ON THE STABILITY OF INTERMETALLIC PHASES

A general methodology using atomic clusters is applied to three proble ms connected to the study of alloy phase stability. The duster method proposed by Alien and Cahn is applied to non-ideal hcp structures unde r tetrahedral approximation using multiatom interactions. The possible ground-state structures which are stable at absolute zero temperature are obtained. A geometrical representation in 4D parameter space of t he possible strengths of multiatom interactions permitted for these st ructures is illustrated in terms of a 2D analogue. Extending these ide as, the cluster variation method (CVM) proposed by Kikuchi is applied to fee structures under tetrahedral approximation to find the effect o f multiatom interactions on the topology of the coherent phase diagram s in which all the phases present are derivable by mere rearrangement of atoms on the parent disordered structure. In addition, the possible invariant reactions are identified in such coherent phase diagrams. F inally the CVM is applied for calculating a model incoherent phase dia gram, that of Ti-Zr system, where disordered hcp and bcc phases are pr esent. The free energies of hcp and bcc phases are formulated using CV M procedures respectively under tetrahedral-octahedral and tetrahedral approximations. The CVM is shown to be in better agreement with the t hermodynamic data and to be able to reproduce the correct value of mea sured enthalpy of transformation compared to that given by the regular solution model, which significantly overestimates the same.