Replacive ordering in alloys

Replacive ordering can occur in an alloy in a continuous mode by the develo pment and amplification of concentration waves of appropriate wave vectors, k. Such processes known as spinodal clustering (k similar to [000]) and sp inodal ordering (k defining the relevant superlattice structure) become ope rative when the alloy system experiences a free energy instability with res pect to the order parameter associated with the corresponding wave vector. The presence of instabilities associated with these processes has been inve stigated in the Al-Li system using the cluster variation approach (CVM) con sidering upto the octahedron-tetrahedron cluster. The interaction energy pa rameters were obtained from the local density based first-principles band s tructure calculations. Predicted instability regimes have been compared wit h those obtained experimentally. The development of different competing sup erlattice structures in the Ni-Mo system has been discussed in terms of the development of appropriate concentration waves. Free energy-order paramete r plots for different k-vectors have been compared for predicting relative stabilities of these superstructures and the evolutionary path of the order ing process. These results have been shown to be consistent with the experi mental findings on this system. (C) 1999 Published by Elsevier Science B.V. All rights reserved.