Microalloying is an effective approach to improve the mechanical properties of γ-TiA1 intermetallic compound.Knowledge about the site occupancy of the ternary alloying element in the crystal lattice of γ-TiAl is highly demanded in order to understand the physics underlying the alloying effect.Previous first-principle methods-based thermodynamic models for the determination of the site occupancy were based on the point defect gas approximation with the interaction between the point defects neglected.In the present work,we include the point defect interaction energy in the thermodynamic model,which allows us to predict the site occupancy of the ternary alloying element in γ-TiAl beyond the point defect gas approximation.The model is applied to the γ-TiAl-Nb alloy.We show that,at low temperature,the site occupancy of Nb atoms depends on the composition of the alloy:Nb atoms occupy the Al sublattice for the Ti-rich alloy but occupy Ti sublattice for the Al-rich alloy.The fraction of Nb atoms occupying A1 sublattice in the Ti-rich alloy decreases drastically,whereas the fraction of Nb atoms on the Ti sublattice in the Al-rich alloy decreases slightly with increasing temperature.At high temperature,Nb atoms occupy dominantly the Ti sublattice for both the Ti-rich and Al-rich alloys.The interaction between the point defects makes the Ti sublattice more favorable for the Nb atoms to occupy.