Percolation and the anti-structural bridge mechanism for diffusion in ordered alloys of the L12 type

Abstract

The antistructural bridge mechanism is a subset of the vacancy mechanism in binary ordered alloys/intermetallic compounds and is an important contributor to diffusion when there is substantial antistructural disorder. In the present study the limits for the antistructural bridge mechanism are established for a number of jump possibilities for both components in A3B alloys taking the L12 structure. The general vacancy model used supposes that the sublattices exist a priori. Three vacancy exchange frequencies describe A jumps from the α-sublattice to the β-sublattice and its reverse, and intra α-sublattice jumps. Another three exchange frequencies describe B atom jumps from the β-sublattice to the α-sublattice and its reverse, and again intra α-sublattice jumps. The limits of the antistructural bridge mechanism are determined analytically by percolation arguments and assessed by computer simulation. The A component does not show percolation threshold behaviour within a useful range of composition. The B component show several types of percolation behaviour depending on which jumps are operative. A particularly interesting interstitialcy-like mechanism is identified in which A atoms diffuse by intra α sublattice jumps which assist B atom diffusion by intersublattice jumps. This is discussed in the context of Ni3Al.