Prediction of substitutional behavior of ternary elements in B2 type NiTi, CoTi, FeTi and NiAl

A method is proposed for predicting the substitution behavior of ternary elements (X) in B2-type intermetallic compounds JAB) having substitutional (antistructure) defects at off stoichiometric compositions. Calculations are carried out using the pseudo-ground state analysis based on the nearest-neighbor, pair-approximation. The results revealed that the site preference of X can be determined by both heat of formation and alloy concentration: X occupies A sites only in case of Delta H-BX < Delta H-AB + Delta H-AX (Delta H-AB stands for the heat of formation between A and B), X occupies B sites only in case of Delta H-AX < Delta H-AB + Delta H-BX, and in cases, X occupies both or either A and/or B sites unfilled by constituent elements depending on alloy concentration. It is shown that: in NiTi, CoTi and FeTi, most 3A- and 4A-group elements occupy Ti sites only, and most 8A-, 4B- and 5B-group elements occupy Ni, Co and Fe sites only; while in NiAl, Co, Rh etc. occupy Ni sites only, and Si occupy Al sites only. These results are in good agreement with available data in the literature.