Study of the Ti-Al-Nb phase diagram

Differential thermal and X-ray diffraction analyses as well as optical and transmission electron microscopy were used to study orthorhombic Ti2AlNb-based alloys, such as Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb. These alloys in the initial state contain an orthorhombic O phase (Ti2AlNb) ordered in terms of all three elements. The differential thermal analysis was performed at different heating and cooling rates, namely, 40 and 80 K/min. The solidus and liquidus temperatures of the Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb alloys have been measured; these are T S = 1707 and T L = 1737°C, T S = 1703 and T L = 1730°C, and T S = 1694 and T L = 1728°C, respectively. The temperature boundaries of phase transformations occurring in these alloys have been determined. The activation energy of phase transformations has been calculated using the Johnson-Mehl-Avrami equation. The results obtained are compared with the available literature data. Causes of the discrepancy between equilibrium phase diagrams of the Ti-Al-Nb system reported by different authors are analyzed.