Doctoral Dissertations

Date of Award

5-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao

Abstract

Although bulk-metallic glasses (BMGs) exhibit low ductility and brittleness at room temperature, large plasticity can be achieved when the feature size is small. Therefore, metallic glasses in a small volume can be exploited in designing components as thin films for structural applications. Magnetron-sputtering process was utilized to prepare multi-component metallic-glass thin films on various substrates, including a 316L stainless steel, a Ni-based superalloy, and a Zr-based alloy. The microstructures of the as-sputtered thin films, including Zr-based, Cu-based, and Fe-based films, were identified to exhibit an amorphous matrix with nanocrystalline phases. Nanoindentation and microscratch studies of the Zr-based films show a high hardness and good film adhesion. The compressive residual stress in the 200 nm-thick Zr-based film was measured to be 2.4 GPa by a slit method.The effects of the metallic-glass films on the fatigue resistance of different substrates were investigated by four-point-bend fatigue experiments. Results indicated that the Zr- and Cu-based metallic-glass films could deliver marked improvements in terms of the fatigue life and fatigue-endurance limit. The thicker film could improve the fatigue resistance more significantly. However, the applications of TiN films, pure Cu films, and Fe-based metallic films could not improve the fatigue resistance of the steel substrate. When the Zr-based metallic-glass films were applied to the Ni- and Zr-based alloys, the fatigue resistance of the substrates could also be improved. The fatigue resistance of steels coated with the Zr- and Cu-based films was investigated by tension-tension fatigue experiments. Results indicate that neither films could improve the fatigue resistance of the steel, which may be related to the poor tensile ductility of metallic glasses.Fractographic and transmission electron microscopy (TEM) studies showed that intrusions and extrusions normally formed at persistent slip bands on free surfaces could be suppressed by the metallic-glass thin films. The fatigue-resistance enhancement is mainly due to the lifetime extension during the fatigue-crack-initiation process. For the metallic-glasses films which could enhance the fatigue resistances of different substrates, their good interfacial adhesion with the substrates, high strength and hardness, improved bending plasticity, and high compressive residual stresses played important roles.

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