FATIGUE DAMAGE IN A WC-NICKEL CEMENTED CARBIDE COMPOSITE
DRAKE, ERIC F.
Doctor of Philosophy
A model WC-Ni cemented carbide was designed to investigate the response of this type of composite to compressive cyclic loading. The material's monotonic stress-strain behavior was studied in the compressive range, revealing considerable micro-strain plasticity and rapid strain-hardening characteristic of this composite family. Specimens were mechanically conditioned at 20 Hz for one million cycles using a zero-compression-zero sinusoidal program in stress control. Peak stress was varied in increments from zero to 1900 MPa, where failure was observed. Bulk hardness, residual stress state, and fracture toughness were monitored as a function of cyclic stress. Only modest shifts in bulk behavior were observed. On a local scale, fatigue effects on dislocation density and structure were found in the binder phase by transmission electron microscopy. As cyclic stress levels were raised, favorably oriented grain-boundary-ledge dislocation sources were activated, leading to precipitate-free bands of intense slip. These bands broadened at the higher stress levels, saturating entire binder phase regions and triggering a change in dislocation arrangements. At the highest stress levels discontinuous cell structures were observed in some binder regions. It is concluded that fatigue damage in this type of composite accrues in the binder in a characteristic manner dictated by short binder-carbide interaction distances and local stress conditions. The principal roles of fatigue in the fracture behavior of this alloy are concluded to be: a source of incipient defects and a means of stable crack extension.
Engineering; Materials science