Calcul de la limite de phases A2/B2 dans le système Nb-Ti-Al par la méthode variationnelle des amas

Abstract
Considerable work has been devoted to Nb-Ti-Al phase diagram investigation to develop high temperature structural materials but the ordered B2 (Ti₂AlNb) phase field remains uncertain. The aim of this work is to calculate this phase diagram with a thermodynamic statistic method : the Cluster Variation Method (CVM). The CVM is used in the irregular tetrahedron approximation and takes into account first and second nearest neighbour interactions. The numerical values for the chemical pair interactions have been chosen to present the main experimental and theorical results available in the literature. Isothermal sections at T = 800, 1000, 1200, 1400 °C are presented. The B2 phase field is large and extended towards the Nb-rich area. The calculated probabilities of sites occupation show an ordered L2₁ (Heussler phase) instead of the B2 phase in the Ti-rich area. A miscibility gap is found between the ordered L2₁ phase and the disordered A2 phase. Another miscibility gap is obtained between the ordered B2 phase and the disordered A2 phase in the Al-rich area (metastable field). The temperatures of the order-disorder B2 → A2 transition and of the L21 → B2 transition have been calculated for different compositions.