Influence of complex point defects in ordered alloys: An ab initio study of B2Fe-Al-B

The point defect (PD) properties in nearly stoichiometric B2 iron aluminides containing boron are studied with density functional energetics and independent-PD thermodynamics. The influences of temperature as well as of intrinsic and B complex PDs are investigated, together with those of the (S)LDA or (S)GGA frameworks. While confirming the importance of magnetism for a proper account of PD properties, this work reveals a strong binding between aluminum antisites (Al-Fe) and iron vacancies (V-Fe), together with vacancy formation energies markedly lower in SGGA than in SLDA. The [Al-Fe+V-Fe] complexes are found to exist in significant thermal amounts, and according to (S)GGA should also be structural defects. Moreover, boron clearly appears as a substitution element, which questions its usually assumed interstitial diffusion, and in SLDA also exhibits non-negligible binding with Fe vacancies.