Quantitative calculations of thermal-expansion coefficient in Fe-Ni alloys: First-principles approach

The thermal expansion coefficients (alpha) of Fe(1-x)Ni(x )alloys are calculated by means of the Debye-Gruneisen model, which uses the input parameters calculated from density functional theory (DFT) with collinear spin alignments. The various atomic configurations of fcc and bcc supercells with x being 0-0.5 are calculated to conduct the composition-specific analysis. Thermodynamic analysis is employed to facilitate the evaluation of alpha in bulk alloys, where the calculated supercells are used as the canonical ensemble. Such calculated alpha exhibits very similar composition-dependency in comparison with the experimental data from literature, particularly providing the well-known Invar effect at 65 wt% of Fe. The calculations also demonstrated that the pressure-derived magnetic frustration, i.e. the magneto-volume effect, is strongly correlated with the Invar effect. The present approach combining the Debye-Gruneisen formalism and the collinear DFT calculation is shown to be a comprehensive theoretical framework for analysis on the thermal expansion properties in metal alloys.