Replacive ordering in alloys

Replacive ordering can occur in an alloy in a continuous mode by the development and amplification of concentration waves of appropriate wave vectors, k. Such processes known as spinodal clustering (k∼〈000〉) and spinodal ordering (k defining the relevant superlattice structure) become operative when the alloy system experiences a free energy instability with respect to the order parameter associated with the corresponding wave vector. The presence of instabilities associated with these processes has been investigated in the Al–Li system using the cluster variation approach (CVM) considering upto the octahedron–tetrahedron cluster. The interaction energy parameters were obtained from the local density based first-principles band structure calculations. Predicted instability regimes have been compared with those obtained experimentally. The development of different competing superlattice structures in the Ni–Mo system has been discussed in terms of the development of appropriate concentration waves. Free energy–order parameter plots for different k-vectors have been compared for predicting relative stabilities of these superstructures and the evolutionary path of the ordering process. These results have been shown to be consistent with the experimental findings on this system.