The first aim of the present work is to design, built and test a device being capable of rapidly heating and subsequently immediately quenching bulk metallic glass (BMG) specimens in a uniform and reproducible manner. The heating and ejection process of the BMG specimen is temperature-controlled, so that the heating rate and temperature to which the BMG is annealed to, can be varied.
BMG composites consisting of B2 CuZr crystalline particles embedded in the glass shall be prepared. The occurrence of the shape-memory B2 CuZr phase and a good glass-forming ability limit the selection of the alloy system to Cu-Zr-Al. The high-temperature B2 CuZr phase is metastable at room temperature, yet rapid cooling of Cu-Zr-Al-based alloys enables to cast B2 CuZr BMG composites. One could speculate that the B2 CuZr phase forms as well during the devitrification of Cu-Zr-Al-based metallic glass at high heating rates. Indeed, a recent work yields hope rendering the preparation of B2 CuZr BMG composites by devitrification possible. Therefore, a rapid heating or flash-annealing device as it is termed here, is developed first. Thereby, flash-annealing is defined as rapid heating to a predefined temperature followed by immediate quenching. Cu-Zr-Al-based BMGs are flash-annealed at different heating rates to temperatures above the crystallization temperature and the influence of the heating rate on the phase formation shall be unravelled. The exact ejection temperature of the BMG specimen determines the volume fraction of the crystalline phase(s) and the crystal size distribution of the so-obtained composite. The immediate subsequent quenching at a high cooling rate is crucial here, since the partially crystallized supercooled liquid must be frozen in. The mechanical properties of the BMG composites and deformation mechanisms involved are studied. Moreover, the microstructure of these BMG composites allows to gain knowledge about crystallization kinetics of highly supercooled liquids during rapid heating.
Contents
1 Abstract
2 Objectives
3 Fundamentals
3.1 Structure of metallic glass
3.2 Glass formation and crystallization
3.2.1 Glass transition
3.2.2 Classical nucleation theory
3.2.3 Crystal growth in undercooled liquids
3.2.4 Isothermal and isochronal devitrification of metallic glasses
3.2.5 Fragility concept
3.2.6 Glass-forming ability
3.3 Mechanical properties
3.3.1 Deformation of bulk metallic glass
3.3.2 Measures to affect the plasticity of bulk metallic glass
3.4 Cu-Zr-Al-based alloys
3.5 Flash-annealing
4 Experimental
4.1 Sample preparation
4.2 X-ray diffraction
4.3 Calorimetry
4.4 Flash-annealing
4.5 Electro-static levitation
4.6 Microscopy
4.7 Mechanical testing
4.8 Optical profilometry
5 Results and Discussion
5.1 Development of the flash-annealing device
5.1.1 Description of the temperature-time heating curve
5.1.2 Inductive heating
5.1.3 Estimation of the cooling rate
5.1.4 Surface oxidation of Cu-Zr-Al-based BMGs during flash-annealing
5.2 Characterization of the as-cast bulk metallic glasses
5.2.1 Development of the Cu44Zr44Al8Hf2Co2 alloy
5.2.2 Amorphicity of the Cu46Zr46Al8 and Cu44Zr44Al8Hf2Co2 BMGs
5.2.3 Mechanical behaviour of as-cast Cu44Zr44Al8Hf2Co2 BMGs
5.3 Structural changes during flash-annealing below Tx
5.3.1 Calorimetric investigation of structural changes
5.3.2 Mechanical properties of flash-annealed bulk metallic glass
5.4 Crystallization of BMGs during flash-annealing
5.4.1 Phase formation of Cu-Zr-Al-based BMGs
5.4.2 Crystallization kinetics of Cu-Zr-Al-based BMGs
5.4.3 Mechanical behaviour of Cu44Zr44Al8Hf2Co2 BMG composites
6 Summary
7 Outlook
8 Appendix
8.1 Chemical analysis
8.2 Device development
8.2.1 Temperature control
8.2.2 Inductive heating - derivation of the skin depth
8.2.3 Estimation of the cooling rate
8.3 Calorimetric analysis
8.3.1 Isochronal transformation kinetics (Kissinger)
8.3.2 Isothermal transformation kinetics (Johnson-Mehl-Avrami-Kolmogorov)
8.3.3 Fragility
8.4 Uniformity of partially crystallized BMG
8.5 Crystal growth rate
8.6 Stereology
Objectives and Research Focus
The primary objective of this dissertation is to investigate the potential of flash-annealing as a novel heat treatment method for Cu-Zr-Al-based bulk metallic glasses (BMGs) to precisely tailor their microstructure and enhance their mechanical properties, specifically by overcoming inherent brittleness through the controlled precipitation of uniformly distributed B2 CuZr nanocrystals.
- Design, development, and testing of a temperature-controlled flash-annealing device.
- Investigation of structural changes and phase formation in Cu-Zr-Al-based BMGs during rapid heating and subsequent quenching.
