Date of Award
Master of Science (MS)
Garrett J Pataky
The single-phase face centered cubic high entropy alloy CoCrFeNi was subjected to tension-tension fatigue and the Paris-Erdogan Law characteristics and underlying deformation mechanisms investigated. The fatigue crack growth experiments were performed at room temperature on single edge notch samples of CoCrFeNi. The experiments consisted of cyclic loading in uni-axial tension with multiple R-ratios and a naturally increasing stress intensity factor. In-situ digital image correlation was used to determine full field displacement maps of the sample surface. This displacement data was used to determine crack tip position, opening load, opening displacement and sliding displacement. With a Paris-Erdogan Law exponent at the lower end of the ductile metal range, 2.5, CoCrFeNi exhibited good fatigue crack growth resistance. This resistance to crack growth is due to significant surface roughness found by post-mortem fractography, and roughness-induced crack closure is supported by opening load data for the load ratio R = 0.05. Fractography also showed step features and surface cracking symptomatic of quasi-cleavage fracture, indicative of limited slip system activation within the material. TEM analysis was performed for further investigation of the step features revealing that the plastic deformation mechanism changed from dislocation slip to the addition of nano-twinning as ΔK increased over crack life.
Williams, William Matthew, "Fatigue Crack Growth Behavior of CoCrFeNi" (2020). All Theses. 3387.