Modelling of site occupancies in ternary B2 compounds in Nb-Ti-Al

In this paper a model is presented for calculating the site occupancies in ternary B2 compounds. The model is based on a simple approach involving che mical rate theory for representing equilibrium in ordered compounds. Compet ing exchange reactions between different combinations of the three elements located on two different sublattices in a ternary B2 compound determine th e equilibrium ordering scheme. The enthalpy change involved in these reacti ons is represented within the framework of the Bragg-Williams mean-field ap proximation using binary interaction parameters. The site occupancies are r epresented in a simple manner using the ordering tie line (OTL) constructio n. This model has been used to compute the site occupancies in ternary B2 c ompounds in Nb-Ti-Al alloys using values of binary interaction parameters a vailable in the literature. The results are in reasonable agreement with ex perimentally determined OTLs for these alloys. The values of the binary int eraction parameters have been identified as significant contributors in det ermining the accuracy of predictions afforded by the model and have therefo re been optimized in order to obtain better agreement between the calculate d and experimentally determined OTLs. The order-disorder transformation tem perature, predicted using the model, is in excellent agreement with the exp erimental results for the Nb-40at.%Ti-15at.%Al alloy. Finally, the model al so predicts secondary ordering effects in those Nb-Ti-Al alloys which have an average composition close to the Heusler composition A(2)BC, in agreemen t with results of electronic structure calculations available in the litera ture.