THERMODYNAMICS OF IRON-CHROMIUM ALLOYS - MASS-SPECTROMETRIC DETERMINATION OF THE EXCESS PROPERTIES IN THE BBC PHASE AND CALCULATION OF THE PHASE-DIAGRAM

Thermodynamic excess properties of iron-chromium alloys in the bcc pha se have been investigated by Knudsen cell mass spectrometry in the tem perature range 1470-1750 K. The molar excess Gibbs energies, entropies and heats of mixing as well as the thermodynamic activities were dete rmined from the ion intensities as a function of composition by fittin g to a sub-regular solution model (2 parameter T.A.P.-series; C(n)H in J/mol: C1H = 16 800, C2H = 4490; C(n)S in J/(mol.K): C1S = 4.155, C2S = 2.065). Solid Fe-Cr alloys in the bcc phase are characterized by po sitive molar excess Gibbs energy G(E), positive molar excess entropies S(E) and endothermic molar heat of mixing H(E). The following maximum excess properties result at 1725 K: H(E) = 4780 J/mol (53.1 at.-% Cr) , S(E) = 1.31 J/mol.K (54.7 at.-% Cr), and G(E) = 2525 J/mol (51 at.-% Cr). The phase diagram has been recalculated by using these data. The optimized new sets of T.A.P.-parameter resulted as: C1H = -7800, C2H = 3500 (C(n)H in J/mol), C1S = -6.8, C2S = 3.8 (C(n)S in J/mol.K) for the liquid phase; as well as C1H = 7800, C2H = 4500 (C(n)H in J/mol), C1S = 3.1, C2S = 3.5 (C(n)S in J/mol.K) for the fcc phase.