Phase transformations in two-phase TiAlTi3Al alloys during continuous heating and cooling, studied by electrical resistivity measurements

The α → γ transformation in a Ti-47.5at.%Al alloy and the α → α2 transformation in a Ti-40at.%Al alloy was studied by a novel computer-interfaced control-cum-data acquisition technique. In-situ, real-time, high-speed simultaneous measurements of resistivity and temperature were made in the system designed and constructed for this purpose. The experiments were made in the system designed and constructed for this purpose. The experiments were conducted in high vacuum. The samples were heated to the completely α region by controlled direct resistance heating, and cooled at various rates utilizing either controlled cooling algorithms or by varying the gas-jet quench. Temperature and resistivity changes were monitored during heating and cooling, and correlated with corresponding phase/microstructural changes. The results have shown that the γ phase is characterized by a low resistivity (about 50 μΩ cm at room temperature), whereas both α and α2 phases have high resistivities (about 190 to 200 μΩ cm at room temperature). Because of the large difference in resistivities of the α and γ phases, large and measurable changes in resistivity can be observed when one of these phases transforms to the other. In the Ti-47.5Al alloy, the lamellar γ morphology was observed at slower cooling rates, the Widmanstatten-like at intermediate cooling rates, and a massive reaction at very high cooling rates. Preliminary results of the transformation start and completion temperatures for the various reactions as a function of cooling rates are presented.