SITE PREFERENCE OF SUBSTITUTIONAL ADDITIONS TO TRIPLE-DEFECT B2 INTERMETALLIC COMPOUNDS

The site preferences of substitutional ternary and higher-order additions to triple-defect type B2 intermetallic compounds are studied. A simple equation that is able to predict the site preference of dilute additions is given. The only parameters needed for this equation are the standard enthalpies of formation at the equiatomic composition. General equations that are able to calculate defect concentrations and substitutional atom distributions for dilute as well as non-dilute additions are also given. These equations use bond energies of atom pairs as parameters. This study shows the site preferences of dilute additions are not always in agreement with predictions based on the direction of the solubility lobes on ternary Gibbs isotherms. The site preference of dilute additions is controlled by the relative strength of bonds formed between the component elements. The addition atoms are distributed in such a way as to favor the formation of stronger bonds over that of weaker bonds. The predictions for dilute additions to NiAl, FeAl and CoAl are compared with experimental results in the literature. There are good agreements between the predictions of the model and the experimental results. The same methodology can be applied to substitutional additions to intermetallic compounds of other crystal structures such as the important L1(2) structure. It is also argued that the calculation of defect concentrations and substitutional atom distributions based on this type of methodology is critical for the understanding of physical and mechanical properties of ternary and higher-order intermetallic compounds.