| Summary: | This work presents a detailed investigation for the effect of Y<sub>2</sub>O<sub>3</sub> and Ni additions on the densification behavior, microstructural evolution and mechanical properties of a WC-Co-TaC-NbC composite. With the aim of obtaining WC-based composites with improved fracture toughness, to be used in severe conditions of high-temperature deformation, different concentrations of Y<sub>2</sub>O<sub>3</sub> were incorporated with and without 5 wt% Ni addition. The consolidated composites were characterized using density measurement, XRD, SEM, hardness, fracture toughness, transverse rupture strength and compression testing. Fully dense composites were obtained through the applied consolidation regime of cold compaction and sintering at 1450 °C for 1.5 h under vacuum with a relative density up to 97%. The addition of 2.5 wt% Y<sub>2</sub>O<sub>3</sub> to the base WC composite increased the relative density and then slightly decreased with the increase of the Y<sub>2</sub>O<sub>3</sub> content. The addition of 5 wt% Ni to the base composites significantly increased the relative density to 97%. The XRD results indicated the existence of the Co<sub>3</sub>W<sub>3</sub>C η-phase after sintering, and the intensity of its peaks was reduced with the addition of 5 wt% Ni. The microstructure of the consolidated composites consisted of three phases: WC, Co<sub>3</sub>W<sub>3</sub>C and Y<sub>2</sub>O<sub>3</sub>. The area fraction of the Y<sub>2</sub>O<sub>3</sub> phase increased as its weight fraction increased. In terms of the fracture toughness, the transverse-rupture strength (TRS) and the compressive strength were significantly improved by the addition of 5 wt% Ni with the 2.5 wt% Y<sub>2</sub>O<sub>3</sub>. Accordingly, this composition was used to manufacture the tools for the friction stir welding of the high-softening-temperature materials, which was successfully used for 25 plunges and about 500 cm of butt joints in nickel-based and carbon–steel alloys.
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