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 from thermodynamic point of view based on the Bragg-Williams method. The model is developed by taking new point defect formation mechanism, random vacancy distribution (RVD) and antisite atom recovering (ASAR) processes, into consideration, which was proposed based on the current findings in X-ray and in situ neutron diffraction studies for B2 FeAl. Free energy expressions for pure states of the antisite defect (ASD), RVD, triple defect (TRD) and ASAR and also those for hybrid state between the RVD, TRD and ASAR are obtained. From these expressions, the condition for appearance of the RVD and ASAR behavior is considered. Numerical results are given for three cases which show the different point defect behaviors due to composition and temperature. The first case indicates a creation of a substantial high concentration 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 antisite atom, not by the TRD process. In the second case, RVD like behavior appears 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 Science Ltd. All rights reserved.