CVM calculation of the phase diagram of b.c.c. FeCoAl

Abstract

The Cluster Variation Method (CVM) in the irregular tetrahedron approximation is used to calculate the phase diagram of the b.c.c. ternary system FeCoAl. At first paramagnetic states are treated and chemical pair interactions between first and second nearest neighbours are taken into account in the calculation. The numerical values for these interactions are taken from separate studies. Complete isothermal sections at T = 1400, 1000, 800, 700, 600, 300 K and several vertical sections are presented. A ternary miscibility gap is found between ordered and disordered phases, (B2 + A2) as well as (D0 3 + A2), and between the two ordered phases B2 and D0 3. This miscibility gap closes at multicritical lines defined by the intersection of the second order critical temperature surfaces with the miscibility gap. In the binary FeCo system the Curie temperature is much higher than the critical temperature of chemical ordering. Therefore magnetic interactions cannot be neglected in this system nor in the Fe- and Co-rich ternary alloys. Magnetic interactions have been considered between nearest 1 2 treatment. Each element was treated as two components, A↑ and A↓, and the CVM treatment thus dealt with a six component system. For T = 600 K a complete isothermal section is presented which includes the magnetic effects.