Preparation and Properties of WC-(Ni, Al) Cemented Carbides
WC-Ni cermets are inferior to WC-Co when measured by their hardness-fracture toughness combination. To enhance system performance, an attempt was made to strengthen the binder in WC-Ni by alloying with Al to produce controlled dispersions of γ′ (Ni3Al) in the binder.
WC-(Ni, Al) cermets with 4 to 12 wt. % Al in the binder were produced by powder metallurgy techniques, liquid phase sintered and hot isostatically pressed. X-ray diffraction and transmission electron microscopy were used to characterize the amount, size and distribution of γ′ in the binder. The volume fraction of γ′ in the binder as measured by X-ray diffraction agreed well with values calculated using available phase diagram data. Dark field TEM studies showed that both cuboidal and heterogeneous γ′ were present in the binder in the as-sintered condition. The size of γ′ varied from 100 to 2000 A. The WC-(Ni, Al) cermets were then solution treated by vacuum heating and quenching in oil. They were then aged in vacuum at different temperatures and times to produce γ′ of controlled size and distribution. Since all these heat treatments were carried out in the solid state, microstructural factors such as carbide fraction, size and contiguity remained unchanged. In this respect, WC-(Ni, Al) cermets provided a model system where only the binder constitution was altered.
Hardness and fracture toughness comparisons of WC-(Ni, Al) in the various heat treated conditions revealed that conventional wisdom regarding strengthening mechanisms in bulk alloys is not directly translatable to binders in cermets, and that the applicability and effectiveness of each strengthening mechanism differs considerably from bulk alloys.
KeywordsFracture Toughness Solution Treatment Crack Resistance Strengthening Mechanism Bulk Alloy
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