The abnormal growth of WC grains and effect of Ni substitution on mechanical properties in sintered WC-Co alloys have been investigated. Powder mixture of WC (5-15) Co composition with powders of different particle size from 0.9㎛ to 10.4㎛ have been also prepared. Cylindrical compacts have been sintered between $1400^\circ{C}$ and $1500^\circ{C}$ for zero to 50 hours. Tungsten wires with 100㎛ or 200㎛ diameter have been introduced into the WC-Co compacts with stoichiometric carbon content of WC, and carburized at $1220^\circ{C}$, and $1500^\circ{C}$ respectively. η phase powder with $W_3Co_3C$ composition has been prepared by heating the powder mixture of 0.9 ㎛ WC, 1㎛ W and 1.5㎛ Co powder at $1400^\circ{C}$ for one hour and introduced into the WC-Co compacts with stoichiometric carbon content of WC.
Abnormal grain growth is observed to occur in two stages: the abnormal grain forming stage and the preferred growth stage of coarse grains. Coarse grain nucleus is formed at the solid state sintering temperature of $1220^\circ{C}$ by carburization of W in the W agglomerated region. Large agglomeration of $\eta$ phase also carburized to coarse WC grains at $1220^\circ{C}$. WC powder with fine grain size carburized at $1500^\circ{C}$ shows significant abnormal grain growth, while WC powders with coarse grain size carburized at $1750^\circ{C}$ or $1950^\circ{C}$ shows normal grain growth in the coarsening of WC-15Co alloy at $1500^\circ{C}$ for 50 hours.
Coarse grains grown abnormally in the WC-Co alloy have been introduced into the WC-15Co alloy with 3.5 ㎛ WC powder which does not show any abnormal grain growth. After annealing at $1470^\circ{C}$ for 27 hours, coarse grains show significant abnormal grain growth. It is confirmed that preferred growth of coarse WC grains in the fine grained WC-Co maxtrix originated from the significant size difference between the growing coarse grains and the consumed fine grains. A significant anisotropy occurs by the spreading the basal plane wider than prismatic plane in the abnormal grain growth. Coalescence of prismatic planes between growing coarse grain and consumed fine grains appears to contribute significantly to abnormal grain growth.
Powder mixtures of WC-10(Co-Ni) and WC-15(Co-Ni) compositions (by wt.%) with different Ni fraction were prepared by milling the required amount of 5.7㎛ Co and 1.5㎛ Ni powders. Rectangular compacts have been sintered for one hour at $1425^\circ{C}$ for WC-10(Co-Ni) specimens and at $1375^\circ{C}$ for WC-15 (Co-Ni) specimens. Ni substitution decreases the density and hardness of the hardmetal, while specimens with equal binder content show homogeneous and equal microstructures and equal level of transverse rupture strength independent of binder composition. Hardness significantly decreases with increase of nickel content in the binder, for example, from HRa 89.2 of WC-10Co alloy to HRa 86.5 of WC-10Ni alloy and from HRa 88.1 of WC-15Co alloy to HRa 85.8 of WC-15Ni alloy. The transverse rupture strength does not vary with binder composition. The decrease of hardness with increase of Ni content is attributed to the lower increasing rate of hardness with decrease of binder layer thickness in the Ni binder than in the Co binder.
