Combined synchrotron X-ray diffraction, dilatometry and electrical resistivity in situ study of phase transformations in a Ti2AlNb alloy

Phase transformations in near Ti2AlNb alloy occurring on cooling of 10 °C min$^{−1}$ from 1020 °C were studied in situ using high energy X-ray diffraction and electrical resistivity measurements coupled with simultaneous dilatometric analysis. From Rietveld refinement of X-ray data, it was established that high temperature range from about 870 to 1020 °C contained partially ordered $β$ phase. The degree of long-range order was of 0.6 at 1020 °C and decreased slowly with temperature decrease and fast when O-phase was formed. The formation of O-Ti$_2$AlNb phase started from about 870 °C and consisted of two stages: (i) fast transformation of partially ordered $β$ into O phase at higher temperatures followed by (ii) slow transformation of fully disordered $β$ into O-phase at lower temperatures. The appearance of O-phase was accompanied by important volume decrease of the alloy as recorded using dilatometry as well as by a decrease of electrical resistivity. The transformations temperatures revealed by high energy X-ray diffraction, dilatometry and electrical resistivity were in good agreement. The obtained results were discussed using thermodynamic simulations carried out with Thermo-Calc software and TCTI2 database