Micromechanical behavior investigation of metallic glasses

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Micromechanical behavior investigation of metallic glasses

 

Author: Liu, Zhiyuan
Title: Micromechanical behavior investigation of metallic glasses
Degree: Ph.D.
Year: 2012
Subject: Metallic glasses.
Metallic glasses -- Testing.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: vii, xi, 139 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2507358
URI: http://theses.lib.polyu.edu.hk/handle/200/6518
Abstract: As a new class of materials, metallic glasses (MGs) have stimulated extensive interest in the academic community, because they possess unique disordered atomic structure lacking long range translational symmetry, and thus have excellent mechanical and chemical properties such as high strength, large fracture toughness and good corrosion resistance. These excellent properties render MGs with a great potential for engineering applications. However, low plasticity at room temperature seriously hinders MGs' application, besides the atomic structure of the MGs has not been well captured and the deformation mechanism has not been fully understood. In this thesis, I start with an investigation of the cooling rate effect on the elastic properties of MGs using quasi-static and dynamic methods, then extend the investigation to the plastic deformation of MGs and characterize the speed of shear band as a function of shear offset. Cooling rate is thought to play the most important role in the vitrification of glass-forming liquids. It is known that the cooling rate can affect MGs' atomic structure and thus may possibly has influence on their mechanical properties. Quasi-static micromechanical study of the cooling-rate effect on Young's moduli and hardness of the as-cast bulks and melt-spun ribbons for Zr₅₅Pd₁₀Cu₂₀Ni₅Al₁₀ MG was carried out. Using the classic nanoindentation method, the Young's moduli of the ribbon samples obtained at higher cooling rates were measured which appeared to be much lower than those of the bulk samples. Through further experiments on slice samples cut from the as-cast bulks MG and finite-element (FE) analyses, we have clearly demonstrated that the measured difference in elastic moduli was mainly caused by the sample thickness effect in nanoindentation tests. To overcome such a confounding effect, microcompression experiments were performed on the as-cast and as-spun MG samples, respectively. Being consistent with the findings from nanoindentation, the microcompression results showed that the cooling rate, as ranging from ~10² to ~10⁶ K/s, essentially has no influence on the Young's modulus and hardness of the MGs.
Then, we turn to using dynamic microcompression test to investigate the cooling rate effect. First of all, to understand and analyze the anelastic deformation of MGs, theoretical framework based on the energy barrier concept was developed. The theoretical results clearly show that the stress-induced reversible local structural transition in MGs is equivalent to a Kelvin-type anelastic deformation process, and this reflects a core-shell atomic configuration in MGs, which consists of free volume zones (FVZs) and surrounding dense packed atomic clusters. Using this theoretical model, dynamic test results of micropillars carved out from bulk and ribbon MGs were analyzed, no discernable difference in effective modulus and viscosity can be revealed, indicating that similar content of dense packed cluster and FVZs in the bulk and ribbon MGs despite the cooling rate difference in about four orders of magnitude. Apart from the cooling rate effect, the shear bands, as the plastic deformation carrier of MGs, were investigated. Important experimental finding and theoretical analysis of the shear-band speed measured in a variety of bulk metallic-glasses (BMGs) were presented. Unlike the other research work, in which the shear-band speed was regarded as a constant, our study, based on carefully designed loading-holding cyclic tests, reveals that the speed of shear band correlates with its resultant shear offset. Such a correlation arises as a 'size' effect, which could be rationalized by the energy balance principle.

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