculated from first principles. Special emphasis will be given to soft modes which are expected to play an important role in starting martensitic transformation. The model will be coupled to a phase-field model of martensitic transformation. Large scale 3-D simulations will be performed to investigate the effect of varying alloy composition and its gradients (segregation bands) on the martensitic transformation temperature. Further-on, nucleation of martensitic variants during the propagation of the martensitic front will be investigated to establish a relation between the scale of the martensitic lamellae, undercooling and alloy composition. The simulations will be compared to available experimental observations of martensitic transformation in Ni-Ti shape memory alloys. The model is expected to contribute to a physically based explanation of the strong dependence of martensite hysteresis on composition. It shall be extendable to different types of martensitic transformations and material classes.