Date of Award

12-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

T.G. Nieh

Committee Members

E.P. George, Y.F. Gao, D. Penumadu

Abstract

Metallic glasses have many unusual properties, such as extremely high strength and large elastic limit. However, they are also brittle, failing in a catastrophic manner, as a result of the formation of highly localized shear bands at room temperature. Such flow localization has been attributed to the influence of strain softening or local heating, or a combination of both, which leads to the local drop in viscosity and, finally, the runaway of the shear band.

In this thesis, a novel method was developed for in situ observation of the localized shear processes. The inhomogeneous deformation of metallic glasses at room temperature and under a slow strain rate was found to be dominated by intermittent sliding on a principal shear plane in the sample. Such sliding results in flow serration in the stress-strain curve in the plastic region, and there is one-to-one correspondence between the intermittent sliding and flow serration.

Using high speed camera and strain gages it was found that the displacement-time curves exhibited micron-sized bursts after the onset of yielding, apparently associated with formation of individual shear bands. Each displacement burst disclosed a three-step (acceleration, steady-state, and deceleration) process during the shear-band propagation. The viscosity of propagating shear bands was evaluated, and a detailed analysis based on free volume model indicated that shear-band propagation was mainly resulted from free volume accumulation. The propagating speed of shear band in a smaller metallic glass specimen was found to be slower, mainly because a lower accumulative strain energy.

The absence of flow serration in compression at high strain rates was a result of the fact that the propagation was overshadowed when crosshead speed approaches the speed of shear-band propagation. Nanoindentation on several metallic glasses at different indentation rates indicates a much higher critical strain rate for the disappearance of flow serration. Brittle metallic glasses were found to have a higher shear-band strain rate as a result of higher shear-band energy.

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