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Microstructure-Property Optimization in Metallic Glasses

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About this book

This thesis consists of an in-depth study of investigating microstructure-property relationships in bulk metallic glasses using a novel quantitative approach by which influence of the second phase features on mechanical properties can be independently and systematically analyzed. The author evaluates and optimizes the elastic and plastic deformation, as well as the overall toughness of cellular honeycombs under in-plane compression and porous heterostructures under uniaxial tension. The study reveals three major deformation zones in cellular metallic glass structures, where deformation changes from collective buckling showing non-linear elasticity to localized failure exhibiting a brittle-like deformation, and finally to global sudden failure with negligible plasticity as the length to thickness ratio of the ligaments increases. The author found that spacing and size of the pores, the pore configuration within the matrix, and the overall width of the sample determines the extent of deformation, where the optimized values are attained for pore diameter to spacing ratio of one with AB type pore stacking.

Table of Contents

Frontmatter
1. General Introduction
Abstract
The demands on materials increase rapidly. To meet these demands, more and more complex microstructures and microstructural architectures are used. However, currently used strategies to develop increasingly complex structures are unsuited to create tomorrow’s materials. For example, the main challenge in determining micro structure–property relationships is that any kind of individual variation in feature properties inevitably changes other properties. This is due to the fabrication methods, which do not permit completely independently vary just one microstructure feature. As an example, if one attempts to alter, for instance, the spacing of a phase in a microstructure, at the same time, length, volume, composition, dispersity, and density of this phase will also change because all the properties are interconnected. This contribution consists of an in-depth study of investigating microstructure-property relationships in bulk metallic glasses using a novel quantitative approach by which influence of the second phase features on mechanical properties can be independently and systematically analyzed. We adopted this strategy to evaluate and optimize the elastic and plastic deformation, as well as the overall toughness of cellular honeycombs under in-plane compression and porous heterostructures under uniaxial tension.
Baran Sarac
2. Fabrication Methods of Artificial Microstructures
Abstract
The exceptional processability and large supercooled liquid region (SCLR) of metallic glasses (MGs) makes them highly promising candidates for thermoplastic processing, especially when replicating features with high precision and tolerance. A lightweight Zr35Ti30Cu7.5Be27.5 MG former (ρ ≈ 3.9 g cm−3), which has the largest SCLR of ΔT = 159 K (at 20 K/min heating rate) of any known bulk glass forming alloy is utilized for heterostructure fabrication. This alloy can be cast into fully amorphous rods of 1.5 cm in diameter, and shows enhanced glass forming ability (GFA). In addition, this alloy exhibits high-yield strength in compression (σ y = 1430 GPa) and relatively better fracture toughness than many other MGs, and a relatively high Poisson’s ratio of ν = 0.37. The undercooled liquid exhibits an unexpectedly high Angell Fragility of m = 65.6. Microreplication methods carried out in open air using relatively low applied pressures (~ 1 atm) demonstrate superior thermoplastic processability of Zr-based alloys for engineering applications. Furthermore, strain rate effects on viscosity of this alloy and similar Zr-based MG alloys have been extensively studied, and based on these measurements, it is demonstrated that Zr35Ti30Cu7.5Be27.5 exhibits exceptional properties for thermoplastic processing.
Baran Sarac
3. Structural Characterization of Metallic Glasses
Abstract
The amorphous nature and the forming characteristics of the cast metallic glass (MG) rod need to be closely monitored to assess the suitability of the cast material for the artificial microstructures. For this reason, three different test methods, namely, formability test, structural and thermal analysis, and bending test were conducted prior to fabrication and testing of MG heterostructures, and the results are presented in this chapter.
Baran Sarac
4. Artificial Microstructure Approach
Abstract
Length scales have been identified as the main criteria in the design of effective metallic glass (MG) heterostructures. Spanning six orders of magnitude (nm to mm), these length scales include different regions like shear transformation zones, where the collective shearing of the atoms in the order of a nanometer in size takes place. The second region we are interested in includes the critical plastic zone size, which varies among MG formers between 10 μm and a few millimeters. This is a very important length scale in the design of MG heterostructures. For example, effective MG composites exhibit a heterostructure with the second-phase spacing, which is comparable to the spacing of the plastic zone size. It was suggested that, in this case, shear bands do not transform into cracks but shear can be absorbed into the softer second phase.
Baran Sarac
5. General Conclusions and Outlook
Abstract
Within this thesis, we introduced a novel approach called artificial microstructures to determine microstructure–property relationships in complex materials. The biggest advantage of this technique over other conventional methods is the systematic data analysis and quantification of results, where we determine the effect of each feature on mechanical properties through completely independent feature variation. We utilized this approach to address two important problems in metallic glasses (MGs). The first problem was to understand the behavior of MGs in hexagonal cellular structures. Here, we found that the deformation can be controlled and manipulated by changing the relative density. As a consequence, three major deformation regions are discovered: collective buckling showing nonlinear elasticity, localized failure exhibiting a brittle-like deformation, and global sudden failure with negligible plasticity. The ideal density for optimal mechanical properties was determined to be ~ 25.0 %, which is within the local failure deformation regime. Enhancement in mechanical properties in MG cellular structures was achieved by stress optimization through corner-fillets, which doubled the strength at the expense of 0.2 % density increase. Besides, energy absorption of MG cellular structures exceeds cellular structures of most other materials due to the utilization of a size effect.
Baran Sarac
Backmatter
Metadata
Title
Microstructure-Property Optimization in Metallic Glasses
Author
Baran Sarac
Copyright Year
2015
Electronic ISBN
978-3-319-13033-0
Print ISBN
978-3-319-13032-3
DOI
https://doi.org/10.1007/978-3-319-13033-0

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