Thermodynamic Investigation of Co-Cr Alloys

Abstract: In a collaborative effort between the Divison of Metallurgyand the Divison of Computational Thermodynamics, RoyalInstitute of Technology (KTH), Stockholm, Sweden, experimentalinvestigations have been carried out in the binary Co-Crsystem. The experimental strategy has been to measure thethermodynamic activities of Cr by the solid oxide electrolytegalvanic cell technique, as well as to measure the molar heatcapacities of the alloys, as function of temperature, by theDSC technique.The thermodynamic properties of four Co-Cr alloys, in thecompositional range 25 to 85 wt. % Cr, have been determined inthe temperature interval 1026 - 1297 K by the solid oxideelectrolyte galvanic cell technique. Equations for the measuredelectromotive force (EMF), E (V) = A + B'T (K), were derived inthe temperature intervals of interest. The Cr activities aswell as the relative partial molar Gibbs energies and entropieswere calculated. The break points in the EMF-temperature linesare compared with the phase diagram data available inliterature for the Co-Cr binary system.The molar heat capacities of several Co-Cr alloys have beendetermined in the temperature interval 318 - 1660 K by the useof the Differential Scanning Calorimetric (DSC) technique. Thecompositional range of the alloys investigated extended from 10to 90 wt. % Cr. Heat capacity equations, Cp (J/mol'K) = A + B'T(K), were derived in the temperature interval of interest tofit the experimental data. The phase transformations observedare compared with the phase diagram data available inliterature for the Co-Cr binary system.Key words:Cobalt, chromium, experimentalthermodynamics, solid state galvanic cell technique,Differential Scanning Calorimetric (DSC) technique,CALPHAD-technique, Thermo-Calc, phase diagram, experimentaldata.