Thermodynamics of Ti–Ni shape memory alloys

The thermodynamics of the Ti–Ni system are reviewed, and CALPHAD descriptions of metastable intermetallic phases are presented. These phases play an important role as precipitates in shape memory alloys. Metastable Ti3Ni4 and Ti2Ni3 are described as line compounds. Their thermodynamic model parameters are optimized with experimental solvus data and molar enthalpies at 0 K from new first-principles analysis. Best results are obtained, when the thermodynamic description of the D024-ordered TiNi3 phase is re-optimized with new thermodynamic data. This also requires adjustment of the other phase descriptions, including B2 austenite and B19′ martensite. The modifications have important consequences on the computed start temperature of the martensitic transformation, which is a crucial property for the shape memory effect. R-phase, a metastable intermediate martensite, is considered in the thermodynamic modeling. The following thermodynamic standard data for the metastable intermetallic phases are obtained at 298.15 K: ΔH°m(Ti3Ni4)=−34,714.5 J/mol, ΔH°m(Ti2Ni3)=−36,742 J/mol, ΔH°m(R-phase)=−35,649 J/mol, S°m(Ti3Ni4)=31.91 J/mol K, S°m(Ti2Ni3)=29.76 J/mol K, S°m(R-phase)=27.87 J/mol K.