Mechanical and thermodynamic properties of intermetallic compounds in the Ni-Ti system

The mechanical and thermodynamic properties of intermetallic compounds in the Ni-Ti system are studied by first-principles calculations. All phases show anisotropic elasticity in different crystallographic directions, in which Ni₃Ti and NiTi₂ are approaching the isotropy structure. The elastic moduli and Vicker's hardness of Ni-Ti system intermetallic compounds decrease in the following order: Ni₃Ti > B2-NiTi > B19′-NiTi > NiTi₂, and Ni₃Ti shows the best mechanical properties. The intrinsic ductile nature of Ni-Ti compounds is confirmed by the obtained B/G ratio. The temperature dependence of linear thermal expansion coefficients (LTECs) of the compounds is estimated by the quasi-harmonic approximation (QHA) method. Ni₃Ti shows the largest values among all Ni-Ti intermetallic compounds. At room temperature, the LTEC for Ni₃Ti is 8.92 × 10-6 K-1, which falls in between the LTEC of zirconia toughened alumina (ZTA) (7.0-9.5 × 10₆ K^-1) and iron matrix (9.2-16.9 × 10⁶ K^-1); i.e., the thermal matching of the ZTA/iron composite will be improved by introducing Ni 3Ti intermetallic compound into their interface. Other thermodynamic properties such as sound velocity and Debye temperature are also obtained.