Phase equilibria and transformations in Ti–(25–52) at.% Al alloys studied by electrical resistivity measurements

Phase equilibria and transformations in Ti–(25–52) at.% Al alloys were studied by electrical resistivity measurements over a range of temperatures using a special device that was constructed for this purpose. The α 2, α, β and γ phases are observed to have distinctly different resistivities and temperature dependencies, owing to which phase transitions could be monitored. The results show that the currently accepted Ti–Al phase diagram is, by and large, accurate, except for minor modifications being required to the phase boundaries in the composition range of Ti–(25–43)Al. Also, the α single-phase field is found to extend to the 25Al composition, which points to the absence of a β+ α 2↔ α peritectoid reaction. The room temperature electrical resistivity of stoichiometric α 2Ti 3Al and γ-TiAl are 118 and 31 μΩ cm, respectively, i.e. show a difference of 87 μΩ cm. The changes in resistivity with temperature are also significantly different in that in α 2 the resistivity saturates to a near-constant value near 750°C, whereas that of the γ phase shows a linear and near-constant slope with temperature like most metallic materials. In order to explain these differences, the electrical resistivity of the α 2 and γ phases has been modeled by fitting the data using the Bloch–Gruneisen formulation with certain simplifying assumptions. Good agreement between the calculated and experimental resistivity–temperature curves, and between calculated and experimental values of the residual resistivity and Debye temperature, were obtained. From the model, parameters such as the Fermi velocity, effective mass of a conduction electron, the number of electrons participating in conduction and electron mean free path have been calculated for the two phases. The calculations reveal that the mean free path is of the order of the lattice parameter in the case of α 2, which leads to high resistivity and resistivity saturation. The resistivity of the α 2 phase is also higher than that of the γ phase due to the fact that the Fermi velocity of the electrons is lower, effective electron mass higher and fewer electrons participate in conduction. These factors, coupled with hybridization and localization effects, cause the different electrical resistivity behavior of the two phases.