Search

There is a competition between several face centered cubic (FCC)-based ordered inter-metallic phases in Ni-Mo alloys containing 8-33 at% Mo. The transformation behavior of these alloys in terms of ordering instabilities has been studied. First-principles tight-binding-linear muffin-tin orbital (TB-LMTO) method coupled with augmented space recursion (ASR) in conjunction with orbital peeling (OP) technique has been employed to extract the concentration dependent effective pair interactions. Further, the mean-field statistical mechanics based static concentration wave (SCW) model has been used to determine the free energies of these ordered phases as functions of temperature, composition and order parameter. This ASR-OP-SCW approach, applied to Ni-Mo alloy system, gives the correct ground state stability sequence as observed experimentally. Furthermore, it has been shown that such an approach can be used to study the complex transformation behavior involving several competing superstructures as well as competing first order and second order ordering processes.

Abstract:: The ground-state geometries, energetics and the stability of ( n =1-12) clusters are studied using ab initio molecular dynamics method. Our results indicate that the ground-state geometries of large clusters () are different from those of clusters where a trivalent impurity Al is added to the same monovalent host Na. Other features observed are an early appearance of 3-dimensional structure and a pentagonal growth path from n =6 up to n =11. As expected, the ground-state geometry of is not an icosahedron but can be viewed as a distorted form of one of the low lying geometries of cluster. In the energetically favored structures impurity atom Mg is never located at the center of the cluster. The stability analysis based on the energetics shows (8 valence electrons) to be the most stable. In addition there is a remarkable even-odd pattern observed in the dissociation energy and the second difference in energy which is absent in earlier studies of and clusters.

The formation of the ordered L12phase in Al-Li binary solid solutions has been studied using first-principleselectronic structure calculations in conjunction with the static concentration wave model. The effective multisite interactions have been determined by using Connolly-Williams prescription in the tetrahedron-octahedron (TO) cluster approximation for the f.c.c. lattice. The resulting Landau plots (free energy versus order parameter) have pointed out the temperatures below which instabilities with respect to ordering develops. Free energy-composition plots have been used for identifying the positions of the phase boundaries and of the critical spinodal. The results of this theoretical investigation have been presented in the form of a phase diagram in which different instability lines have been marked.