Heat capacities of several Al-Ni-Ti compounds

The heat capacities of several Al-Ni-Ti compounds were determined by drop calorimetry over the temperature range of 500-1500 K. A modified Einstein model and a two-parameter polynomial model provide reasonable representations of the experimental heat capacity data. The heat capacities, Cp, using a two-parameter polynomial representation are as follows: Ni0.5Ti0.5, Cp = 22.39 + 8.24 × 10−3 T (J/(mol K)); Al0.45Ni0.5Ti0.05, Cp = 23.01 + 5.12 × 10−3 T (J/(mol K)); Al0.16Ni0.74Ti0.10, Cp = 18.36 + 10.76 × 10−3 T (J/(mol K)); and Al0.25Ni0.5Ti0.25, Cp = 25.38 + 1.088 × 10−3 T (J/(mol K)). The experimental data are compared with the values derived from a thermodynamic database of Gibbs energy functions. The analysis shows that (1) either model is a good representation of the data; (2) it is not adequate to assume the Neumann-Kopp rule for the description of the heat capacities of Al0.45Ni0.5Ti0.05, Al0.16Ni0.74Ti0.10 and Al0.25Ni0.5Ti0.25; (3) it is not appropriate to determine a compound Cp from a thermodynamic database of Gibbs energy functions if the compound is modeled by using the Neumann-Kopp rule and any of its components undergoes melting in the temperature range of interest.