Thermodynamic investigation of the τ4τ4-Al–Fe–Si intermetallic ternary phase: A density-functional theory study

• The heat capacity of τ4τ4-Al–Fe–Si is calculated ab initio via DFT and QHA.
• Calorimetry measurements of the heat capacity between 930 and 1050 K are shown.
• The ab initio method is more accurate than the Neumann–Kopp empirical formula.
• Thermal and elastic properties are calculated.

With the increasing use of light materials, controlling the purity and composition of these materials during production becomes more important. Phase diagrams calculated using thermodynamic data are an important tool in modeling of phase formation and composition. Here we discuss the measurement and calculation of thermodynamic data for the ternary τ4τ4 compound from the Al–Fe–Si system. We focus on the heat capacity, a central quantity which is used for temperature extrapolations of the Gibbs energy. The measurement is restricted to a narrow temperature interval, therefore calculations are used to gain data over a wide temperature range. Calculations using density-functional theory in combination with the quasi-harmonic approximation (DFT–QHA) are compared to a simple empiric formula known as the Neumann–Kopp (N–K) rule. The DFT–QHA calculations are in good agreement with the differential scanning calorimetry (DSC) measurement. Both calculations reproduce the same order of magnitude of the heat capacity as seen in measurements. However the N–K approximated cpcp shows systematic deviations from the expected behavior, and the difference between the two calculations increases with temperature. Above the melting point of Al, which is still below the melting point of the compound τ4τ4, the N–K method is not applicable.