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E. Povoden-Karadeniz, D. Cirstea, P. Lang, E. Kozeschnik:
"Thermodynamic modeling of metastable phases for precipitation simulation in NiTi shape memory alloys";
Poster: Calphad XLI, Berkley, USA; 2012-06-03 - 2012-06-08.

Shape memory alloys (SMAs) are martensitic metals that can "remember" the original shapes of their parent modification. In classical Ni-(46-49) at% Ti shape memory alloy with B2-structured parent matrix, metastable precipitates play an important role for the shape memory behavior, as they influence the martensitic phase transition [1,2]. Thermokinetic simulation of the precipitate evolution assists our knowledge of the influence of precipitates on materials properties after different heat treatments. This simulation requires thermodynamic and diffusion mobility databases. In this work, Gibbs energy functions of the metastable intermetallic phases Ni4Ti3 and Ni3Ti2 are optimized with experimental solvus data and 0K molar enthalpies from first principles analysis. Furthermore, diffusion coefficients of Ni-Ti B2 phase are used for the construction of a mobility diffusion database for thermokinetic precipitation simulations. As the metastability of Ni4Ti3 and Ni3Ti2 is related to the thermodynamics of other phases in the system, the available thermodynamic equilibrium assessments of the Ni-Ti system are also checked. We found inconsistency of calculated Gm(Ni3Ti) with new electromotive force experiments. Also, Hm(Ni3Ti) deviate considerably from most published calorimetric and first principles data. New first principles calculations confirm this inconsistency. Correct molar enthalpies data are of particular importance for thermokinetic precipitation simulations with MatCalc, since they occur in the generalized broken-bond model [3] for calculation of interfacial energies. Thus thermodynamic model parameters of D024-ordered Ni3Ti are modified, aiming on a better agreement with new thermodynamic data. In order to maintain the agreement with the experimental phase boundaries, also slight correction of the model parameters of the fcc alloy phase is required. Besides classical NiTi shape memory alloy, ternary Ni-Ti-based SMAs have been investigated due to their potential of improving the shape memory effect [4-6]. These are Ti-Ni-Zr, Ti-Ni-Hf, and Ti-Ni-Cu alloys. The new parameters are thus included in an open-source thermodynamic Ni-Ti-Zr-Hf-Cu database.

[1] S. Miyazaki S, Y. Ohmi Y, K. Otsuka and Y. Suzuki, J.Phys., 43(C4) (1982) 255-260.
[2] T. Saburi, T. Tatsumi and S. Nenno, J. Phys., 43(C4) (1982) 261-266.
[3] B. Sonderegger and E. Kozeschnik, Metall. Mater. Trans., 40A (2009) 499-510.
[4] P.J. McCluskey, C. Zhao, O. Kfir and J.J. Vlassak, Acta Mater., 59 (2011) 5116-5124.
[5] Y. Motemani, P.J. McCluskey, C. Zhao, M.J. Tan, J.J. Vlassak, Acta Mater., 59 (2011) 7602-7614.
[6] T. Saburi, Shape Memory Materials, Cambridge University Press, Cambridge, 1998.

Ni4Ti3, Ni3Ti2, Ni3Ti, metastable, precipitation