Effect of microstructure on fatigue properties of WC-Co hard metals

Abstract

Due to the inherent hardness and brittleness of WC-Co hard metals the determination of mechanical properties such as yield strength, ultimate strength and fatigue strength is a difficult task. A testing setup to determine stress-strain curves of hard metals under static and cyclic uniaxial compression and tension loading was developed. The investigated hard metal grades varied in WC grain size and Co-binder content ranging from 0.2 to 1.3 μm and 6 to 12 wt. %, respectively. The static stress-strain behaviour was analysed in uniaxial compression and tensile tests and specific material data like Young’s modulus and yield strength were determined. The cyclic stress-strain response along with the ratchetting behaviour as well as subcritical fatigue crack growth were investigated under uniaxial loading conditions.Fractographic studies of the fracture surfaces were conducted using scanning electron microscopy to identify the nature of crack origins. Based on Murakami’s concept the threshold of the stress intensity factor range at which fatigue crack growth starts from inhomogeneities was determined to be about 4.3, 6.2 and 9MPa√m at stress ratios of 0.1, −1 and −3 respectively.