Influence of carbon on the formation of γ/γ′ microstructure and κ-phase in the WC/Co–Ni–Al–W system: ab initio calculations and experimental studies

Abstract

Modifications of the binder phase (γ) of cemented carbides have the potential to increase the hardness and wear resistance of the whole material. Partially, coherent precipitations with L12 structure (γ’) promise these improved properties without sacrificing tensile strength or toughness. γ’ is a metastable phase in the Al–Co–W ternary system in the form of Co3(Al,W) which is stabilized by the substitution of cobalt with nickel. Superalloys of the composition Co–(30Ni)–9Al–7 W with different carbon contents were prepared by inductive melting, and the resulting microstructures were analysed using SEM–EDS, XRD and Vickers hardness. Cemented carbides with γ/γ’ binder microstructure were prepared via DTA, and the phase equilibria in the composite material were investigated experimentally and in silico. It was shown that nickel stabilizes the γ’ phase in superalloys as well as in cemented carbides. Carbon leads to the formation of an additional phase with E21 structure (κ). DTA measurements of cemented carbides with different aluminium–cobalt–nickel mixtures as binder gave an overview of the compositional influence. Enthalpies of formation for compounds with L12 and E21 structure were calculated using ab initio methods and compared to experimental results.Graphical