PHASE-EQUILIBRIA AND TRANSFORMATIONS IN TI-(25-52) AT-PERCENT 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 temper atures using a special device that was constructed for this purpose. T he alpha(2), alpha, beta and gamma phases are observed to have distinc tly different resistivities and temperature dependencies, owing to whi ch phase transitions could be monitored. The results show that the cur rently 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 2 single-phase field is found to extend to the 25Al composition, which points to the absence o f a beta + alpha(2) <----> alpha peritectoid reaction. The room temper ature electrical resistivity of stoichiometric alpha(2)Ti(3)Al and gam ma-TiAl are 118 and 31 mu Omega cm, respectively, i.e. show a differen ce of 87 mu Omega cm. The changes in resistivity with temperature are also significantly different in that in at the resistivity saturates t o a near-constant value near 750 degrees C, whereas that of the gamma phase shows a linear and near-constant slope with temperature like mos t metallic materials. In order to explain these differences, the elect rical resistivity of the alpha(2) and gamma phases has been modeled by fitting the data using the Bloch-Gruneisen formulation with certain s implifying assumptions. Good agreement between the calculated and expe rimental resistivity-temperature curves, and between calculated and ex perimental values of the residual resistivity and Debye temperature, w ere obtained. From the model, parameters such as the Fermi velocity ef fective mass of a conduction electron, the number of electrons partici pating 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 alpha(2), which leads to high resistivity and resistivity saturation. The resistivity of the ar phase is also higher than that of the gamma phase due to the fact that the Fermi velocity of the electrons is low er, effective el ectron mass higher and fewer electrons participate in conduction. Thes e factors, coupled with hybridization and localization effects, cause the different electrical resistivity behavior of the two phases. (C) 1 998 Acta Metallurgica Inc.