Sintering of WC hardmetals with Ni-Co-Cr-Ti-Al multi-component alloys

Cemented carbides with Ni-Co-Cr-Al-W binder phases have been obtained from WC-Ni-Co-Cr3C2-TiAl3 powder mixtures by using standard hardmetal processing routes. Dilatometric and DSC experiments confirm that melting events and shrinkage in these materials are shifted towards higher temperatures as the Al content increases. It is also shown that sintering in Argon is enhanced at very low Al levels (0.63 wt%), which is likely due to the efficient reduction of powder oxides via direct reaction with TiAl3. For higher Al additions, HIP treatments are needed in order to remove the residual porosity left after vacuum sintering. Microstructural characterization confirms that Al additions induce two types of precipitation phenomena in the metallic binder phase of these hardmetals: one consisting of very fine Al-rich oxides (product of the aforementioned powder oxide reduction) and the other comprising even finer γ' type cuboids (similar to those found in Ni superalloys). As confirmed by TEM-EDS analyses, the binder phase of these cemented carbides is a multicomponent alloy incluiding Co, Ni, Cr, W and increasing amounts of Al as TiAl3 addition increases. About 0.5 wt% Al is lost to the powder oxides reduction process. TiAl3 additions have a significant hardening effect on WC-Ni-Co-Cr3C2 alloys. This is due to its effective role as WC grain growth inhibitor. Although, solution-aging treatments induce a finer and more homogeneous precipitation of the γ' phase in these materials, their hardness is slighlty reduced if compared to that of as-HIPed samples.