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Abstract

NiTi shape-memory alloys (SMAs) are an important subject of study because of their shape memory capability and extraordinary superelasticity. They have many application today, especially in the medical sector. The austenite/martensite transformation temperature of NiTi depends dramatically on composition (1 at% change can result in a shift of approximately 200 K [1]).

Ni₄Ti₃ precipitates, which are metastable phases in the desired composition range, have significant influence on the composition of the matrix. Therefore an exact knowledge about growth of these precipitates is key factor for designing the materials composition to the wanted transformation temperature, but also to tune elastic properties. In the past there were several phase-field modeling attempts [2,3] that tried to reproduce measurements of concentration profiles around the stoichiometric precipitates [4], which in fact are considered to be the most important factor for the martensitic transformation temperature. In order to overcome the disagreement between this simulations and the measurements we introduce diffusive fluxes based on elastic energy gradients in the matrix around the precipitate. Simulations are presented were performed by our open source phase field software OpenPhase.

[1] J. Frenzel et al., Journ. of Alloys and Compounds 385 (2004), 214
[2] N. Zhou et al., Acta Mater. 58 (2010), 6685
[3] W. Guo et al., Acta Mater. 59 (2011), 3287
[4] Z. Yang et al., Micron. 37 (2006), 503