Site occupancy of alloying elements in Ti3Al compounds

A method is proposed to predict the site occupation behavior of alloying elements in Ti3Al. The electronic structures of 20 alloying elements (M) occupying both Ti and Al sites are calculated using the discrete variational-Xa cluster method. A diagram is drawn using Eb,Ti and Eb,Al, the binding energy of clusters when M taking Ti or Al sites. The elements are divided into three groups by two straight lines. The M located outside the two lines occupy either Ti or Al sites regardless of the composition. The substitution behavior of M in between are affected by the composition. The driving force for an element to order on one sublattice is related to its position in the diagram. The effects of composition, multi-element alloying and ordering process are discussed. The agreement with other results is generally good. The site competition phenomena and a new phase transition with stepped ordering process are predicted.