Possibilities of random vacancy distribution and antisite atom recovering in the point defect mechanism in B2-type intermetallics

Point defect behavior in B2-type intermetallic compounds is investigated fr om thermodynamic point of view based on the Bragg-Williams method. The mode l is developed by taking new point defect formation mechanism, random vacan cy distribution (RVD) and antisite atom recovering (ASAR) processes, into c onsideration, which was proposed based on the current findings in X-ray and in situ neutron diffraction studies for B2 FeAl. Free energy expressions f or pure states of the antisite defect (ASD), RVD, triple defect (TRD) and A SAR and also those for hybrid state between the RVD, TRD and ASAR are obtai ned. From these expressions, the condition for appearance of the RVD and AS AR behavior is considered. Numerical results are given for three cases whic h show the different point defect behaviors due to composition and temperat ure. The first case indicates a creation of a substantial high concentratio n of the A-vacancy only in the B-rich composition region, as observed in B2 NiAl. This situation, however, is interpreted by the ASAR process by B ant isite atom, not by the TRD process. In the second case, RVD like behavior a ppears in the A-rich region. This resembles the observation in B2 FeAl, but it is not by an appearance of the pure RVD state. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.