THERMODYNAMIC PROPERTIES AND PHASE-EQUILIBRIA IN THE FE-P SYSTEM

Differential scanning calorimetry (DSC) has been used to measure the h eat capacities of iron phosphides Fe3P, Fe2P and FeP in the temperatur e range 113-873 K. Simultaneously a Knudsen effusion method with mass- spectrometric analysis of the gaseous phase has been applied to invest igate the thermodynamic properties of iron phosphides (1041-1549 K), b cc solid solutions Fe-P (1384-1619 K) and Fe-P liquid alloys (1349-181 1 K). Delta(f)S of the iron phosphides calculated according to the sec ond and third laws of thermodynamics agree within the limits of experi mental error. Delta(f)G of the phosphides produced from gamma-Fe and P -2 gas have been approximated with the following equations (in J mol(- 1)): Delta(f)G(Fe3P) = - (239 076 +/- 2269) + (80.44 +/- 1.70)T, Delta (f)G(Fe2P) = - (231 728 +/- 1110) + (79.95 +/- 0.81)T, Delta(f)G(FeP) = -(193 892 +/- 2700) + (82.10 +/- 2.30)T. The thermodynamic behaviour of P diluted in alpha-Fe obeys Henry's law, the non-magnetic part of the excess Gibbs energy of mixing of the bcc solid solution being equa l to (standard states: alpha-Fe and white P) Delta(f)G(ex)(nm) = (-280 .4T + 0.195T(2) - 3.70 x 10(-5)T(3))X(p)X(Fe) J mol(-1). The thermodyn amic 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 the rmodynamic data agrees with the published information.