DIFFUSION MECHANISM AND DEFECT CONCENTRATIONS IN BETA'-FEAL, AN INTERMETALLIC COMPOUND WITH B2 STRUCTURE

A statistical thermodynamic model of the Wagner-Schottky type is applied in order to determine the defect pattern in the beta'-FeAl phase. Model calculations based on the concentration dependence of the thermodynamic activity of Al yield the following defect concentrations for the composition Fe50.5Al49.5 at 1338 K: 176 +/- 0.20% of all lattice sites are vacant, 3.36 +/- 0.40% of the Al-sublattice sites are occupied by Fe atoms, and 0.84 +/- 0.40%, of the Fe-sublattice sites are occupied by Al atoms. The obtained concentration of anti-structure Fe atoms is close to the number derived from Mossbauer spectroscopy studies which were carried out to determine the diffusion jump mechanism of Fe atoms in the ordered alloy. Experimental results and model calculations imply vacancies and anti-structure atoms on both sublattices. Therefore, although beta'-FeAl in principle belongs to the triple-defect B2-type, a certain degree of 'hybrid behaviour' must be taken into consideration.