Tesis doctoral de Nele Van Steenberge
The exceptionally high yield strength, close to the theoretical limit, and yield strain of amorphous metallic systems in bulk offer them potential for structural applications. However, plastic deformation at room temperature occurs in a highly localized manner by the formation of a few shear bands. Instead of workhardening, metallic glasses soften upon deformation, which prevents stable plastic elongation. Although bmgs possess a high fracture strength, once yielding has set in, early failure after a small percentage of macroscopic deformation appears, often along one single shear band. This inhomogeneous deformation mechanismat ambient temperature still limits the reliability of bmgs for structural applications. logically, the enhancement of ductility of this type of materials has been the subject of many research works in the last decade. It was found that when increased plasticity levels were obtained, it involved always multiplication of shear bands, indicating that this multiplication is a powerful toughening mechanism in metallic glasses, regardless of how it is accomplished. Probably the most explored concept to avoid catastrophic failure has been the development of a heterogeneous microstructure, with a second phase on different length scales, both crystalline and amorphous. Various routes have been tried out to obtain this second phase in the amorphous matrix: physically adding a reinforcing phase to the melt, by direct precipitation from the melt of a properly designed composition or by (partial) nano-crystallization of the glass after casting. upon annealing below the glass transition, changes in both topological and chemical short range order have been reported. The former is believed to deteriorate plasticity due to structural relaxation of the amorphous structure, reducing its «free volume», necessary for local shear transformations, and redistribution of local stresses frozen in during rapid quenching. The latter effect, changes on chemical short/medium range order, has hardly been studied into detail. Besides annealing, deformation has been reported to induce structural and microstructural changes. Where annealing always involves the risk of embrittlement due to the second annihilating effect of relaxation, deformation involves creation of free volume and does not suffer from this counterbalancing effect. the (microstructural changes, induced by annealing and deformation, form the main topic of the work presented in this thesis. Various techniques have been applied to observe/visualize in the first place possible changes. In a next step, their influence on thermal behavior and mechanical behavior will be exposed. Besides the already known techniques like x-ray diffraction with a high-brilliance synchrotron source and high-resolution transmission electron microscopy, more accessible techniques with less requirements on sample preparation like differential scanning calorimetry can already give an indication of such structural changes due to the immediate link discovered between these structural changes at low temperature and the change in crystallization behavior at higher temperatures. Besides, acoustic measurements of the elastic properties is a technique extremely sensitive to the local structure. Finally, depth-sensing techniques have become widely used for the mechanical characterization of solid materials since they allow the evaluation of the mechanical properties from a relatively small volume of material. In particular, indentation techniques permitted the determination of the mechanical properties of thin ribbons of metallic glasses. They are becoming to be of inestimable value in the study of the mechanical behavior of metallic glasses, due to their resolution down to the nanometric scale. however, in a first part of this thesis, it will be shown that one should be aware when applying this technique, of the existence of a so-called size-effect, directly linked with the structural changes upon deformation.
Datos académicos de la tesis doctoral «Study of structural changes in zr-based bulk metallic glasses upon annealing and deformation treatments«
- Título de la tesis: Study of structural changes in zr-based bulk metallic glasses upon annealing and deformation treatments
- Autor: Nele Van Steenberge
- Universidad: Autónoma de barcelona
- Fecha de lectura de la tesis: 20/06/2008
Dirección y tribunal
- Director de la tesis
- María Dolores Baró Mariné
- Tribunal
- Presidente del tribunal: Santiago Suriñach cornet
- alex Zhilyaev (vocal)
- yolanda Calventus (vocal)
- marcello Baricco (vocal)