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). ΔfS of the iron phosphides calculated according to the second and third laws of thermodynamics agree within the limits of experimental error. ΔfG of the phosphides produced from γ-Fe and P2 gas have been approximated with the following equations (in J mol–1): ΔfGFe3 P=–(239 076 ± 2269)+(80.44 ± 1.70)T, ΔfGFe2 P=–(231 728 ± 1110)+(79.95 ± 0.81)T, ΔfGFeP=–(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)ΔfGex(nm)=(–280.4T+ 0.195T2–3.70 × 10–5T3)XPXFe 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.