Identification of optimum binder phase compositions for improved WC hard metals

Abstract

The possibility has been investigated of using measurements of the mechanical properties of alloys with binder phase compositions to identify binder phases that will provide high hardness and toughness in WC hard metals based on cobalt, nickel, CoNi, NiAl, NiCrMo and NiCrMoAl. The results showed that, in WCCo and WC(CoNi) hard metals with constant grain size and binder phase content, an optimum grain size and binder phase content, an optimum combination of fracture toughness and compressive strain could be obtained at particular tungsten and carbon contents which could be determined from measurements of magnetic saturation and lattice parameters. As predicted from tests on NiWC alloys, it was found that WCNi hard metals had lower toughness and strength than WCCo hard metals do. Additions of aluminium to nickel binder phases to form γ′ precipitates raised the strength and conferred creep resistance but decreased the fracture toughness. Solid solution strengthening by chromium and molybdenum raised the hardness without reducing toughness as measured by Palmqvist tests and gave properties which matched those of WCCo hard metals. By combining γ′ hardening and solid solution hardening by chromium and molybdenum in nickel-based binder phases, better values of hardness and toughness were obtained than those of WCCo hard metals. The new compositions also offered the possibilities of enhanced resistance to creep and corrosion, and properties that could be varied by heat treatments to meet specific requirements.