Thermodynamics of solid Co-Cr alloys by Knudsen cell mass spectrometry and computation of the phase diagram

Thermodynamic excess properties of cobalt-chromium alloys in both the bcc solid and fee solid phases have been investigated by Knudsen cell mass spectrometry in the temperature range 1580-1790 K. The molar excess Gibbs energy, excess entropy and heat of mixing as well as the thermodynamic activities were determined from the ion intensities as a function of composition and temperature by fitting to a sub-regular solution model and optimised to the phase equilibrium data (2 parameter T.A.P.-series; Fee phase: C-n(H) in J/mol: C-1(H)=-10600, C-2(H)-26500; C-n(S) in J/(mol.K): C-1(S)=4.15, C-2(S)=-26.0; Bcc phase: C-n(H) in J/mol: C-1(H)=19500, C-2(H)=-29500; C-n(S) in J/(mol.K): C-1(S) = 19.4, C-2(S)=-32.0). Solid Co-Cr alloys at 1673 K are characterized in the fcc phase by negative molar excess Gibbs energy G(E), prevailing negative molar excess entropies S-E and exothermic molar heats of mixing H-E, and in the bcc phase by negative and positive (at high Cr content) molar excess Gibbs energy G(E), positive and slight negative (at higher Cr content) molar excess entropies SE and endothermic and slight exothermic (at higher Cr content) molar heats of mixing H-E. At 1673 K the following minimum and maximum excess properties result: a) Fee phase, minimum values: H-E=-6400 J/mol (62 at.-% Cr), S-E=-2.95 J/mol.K (70 at.-% Cr), and G(E)=-2650 J/mol (34 at.-% Cr); b) Bcc phase, maximum values: H-E=2300 J/mol (26 at.-% Cr), S-E=2.15 J/mol K (26 at.-% Cr), and G(E)=1090 J/mol (80 at.-% Cr); bcc phase, minimum values: H-E=-740 J/mol (84 at.-% Cr), S-E=-1.0 J/mol K (82 at.-%Cr), and G(E)=-1300 J/mol (22 at.-% Cr). The phase diagram has been recalculated by using these data, and the results of mass spectrometric investigations on the liquid phase.