Equilibrium aspects of Cr-alloyed cemented carbides

Sammanfattning: In view of the extensive use of Cr as a grain growth inhibitor in WC-Co cemented carbides this thesis comprises a combined experimental and ab initio study of a number of critical issues pertaining to phase equilibria of the subsystems to the W-Co-Cr-C system.To be able to predict which of the stable Cr-carbides forms above the solubility limits of Cr and C in the fcc phase the respective solubility of Co in Cr23C6, Cr7C3 and Cr3C2 are investigated experimentally. Furthermore, the site occupancies of Co in Cr7C3 are investigated by neutron diffraction as a first step towards a more realistic model for this phase.The energetics of the ternary intermetallic R-phase and the unstable Co3C2 end-member compound are investigated by density functional theory. For Co3C2, the quasi-harmonic vibrational Gibbs energy is also calculated.By subsequent CALPHAD assessment an improved thermodynamic description is developed and its agreement with experiment is investigated. The resulting thermodynamic description allows for improved accuracy in predicting the formation of carbide phases as a function of C content and Co/Cr ratio as well as liquidus and solidus temperatures.Remaining issues may be attributed to the thermodynamic description of the W-Cr-C system and the stability of the Cr-based carbides in the Cr-C system. In the case of the Cr-based carbides, severe experimental scatter prevents an accurate determination of the stability of either of them. As a first attempt to resolve the situation, a state of the art ab initio approach is applied to calculate the finite temperature thermodynamic properties of Cr3C2, benchmarked with reported heat capacity and relative thermal expansion measurements.