Thermodynamic properties and phase equilibria in the Fe–P system

Abstract

Differential scanning calorimetry (DSC) has been used to measure the heat capacities of iron phosphides Fe₃P, Fe₂P and FeP in the temperature range 113–873 K. Simultaneously a Knudsen effusion method with mass-spectrometric analysis of the gaseous phase has been applied to investigate the thermodynamic properties of iron phosphides (1041–1549 K), bcc solid solutions Fe–P (1384–1619 K) and Fe–P liquid alloys (1349–1811 K). Δ_fS of the iron phosphides calculated according to the second and third laws of thermodynamics agree within the limits of experimental error. Δ_fG of the phosphides produced from γ-Fe and P₂ gas have been approximated with the following equations (in J mol^–1): Δ_fG_{Fe3 P}=–(239 076 ± 2269)+(80.44 ± 1.70)T, Δ_fG_{Fe2 P}=–(231 728 ± 1110)+(79.95 ± 0.81)T, Δ_fG_FeP=–(193 892 ± 2700)+(82.10 ± 2.30)T. The thermodynamic behaviour of P diluted in α-Fe obeys Henry's law, the non-magnetic part of the excess Gibbs energy of mixing of the bcc solid solution being equal to (standard states: α-Fe and white P)Δ_fG^ex(nm)=(–280.4T+ 0.195T²–3.70 × 10^–5T³)X_PX_Fe J mol^–1. The thermodynamic properties of liquid solutions have been described with the ideal associated-solution model assuming that Fe₃P, Fe₂P and FeP complexes exist in the melt. The phase diagram computed with the help of the thermodynamic data agrees with the published information.