Room temperature four-point-bending fatigue studies of the unidirectionally solidified Ni-Ni3Cb eutectic composite have been conducted to better understand fatigue behavior in composite materials. Fatigue crack propagation (FCP) data for this eutectic composite alloy revealed a power relationship between the stress intensity factor range and the crack growth rate from 10-7 to 10-4 in./ cycle. This application of fracture mechanics concepts to FCP data represents the first such reported information of its kind for eutectic composites. Fatigue behavior of this eutectic alloy was found to be sensitive to solidification parameters, thermal history, and the mean stress intensity level.
Metallographic and electron fractographic examination of the fatigue fracture revealed the FCP mechanism to be a function of the prevailing stress intensity factor at the crack tip with a fatigue fracture mechanism transition occurring between 5 × 10-6 and 1.5 × 10-5 in./cycle. At low growth rates, the Ni(γ) phase exhibited faceting indicative of Stage I propagation along active slip planes, while above 5 × 10-5 in./cycle, the γ fracture surface was characterized by the presence of fatigue striations which laid parallel to the Ni-Ni3Cb interface. At both high and low growth rates, the fatigue response of the Ni-Ni3Cb composite was controlled by the γ lamellae.