The anti-structural bridge mechanism for diffusion in ordered alloys of the B2 type

The antistructural bridge mechanism has recently been proposed as a major contributor to diffusion at compositions away from stoichiometry in B2 ordered alloys/intermetallic compounds. In the present study the connectivity of the mobile atom distribution on the sublattices is first analysed using the authors' adaptation to ordered structures of Manning's formalism for diffusion in the random alloy. For percolation to occur, the analytical formalism predicts that the threshold concentration of antistructural (with respect to the sublattice) atoms must be greater than 0.273 at all levels of order. Computer simulation of the same model shows that for percolation to occur, the threshold concentration of antistructural atoms must be greater than 0.245 (for complete disorder) but this decreases to 0.13 (for complete order). This means that at the typically high levels of order observed in many ordered alloys and intermetallic compounds, the ASB mechanism cannot be a contributor to diffusion unless the overall composition is greater than 0.565. This composition is outside the stability range of most of these materials. (C) 1997 Elsevier Science Limited.