Date of Award

August 2020

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Garrett J Pataky

Committee Member

Paul Joseph

Committee Member

Huijuan Zhao

Abstract

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.

Share

COinS
 
 

To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.