Date of Award

12-2012

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Hahn Choo, Yanwen Zhang

Abstract

Research was performed on the Al9Co18.2Cr18.2Cu18.2Fe18.2Ni18.2 [subscripts atomic percent] and Al7.5Cr22.5Fe35Mn20Ni15 high-entropy alloys (HEAs) in an attempt to study their fatigue behavior. The present investigation shows encouraging fatigue-resistance characteristics due to their high fatigue lives of various samples at relatively high stresses. The current results indicate that the fatigue behavior of HEAs compares favorably to many conventional alloys, such as steels, titanium alloys, and advanced bulk metallic glasses with a fatigue-endurance limit of the Al9Co18.2Cr18.2Cu18.2Fe18.2Ni18.2 HEAs between 540 and 945 MPa and a fatigue-endurance limit to ultimate tensile strength ratio between 0.402 and 0.703. The Al7.5Cr22.5Fe35Mn20Ni15 HEAs were found to have a fatigue-endurance limit between 540 and 630 MPa.

Some unpredictability in the fatigue life of the samples was observed by scattering in the stress versus life plot. Weibull models were applied to predict the fatigue data and to characterize the variability seen in Al9Co18.2Cr18.2Cu18.2Fe18.2Ni18.2 HEAs. A Weibull mixture predictive model was used to separate the data into two, strong and weak, groups. This model predicts that at stress ranges above 858 MPa, the median time to failure of specimens in the strong group will be greater than 107 cycles.

Large oxide particles were found on the tensile surfaces of both samples with microcracks forming at these sites. It was shown that these microstructural defects may have a significant effect on the fatigue behavior of HEAs. A comparison of the endurance limits and fatigue ratios of HEAs to conventional structural alloys shows that HEAs may outperform many conventional alloys under fatigue conditions. It is believed that a reduction in the number of defects introduced during fabrication and processing may result in a superior fatigue behavior, which exceeds that of conventional alloys.

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