Rate and microstructure influence on the fracture behavior of cemented carbides WC-Co and WC-Ni

Abstract

Tungsten carbide has both industrial and military applications, from high strength end mill dies and geological drilling, to kinetic energy penetrators. In these extreme environments, an understanding of the dynamic fracture properties and the potential influence of grade microstructure is necessary. The present work investigates fracture behavior of cobalt and nickel cemented tungsten carbide with varying grain size and binder content. Notched hardmetal WC-Co and WC-Ni samples are impacted under mode-I (opening) fracture conditions, and the dynamic stress intensity factor is determined from digital image correlation using ultra high-speed imaging, and compared with quasi-static values. In both grain size and binder content variants examined, the dynamic fracture toughness increased from the quasi-static by a factor of 1.51–2.44. In addition, a 7% increase in cobalt binder content (while maintaining nominally identical average grain size) resulted in a 20% increase in quasi-static fracture toughness, from 8.62 to 10.38 MPa $$\sqrt{\text {m}}$$ ; while the same binder increase resulted in a 34% decrease in critical SIF from 21.07 to 15.72 MPa $$\sqrt{\text {m}}$$ . The 6% nickel binder WC was found to have a 4.5% higher quasi-static fracture toughness than the 6% cobalt binder WC of the same grain size, but a statistically insignificant difference under dynamic loading. Overall, there is a 28% increase in the quasi-static fracture toughness of tungsten carbide samples with an increase of average grain size from 1 to 3 $$\upmu $$ m, and under dynamic loading the larger grain WC shows a nominally identical increase in fracture toughness. These findings are discussed within the theory of classical dynamic fracture mechanics, the implications of the experimental configurations pursued, and the microstructural features are examined using fractography.