Characterization of deformation defects in metallic glasses and metallic glass composites : from experiment to computer modeling

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Characterization of deformation defects in metallic glasses and metallic glass composites : from experiment to computer modeling

 

Author: Shen, Ying
Title: Characterization of deformation defects in metallic glasses and metallic glass composites : from experiment to computer modeling
Degree: Ph.D.
Year: 2012
Subject: Metallic glasses.
Metallic glasses -- testing.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xv, 166 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2615878
URI: http://theses.lib.polyu.edu.hk/handle/200/6980
Abstract: With its excellent mechanical properties, metallic glass has great potential to be used as structural materials for engineering application. However, shear deformation confined in the localized narrow shear zones in bulk metallic glasses (BMGs) always leads to catastrophic failure, which results in little overall plasticity and low ductility and limits their further engineering applications in a broad area. To overcome this drawback, it is necessary to deeply understand the deformation mechanism in these amorphous alloys. However, because of the difficulty of direct characterization of fundamental deformation units, the defects, and their responses to shear deformation in the metallic glass, the deformation mechanisms in BMGs are not fully understood yet. To deeply interpret the deformation mechanisms in metallic glasses, it is necessary to understand the internal atomic structures of defects and their properties under thermal or/and mechanical stimulation. In metallic glasses, atoms are disorderly organized to form an amorphous structure without crystalline lattices. Due to the rapid quenching from liquid state, structural heterogeneity would be inevitably introduced into the amorphous matrix. Some local atomic units can be weaker or sparser than their surrounding matrix, which can be considered as defects in metallic glasses. As loose atomic units, the defects can be the sources for atomic rearrangements under thermal stimulation. We focus on the characterization of as-synthesized defects generated from rapid quenching and on their roles in structural relaxation. We use dynamic mechanical analysis (DMA) to measure the internal friction Q-1 of metallic glass, which is scaled as Q-1~ω-n, where ω is the testing frequency. The reversible and irreversible structural relaxations in metallic glass are characterized, through the index n. It is found a maximum index n is associated with reversible β relaxation, and the index n at glass transition temperature is a universal value of around 0.25-0.28. Chemical short range ordering (CSRO) and topological short range ordering (TSRO) are used to interpret such scaling.
We then characterize the defects which response to shear deformation and define them as deformation defects. Based on our experimental studies, annihilation or creation of the deformation defects is found to be a process of first-order structural transition. The activation energy Ef for such transition and the surface energy of these defects are characterized. Besides, important parameters such as the size of the deformation defects and activation energy for plastic flow are also obtained, which are all crucial parameters for computational modeling of shear banding in metallic glass. Evidences from the experiments show that local plastic deformation of metallic glasses is associated with the structural change of deformation defects, which is a first-order phase transformation. Based on the experimental study, phase field modeling with the parameters obtained from internal friction studies is applied to characterize the macroscopic mechanical properties of the metallic glasses and their composites. Systematic study on the shear banding, crack initiation and propagation in BMGs and fiber-reinforced BMG composites is carried out with phase field modeling. The fracture toughness and the modes of failure obtained from the simulation are consistent with experiments, suggesting the successful application of the phase-field approach on the understanding of shear banding and fracture processes in BMGs and BMG composites. The enhanced fracture toughness of the composite is found to relate with the bonding conditions between the BMG and the reinforcements and shear band can be initialized from their interface, and the relationship is quantitatively determined in the phase field modeling. From the simulation study it is found that secondary shear band can be generated from the integration of two attracting shear bands. What we observe from the modeling of shear band multiplication and the generation of secondary shear bands is remarkable for the enhancement of the plasticity and fracture toughness of the BMG composites. The mechanical properties of BMGs and BMG composites are thus successfully connected with the characteristics of microscopic deformation defects.

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