Doctoral Dissertations

Date of Award

5-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, John D. Landes, James R. Morris

Abstract

The cyclic-compression behavior of a Cu45Zr45Al5Ag5 bulk metallic glass (BMG) was investigated in order to elucidate the damage initiation and growth mechanisms. The present Cu45Zr45Al5Ag5 BMG was found to have the highest fatigue-endurance limit for BMGs reported to date. Fracture under cyclic compression occurred in a pure shear mode. In addition to many shear bands and cracks, areas of “chipping” were commonly found on the outside surfaces of the fatigue specimens. Crack growth rates were found decrease with cycles.

The effects of the as-cast specimen size, cooling rate, and the free volume content on the monotonic and cyclic compression behavior of a Zr-based BMG was investigated. The smaller samples experienced a faster cooling rate, resulting in a higher free volume content. The smaller samples displayed superior monotonic compression and cyclic compression properties. This trend was attributed to a higher free volume content.

The effect of the sample aspect ratio (height/diameter) on the cyclic compression behavior of a Zr-based BMG was explored. For smaller aspect ratios (0.5), the yield strength and compressive plastic strain significantly increased when compared to that for an aspect ratio of 2. In general, when the aspect ratio was 0.5, the fatigue lives were longer than when the aspect ratio was 2. The dramatic effect of the sample aspect ratio was attributed to the development of a hydrostatic stress state from the interaction of the uniaxial applied load and the friction stress developed at the interface of the top and bottom specimen surfaces and the platens.

The stress-life fatigue behavior and fracture morphology of a (Cu60Zr30Ti10)99Sn1 BMG alloy was investigated under both 3-point and 4-point bending conditions. For all stress levels tested, the fatigue lifetimes tended to be higher for the 3-point loading condition. All fracture surfaces were found to be comprised of four main regions: a crack-initiation site, a stable crack-growth region, an unstable fast-fracture region, and a melting region. Finely spaced parallel marks oriented somewhat perpendicular to the direction of crack propagation were observed in the stable crack-growth region. Analyses of these marks found that their spacing increased with increasing stress intensity- factor range.

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