Order-disorder transition in hcp binary alloys: Next-nearest-neighbor interactions

The order-disorder transition in hcp binary alloys is investigated by Monte Carlo simulations and the renormalization-group methods. The inclusion of the interactions beyond the nearest neighbors induces qualitative changes of the phase boundaries. Especially, the contribution from second-neighbor interactions explains the shift along the concentration axis of the congruent point of order-disorder transition observed in the $\ensuremath{\alpha}/{\ensuremath{\alpha}}_{2}$ phase boundaries in the Ti-Al binary system.