Atomic-scale study of low-temperature equilibria in iron-rich Al-C-Fe

Abstract

The capability of the thermodynamic approach based on the independent point defect approximation to describe low-temperature phase equilibria is investigated and applied to the Al-C-Fe system. The method gives a reasonable description of the multicomponent and multisublattice Fe-rich corner and evidences numerous peculiarities concerning the ordered phases as well as the density-functional-theory (DFT) energy models. The study of ${\text{Fe}}_{3}\text{Al}(\text{-C})$, revealing strong defect-induced instabilities, rules out the LDA, SLDA and GGA schemes and leaves (spin-polarized) SGGA as the only valid one. C stabilizes $L{1}_{2}$ ${\text{Fe}}_{3}\text{Al}$ with respect to $D{0}_{3}$, which justifies the fcc-type structure of the $\ensuremath{\kappa}$ ${\text{Fe}}_{3}\text{AlC}$ compound. The present work also helps in justifying the experimentally observed depletion of C in the $\ensuremath{\kappa}$ phase. Finally, a correct description of both ${\text{Fe}}_{3}\text{C}$ and $\ensuremath{\kappa}$ requires inclusion of interstitial carbon at low temperature, emphasizing the unexpected importance of interstitial defects in ordered phases.