Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Dr. Robert J. Klassen

Abstract

In this thesis length and time scale dependence of the operative plastic deformation mechanisms in Au is studied. Uniaxial compression tests were performed at various loading rates on FIB-milled Au micropillars and single-crystalline Au microspheres of diameter ranging from 0.8 to 6.0 µm to investigate the incipient and bulk plasticity events. Constant-load ambient-temperature creep tests were performed on the micropillars to study the time-dependent plasticity at very slow strain rates. Uniaxial compression tests were also performed on coated Au microspheres to study the effect of extrinsic constraint on the deformation mechanisms.

During uniaxial compression, both the Au micropillars and microspheres displayed strain jumps, the frequency of which decreased with increasing sample diameter and increasing resolved shear stress. The bulk flow stress, corresponding to 5% – 20% average compressive strain, was dependent upon both the strain rate and the specimen diameter. Analysis of the apparent activation volume, V*, and energy, Q*, of the deformation process indicated that the operative deformation mechanism for the small 0.8 µm diameter pillars and spheres was characteristic of a mechanism limited by surface nucleation of dislocations while larger diameter samples displayed values indicative of the more common dislocation-obstacle interaction limited deformation mechanism.

The deformation-rate dependence of incipient plastic deformation of the Au micropillars and microspheres was also dependent upon the strain rate and sample diameter. For the smallest, sub-micrometer size, samples the incipient plasticity was controlled by heterogeneous dislocation nucleation events, while a dislocation-obstacle interaction limited glide process was found to be operative in the larger specimens.

In the extrinsic constraint study, Au microspheres that were coated with a 40 – 80 nm thick Ni layer displayed a slightly increased flow stress compared to similar size uncoated microspheres. The estimated V* and Q* values for the coated microspheres suggest that the mechanism responsible for the initiation of first dislocation motion is essentially the same regardless of the presence of a constraining coating.

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