MASS-SPECTROMETRIC DETERMINATION OF THE THERMODYNAMIC MIXING BEHAVIOROF LIQUID TERNARY FE-NI-CR ALLOYS

Thermodynamic investigations on liquid ternary Fe-Ni-Cr alloys have be en performed by means of the computer-aided Knudsen cell mass spectrom etry. Ternary thermodynamically adapted power (TAP) series are used fo r the algebraic representation of the thermodynamic excess properties. At 1950 K, the molar excess Gibbs energy G(E) is negative in the pred ominant part of Gibbs triangle (minimum value: -2980 J/mol at x(Ni)=0. 55, x(Cr)=0.45), and positive only near the liquid binary Cr-Fe alloys (maximum value: 940 J/mol at x(Cr)=0.375, x(Fe)=0.625). The molar hea t of mixing H-E is exothermic for nearly all Fe-Ni-Cr melts (minimum v alue: -4770 J/mol at x(Fe)=0.38, x(Ni)=0.62). Only the Cr-richest tern ary melts near the Ni-Cr binary alloys are slight endothermic (maximum value: 80 J/mol at x(Ni)=0.125, x(Cr)=0.87). The molar excess entropy S-E is slight negative for ternary Fe-Ni-Cr melts near the two binary boundary systems Fe-Ni and Cr-Fe with a minimum SE value of -1.27 J/m ol K at x(Cr)=0.40 and x(Fe)=0.60, and slight positive at the side of binary Ni-Cr melts (maximum S-E value: 1.35 J/mol K at x(Ni)=0.50, x(C r)=0.50). The Fe-activities of liquid ternary Fe-Ni-Cr alloys as deter mined, in this work, show slight negative deviations from the ideal be havior in the range of composition below the section of constant mole fraction x(Cr)/x(Ni)=1, and slight positive deviations from Raoult's l aw above this section line. The Ni-activities of all ternary Fe-Ni-Cr melts show negative deviation from the ideal behavior. The Cr-activiti es show slight positive deviations from Raoult's law in the range of c omposition on the left-hand side of the section of constant mole fract ion x(Ni)/x(Fe)=1, and slight negative deviations from the ideal behav ior on the right-hand side of this section line. The results of this w ork can be used successfully for phase-diagram calculations. (C) 1998 Elsevier Science B.V.