  By combining ab-initio electron theory and statistical mechanics, the
physical properties of the ternary intermetallic system Ni-Fe-Al in the ground
state and at finite temperatures were investigated. The Ni-Fe-Al system is not
only of high technological interest, but exhibits also rich physics, e.g., a
delicate interplay between structure and magnetism over a wide composition
range and substantial electronic correlations which is challenging for modern
electronic structure methods. The new Stuttgart ab-initio mixed-basis
pseudopotential code in the generalized gradient approximation (GGA) was used
to determine the energetics in the ground state. Therewith, in combination with
the cluster expansion (CE) method a representation of the energy landscape at
$T$=0 over the whole Gibbs triangle was elaborated. At finite temperatures, the
cluster variation method (CVM) in tetrahedron approximation was employed in
order to calculate the ab-initio ternary phase diagram on the bcc and fcc
lattice. Thereby, a miscibility gap in the ternary B2 phase was theoretically
verified.
