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

摘要

Considerable work has been devoted to Nb-Ti-Al phase diagram investigation to develop high temperature structural materials but the ordered B2 (T^AlNb) phase field remains uncertain. The aim of tliis work is to calculate this phase diagram with a thermo dynamic 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 pah' interactions have been chosen to present the main experimental and theorical results available hi 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 hi the Ti-rich area. A miscibiJity 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 (inetastable field). The temperatures of the order-disorder B2 -> A2 transition and of the L21 -> B2 transition have been calculated for different compositions.