Prediction of the ordering behaviours of the orthorhombic phase based on Ti2AlNb alloys by combining thermodynamic model with ab initio calculation

The order–disorder transformation of intermetallics is of fundamental and technical importance. The ordering behaviours of the O phase based on Ti2AlNb alloys are predicted by combining thermodynamic model with ab initio calculation. The site occupying tendencies of the constituent elements are studied for the first time theoretically without referring experimental data as input. The predicted results show that Al atoms always tend to occupy the γ (4c1) sublattice, Ti atoms tend to occupy the α (8g) sublattice and Nb atoms the β (4c2) sublattice. The ordering tendencies of Ti and Nb atoms decrease with the increase of temperature, while the site occupation of Al atoms is weakly dependent on the temperature. The order–disorder transformation belongs to a second-order transition with a continuous character. It is also predicted that for the nonstoichiometric O phase with Al contents higher than 25 at.%, the site occupancies of the excess Al atoms prefer the β sublattice. The predicted site occupancy fractions and order parameters agree well with the reliable experimental data. The prediction has been improved compared with the Gorsky–Bragg–Williams model, as well as our early LMTO-ASA calculations.