Modeling of the ordering schemes in B2 niobium-titanium-aluminum alloys

A model is developed for calculating the site occupancies in ternary B2 compounds. The model is based on a simple approach involving chemical reaction rate theory for representing equilibrium in ordered compounds. Competing exchange reactions between different combinations of the three elements located on two different sublattices in a ternary B2 compound determine the equilibrium ordering scheme. The enthalpy change involved in these reactions is represented within the framework of the Bragg-Williams mean-field approximation using binary interaction parameters. The site occupancies are represented in a simple manner using the Ordering Tie Line (OTL) construction. This model has been used to compute successfully the site occupancies in ternary B2 compounds in Nb-Ti-Al alloys, FeAl and NiAl alloys. The model also predicts possible secondary ordering effects in those Nb-Ti-Al alloys which have an average composition close to the Heusler composition A₂BC, in agreement with results of electronic structure calculations available in the literature. Additionally, the model has been extended to quaternary B2 alloys.; A number of factors effect the site occupancies obtained by using the ALCHEMI technique such as the conditions of experimentation, ionization delocalization and the ordering state of the alloy itself. These factors would considerably influence the fast electron density distribution on the sublattices of an ordered crystal. The fast incident electron densities on the sublattices have been simulated using a many beam Bloch-wave model to explain the effect of sample orientation, and influence of ordering state of the alloy itself on the ALCHEMI measurements.; Finally, as an attempt to share the developed model and be able to use other models to obtain accurate descriptions of material phenomena, a distributed software model for the integration of legacy codes over the increasingly fast network has been developed. The basics of the model as are described in the text.