Lu, Jun (2002) Mechanical behavior of a bulk metallic glass and its composite over a wide range of strain rates and temperatures. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-06082005-151713
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The development of bulk metallic glasses (BMG), which have exceptional mechanical properties such as high strength, high hardness and corrosion resistance, as well as good glass forming and shaping abilities, using relatively expensive materials and processing techniques, offers great opportunities to use this class of solids as structural amorphous materials (SAM). In this thesis, the mechanical behavior of a bulk metallic glass [...] (Vitreloy 1) and its composite [beta]-phase Vitreloy 1 composite, i.e., [...]) is investigated.
The stress-strain relations for Vitreloy 1 over a broad range of temperatures (from room temperature up to the crystallization temperature) and strain rates [...] were established in uniaxial compression using both quasi-static and dynamic Kolsky pressure bar loading systems. The effect of strain rate and temperature on steady state flow stress, viscosity and peak stress, as well as the effect of jump-in-strain-rate on the stress-strain behavior, were investigated. Based on the experimental results, boundaries between three main deformation modes are proposed, namely, Newtonian flow and nonlinear flow resulting in homogeneous deformation and shear-localized failure constituting inhomogeneous deformation. To characterize the constitutive behavior of the bulk metallic glass, a free volume based model as well as a fictive stress model are utilized to analyze the stress-strain behavior and a mechanism for shear band formation.
A unique deformation characteristic of a bulk metallic glass is the shear localization of the material in response to external mechanical loading, which may lead to catastrophic shear failure immediately after yielding under uniaxial loading and at low temperatures. A dynamic indentation experimental setup was developed to evaluate the high-strain-rate inelastic post yield deformation behavior of Vitreloy 1 and its [beta]-phase composite. Time-resolved depth and load responses during the process of indentation on the materials were obtained. Both materials are found to be strain rate insensitive up to 2,000 [...]. Numerical simulations of the indentation experiments, using both pressure insensitive (J2 von Mises) and pressure dependent (Drucker-Prager) flow models, reveal that both materials are pressure (or normal stress) dependent. Intense multiple shear bands are observed in the indentation craters and are responsible for the observed overall inelastic deformation.
To further examine the inelastic deformation and as well as whether a pressure sensitive or normal stress is more appropriate for Vitreloy 1, multiaxial compression experiments using a confining sleeve technique were performed. In contrast to the catastrophic shear failure behavior in uniaxial compression, Vitreloy 1 exhibits large inelastic deformation of more than 10 percent under confinement, indicating the nature of ductile deformation under constrained conditions. It is found that the metallic glass follows a pressure dependent Tresca criterion, [...], and the coefficient of the pressure dependence, [beta], is 0.17. Multiple parallel shear bands are observed on the outer surfaces of the deformed specimens.
Motivated by potential use of Vitreloy 1 in impact related applications, the shock compression characteristics of both Vitreloy 1 and [beta]-Vitreloy composite were studied using planar impact loading. A surprisingly low amplitude elastic precursor bulk wave, corresponding to the elastic response of the 'frozen structure' of the intact metallic glasses, was observed to precede the rate-dependent large deformation shock wave. A concave downward curvature after the initial increase of the [...] shock Hugoniots suggests that a phase-change-like transition occurred during shock compression. In addition, compression damage occurred due to the shear localization. The spalling inside Vitreloy 1 was induced by shear localization, while in [beta]-Vitreloy 1, it was due to debonding of the [beta]-phase boundary from the matrix. The spall strengths at strain rate of [...] were 2.35 GPa and 2.11 GPa for Vitreloy I and [beta]-Vitreloy 1, respectively.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Engineering and Applied Science|
|Major Option:||Mechanical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||20 March 2002|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||08 Jun 2005|
|Last Modified:||26 Dec 2012 02:52|
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