- Analysis of the influence of heating rates on crystallization kinetics and nucleation processes.
- Evaluation of mechanical properties, deformation mechanisms, and the impact of structural heterogeneities and nanocrystal precipitation on BMG plasticity.
Excerpt from the Book
3.1 Structure of metallic glass
The lack of long-range order is the defining structural characteristic of glass [68]. For this reason, no sharp reflections, which could be attributed to lattice planes, are present in the X-ray diffraction (XRD) pattern. Instead, one can observe two characteristic broad “maxima” (Fig.3.1.1, marked with black arrows). Metallic glass shows no discernable microstructure like for instance grains, precipitates, interfaces, twins or dislocations as they are typical characteristics of polycrystalline alloys. Instead, high-resolution transmission electron micrographs display mazelike patterns (Fig. 3.1.1b).
Bernal and Scott have been the first to propose a model to describe disordered structures [60, 61]. They approximated the atoms in metals as identical hard spheres and arranged them as densely as possible in a random manner. The three-dimensional space, however, cannot be filled solely with these densely randomly packed clusters without introducing long-range translational symmetry. This problem is addressed in literature as “packing frustration” [59, 62, 69]. Therefore Bernal suggested that holes smaller than a hard sphere have to be incorporated into the dense random packing. Bernal termed them “canonical holes” [61]. Altogether, according to this model five different types of holes with edges of equal length are present as can be seen from Fig. 3.1.2. This pioneering concept known as “the dense random packing of hard spheres” (DRPHS) model [69], has been analyzed by means of molecular dynamics (MD) simulation to determine the fraction of each hole. The “empty” space necessary to avoid packing frustration is also termed as “free volume” [59] and is redistributed among all hole types whereas the tetrahedra are found to be dominant [70].
Summary of Chapters
3 Fundamentals: Provides a comprehensive overview of metallic glass structures, glass formation, crystallization kinetics, and the mechanical properties of BMGs and BMG composites, establishing the theoretical framework for the study.
4 Experimental: Describes the materials, sample preparation techniques, the custom-built flash-annealing device, and the analytical methods including X-ray diffraction, calorimetry, levitation, and mechanical testing used to investigate the BMG samples.
5 Results and Discussion: Details the development of the flash-annealing device, characterizes the as-cast BMGs, and presents an in-depth investigation of structural changes, crystallization kinetics, and mechanical behaviors observed during flash-annealing.
Keywords
Bulk Metallic Glasses, Flash-annealing, Cu-Zr-Al alloys, Crystallization kinetics, B2 CuZr nanocrystals, Mechanical properties, Plasticity, Shear bands, Free volume, Structural heterogeneities, Nucleation, Quenching, Calorimetry, Phase formation, Amorphous materials.
Frequently Asked Questions
What is the core subject of this doctoral thesis?
The thesis explores the application of flash-annealing as a rapid thermal treatment for Cu-Zr-Al-based bulk metallic glasses (BMGs) to improve their mechanical performance by inducing controlled crystallization.
What are the primary themes addressed in the work?
The work focuses on understanding the crystallization kinetics, structural relaxation, rejuvenation processes, and the formation of BMG composites with improved ductility and work-hardening capabilities.
What is the central research question?
The research investigates whether flash-annealing can be effectively used to reproducibly generate uniformly distributed nanocrystals within BMGs to enhance their plasticity while maintaining high strength, thereby overcoming their intrinsic brittleness.
Which scientific methods are utilized?
The study employs a custom-built flash-annealing device, X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electrostatic levitation, and mechanical testing (compression and tension).
What does the main body cover?
The main body examines the structural changes of BMGs subjected to rapid heating, develops models for the flash-annealing process, characterizes phase formation, evaluates the resulting microstructures, and analyzes how these microstructures influence mechanical deformation mechanisms.
Which keywords define this research?
Key terms include Bulk Metallic Glasses, Flash-annealing, Cu-Zr-Al-based alloys, crystallization kinetics, B2 CuZr nanocrystals, plastic deformation, shear bands, and structural heterogeneities.
How does flash-annealing differ from traditional casting?
Unlike casting, where uncontrolled temperature gradients often lead to heterogeneous and large crystal distributions, flash-annealing allows for rapid, uniform heating and quenching, which enables the tailored and reproducible precipitation of small, uniformly distributed nanocrystals.
What role does the B2 CuZr phase play in mechanical properties?
The B2 CuZr phase is a shape-memory crystalline phase. Its precipitation within the amorphous matrix enables work-hardening through a stress-induced martensitic transformation, significantly improving the plasticity and ductility of the composite material.
- Quote paper
- Konrad Kosiba (Author), 2017, Flash-Annealing of Copper-Zirconium-Aluminium based Bulk Metallic Glasses, Munich, GRIN Verlag, https://www.grin.com/document/369387
