Fracture of coherent interfaces between a FCC metal matrix andthe Cr23C6 carbide precipitate from first principles
Résumé
It is known that microcrack initiation in metallic alloys containing second-phase particles maybe caused by either an interfacial or an intra-precipitate fracture. So far, the dependence of these features on properties of the precipitate and the interface is not clearly known. The present study aims at determining key properties of carbide-metal interfaces controlling the energy and critical stress of fracture, based on density functional theory (DFT) calculations. We address coherent interfaces between a FCC iron or nickel matrix and a frequently observed carbide, the M23 C6, for which a simplified chemical composition, the Cr23 C6 is assumed. The interfacial properties such as the formation and Griffith energies, and the effective Young modulus are analyzed as functions of the magnetic state of the metal lattice, including the paramagnetic phase of iron. Interestingly, a simpler antiferromagnetic phase is found to exhibit similar interfacial mechanical behaviour to the paramagnetic phase. A linear dependence is determined between the surface-interface energy and the variation of the number of chemical bonds weighted by the respective bond strength, which can be used to predict the relative formation energy for the surface-interface with various chemicalterminations. Finally, the critical stresses of both intra-precipitate and interfacial fracture due to a tensile loading are estimated via the UBER (universal binding energy relationship) model, parameterized on the DFT data. The validity of this model is verified in the case of intra-precipitate fracture, against results from DFT tensile test simulations. In agreement with experimental evidences, we predict a much stronger tendency for an interfacial fracture for this carbide. In addition, the calculated interfacial critical stresses are fully compatible with available experimental data in steels, where the interfacial carbide-matrix fracture is only observed at incoherent interfaces.