THE EFFECT OF LATTICE-RELAXATION AND CHARGE-TRANSFER ON MIXING ENERGIES AND CONFIGURATIONAL INTERACTIONS IN ALLOYS - CLUSTER CALCULATIONS FOR THE DILUTE BCC TI1-XV(X) AND FCC NI1-XAL(X) SYSTEMS

We present self-consistent LMTO-ASA cluster calculations of the solution energy Q and the nearest- and next-nearest-neighbour configurational interactions, V1 and V2, for the dilute BCC Ti1-x V(x) and FCC Ni1-xAl(x) alloys. The lattice relaxation effects (LRE) are described with a proper choice of atomic volumes for the impurity and its neighbours, which are found employing the conventional phenomenological estimates for atomic displacements near an impurity. For the Ti-V system LRE are found to be important, while the charge transfer effects (CTE) are small. The results of our calculations for this system seem to indicate that the phenomenological estimates overestimate LRE contributions to Q and V(i) by a factor of about two. We also present phenomenological estimates of LRE contributions to Q for impurities in 24 cubic host metals. For the Ni-Al system LRE are found to be small while CTE are significant, and taking them into account in the cluster calculations of Q and V(i) seems to noticeably improve their accuracy as compared with single-site approaches.