Masters Theses

Date of Award

5-2002

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Raymond A. Buchanan, Peter K. Liaw

Committee Members

Chain T. Liu, Charlie R. Brooks, Charles S. Feigerle

Abstract

Exciting recent advances have led to "bulk metallic glasses" (BMGs) with fabrication diameters as large as 80 mm [1]. Although the mechanical behavior of BMGs has been studied, fatigue studies in controlled environments have not been adequately performed or understood. An early fatigue investigation had resulted in fatigue strengths lower than anticipated [2,3]. It is suspected that environmental effects degrade the fatigue life. Fatigue testing of a zirconium-based bulk metallic glass has been performed in distilled water and a 0.5 M NaCl electrolyte at room temperature [4]. Results of these tests have shown a substantial decrease in fatigue lifetimes when compared with those from tests conducted in air. Tensile tests of another similar zirconium-based bulk metallic glass conducted in various environments, including vacuum, showed very little effect due to the change in the environment [5]. This may indicate that changes in chemical composition or the test duration may play a significant role in the fatigue stresslife behavior. The research at hand focused on the effects that the environment had on the degradation of a zirconium-based bulk metallic glass, Zr52.5Al10Ti5Cu17.9Ni14.6 (at. %). Corrosion and fatigue experiments were conducted. By testing in both fields of research, the synergisms of the chemical and mechanical degradation of these new advanced materials were better understood. Polarization curves of amorphous and crystalline samples were compared. By performing this experiment, the influence of atomic homogeneity on the corrosion resistance of a material was better understood. Corrosion investigations proved the amorphous samples to have some electrochemical properties better than their crystalline counterparts while both proved to be highly corrosion resistant to both 0.6 M NaCl and 0.05 M Na2SO4 electrolytes. Fatigue testing was conducted in air and vacuum environments. By comparing the results, the environmental effects due to water vapor (in air) on the fatigue lifetime of BMG-11 were evaluated. It was concluded that water vapor does not have a major effect on the fatigue lifetime of BMG-11. Indeed, the observed lifetimes in vacuum were shorter than those in air. Subsequent testing indicated that the dissociation of the residual water vapor to atomic hydrogen in vacuum via a hot tungsten-filament ionization gauge could be a factor in the shorter fatigue lifetimes in vacuum than in air. In air, the fatigue endurance limit and the fatigue ratio for BMG-11 was found to be 907 MPa and 0.534, respectively. These values are much higher than those values reported from a previously conducted fatigue study on a similar zirconium-based BMG, and better than many conventional high-strength crystalline alloys.

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