Tracking the evolution of microstructure and phases of WCoB-Co cermets during sintering

Transition metal boride enable applications in fields of cutting tools as the hard phase, but their fabrication suffers from shortcomings in high temperature for binary and hard/soft phase balance. Here, we leveraged a WCoB-Co cermets prepared by using elemental powders of W, Co and B powders via in situ synthesis method and investigated the microstructure evolution of a composite structure of WCoB/Co. The phase evolution, microstructure evolution and mechanical properties were investigated using X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The complete sintering process consist of three solid-phase reaction stages and one liquid-phase sintering stage. The solid-phase reaction stage includes the following three reactions: B + 2Co -> Co2B, 11 W +15 Co2B -> 9WCoB + W2Co21B6, and 14 W + 2W(2)Co(21)B(6) -> 12WCoB + 23Co + Co7W6. The eutectic liquid phase, essential for the densification, forms at 1323 degrees C. The Rockwell hardness and density value reach 87.5HRA and 12.11 g/cm(3), respectively. At 1380 degrees C, the traverse rupture strength (TRS) reaches the peak value of 1495 MPa. Moreover, the results show that transgranular fracture is the main fracture mode. The densification stage was completed at similar to 1400 degrees C. In particular, Co7W6 phase was discovered during the study of the evolution of WCoB-Co cermets. An increase of the B content was found to be beneficial to eliminate the Co7W6 phase that proved to be detrimental to mechanical properties.

Automated summary

Transition metal borides have enabled applications in cutting tools, but face challenges in fabrication due to high temperature requirements and phase balance issues. A WCoB-Co cermets was prepared using in situ synthesis with elemental powders and studied for microstructure evolution, phase evolution and mechanical properties. Results indicated that increasing boron content was beneficial in eliminating the detrimental Co7W6 phase for mechanical properties optimization.