Micro/nano-mechanical study of deformation mechanisms and structure in amorphous metal

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Micro/nano-mechanical study of deformation mechanisms and structure in amorphous metal

 

Author: Zeng, Jifang
Title: Micro/nano-mechanical study of deformation mechanisms and structure in amorphous metal
Degree: Ph.D.
Year: 2013
Subject: Metallic glasses.
Metallic glasses -- Mechanical properties.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xxii, 165 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2639080
URI: http://theses.lib.polyu.edu.hk/handle/200/7141
Abstract: In this thesis, micro- and nano-mechanical experiments combined with theoretical analyses were performed to investigate the mechanical heterogeneity in metallic glasses (MGs). Firstly, the anelastic deformation of MGs at ambient temperature was studied through spherical nanoindentation. The anelastic deformations in MGs were successfully revealed experimentally before shear banding occurs at room temperature. A general rheological model was proposed to rationalize the experimental results. Through the fitting of the experimental data to the model, we are able to quantify the structural inhomogeneity intrinsic to MGs in terms of local modulus and viscosity. The outcome of this investigation provides an important insight into the structure-property relation in MGs, and also shed light on the overall atomic packing in them. Secondly, we developed an experimental scheme to investigate the structural and mechanical origin of shear-banding-induced softening in a Zr-based BMG. Three BMG plates were plastically bent to different curvatures by using the mandrels of different diameter. The Young's modulus was measured via the Joslin-Oliver based Berkovich nanoindentation approach at a spatially resolved scale, of which the magnitudes remain nearly at a constant across the width of the BMG bent bars; in contrast, the hardness varies with the indentation location. As compared to the hardness measured before bending, the post-bending hardness is reduced significantly on the compressive side of the bend bars while remains almost unchanged on the tensile side. Interestingly, from the compression of micropillars, we didn't find any prominent change of the yield strengths and moduli of the BMG bend bars regardless of the pillar location. These findings suggest that residual stress plays a vital role in the plasticity-induced softening in the BMGs. In the last part of my thesis, we present our recent findings obtained through the high-resolution atomic force microscopy (HRAFM) on the evolution of the nanoscale structures in a metallic glass (MG) thin film. To reach the unprecedented spatial resolution of ~ 1nm, we have developed an experimental scheme to correct the possible AFM artifacts arising from surface adhesion and topography. After that, we systematically investigated the fractal growth of the dense-packing phases during the atomic packing evolves in an annealed Zr-Ni MG thin film. Through our HRAFM experiments, the annealing process of MGs was found to be associated with a fractal growth of some dense-packing phases. Based on these findings an important implication can be drawn that, with decreasing the cooling rate, the atomic structure of MGs become denser with a fractal packing of dense-packing phases. This fractal packing of the dense-packing phases constitutes the major overall structure feature of MGs and might dictate their structural revolution when subjected to mechanical deformation.

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