Calculation of thermodynamic functions for the nickel-aluminum system

Abstract

Now in recent years considerable progress has been made in the calculation of phase diagrams by a method based on modern concepts of the quantum theory of metals and alloys [3-5]. In this method thermodynamic functions are expressed in terms of microscopical characteristics of electron-ion interaction in metallic systems. This approach makes it possible to classify alloys according to the intersolubility of their components and discover the conditions for the existence and the causes of formation of intermediate phases in binary alloys [4, 5]. Initially, this method was employed for the examination of alloys based on nontransition metals. In a more recent work [6], however, electron-ion interaction parameters are given for transition metals. As a result, the possibility has been opened up of calculating the thermodynamic properties of transition metal alloys. In view of the extensive use of alloys of the Ni -A1 system as coatings based on intermetallic compounds, matrices of composite materials, and strengthening-phase matrices of heat-resisting alloys, the present work was undertaken with the aim of determining, by the pseudopotential method, the energy of a disordered nickelbase solid solution and of calculating possible intermediate phases. It was considered of particular interest that in an experimental phase diagram there exists a NiA1 (B2) phase having a bcc lattice, whereas the pure components of the alloy - nickel and aluminum - have fcc lattices. The authors were unable to find a theoretical explanation, supported by calculation, of this phenomenon in the literature. Disordered Solid Solutions To understand the nature of phase equilibria in alloys, it is extremely important to know the character of the electron-ion interaction occurring in them. In this connection, numerous thoeries of phases in alloys utilize fairly crude hypotheses. Of the various theories explaining the contribution from the conduction electrons to the full bond energy, the best-known is the Brillouin zone effect theory proposed by Jones and Goodenough. With the aid of this theory it has proved possible, inter alia, satisfactorily to explain the HumeRothery rule concerning the influence exerted by the effective charge carried by an ion in an alloy on a phase equilibrium. Nevertheless, in the employment of this theory for the explanation of phase diagrams certain difficulties arise. Jones' theory is based on a coarse zonal model neglecting all properties of the atoms corn