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Study of Structural and Mechanical Properties of Zr-, Ti- and La-based bulk Metallic Glasses

Asadi Khanouki, Mohammad Taghi | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50974 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Aashuri, Hossein; Tavakoli, Rouhollah
  7. Abstract:
  8. Bulk metallic glasses (BMGs), in contrast to conventional crystalline materials, are defined as metals with an amorphous and disordered atomic-scale structure. Due to the absence of dislocations and grain boundaries, BMGs have considerably unique mechanical properties such as high strength and elastic strain, high wear resistance and desirable corrosion resistance. However, they generally suffer from poor plasticity caused by an inhomogeneous deformation which leads to catastrophic failure by localization of strain into narrow regions, known as shear bands. This factor has extremely restricted their application as advanced structural materials. Furthermore, the recently discovered phenomenon known as intermediate temperature ductility minimum (ITDM), has more restricted their applications. In the present research, it has been tried to illustrate the dominant deformation mechanisms in BMGs. One of the main goals of this work is to gain a clear understanding of the ITDM mechanism in BMGs. In this purpose, the effect of temperature and strain rate, as main factors which affecting the microstructure evolution, fracture surface and hence ductility, have systematically been investigated. Three different alloy systems (Zr-, Ti- and La-based BMGs), which have different mechanical properties were prepared by vacuum arc-melting. Then, plates with a thickness of 1.0 mm and the width of 10 mm were produced by the suction casting into a water-cooled copper mold. In order to study the plastic deformation, three point bending tests were conducted at a wide range of temperature and strain rate. Also, the tensile side of specimens and their fracture surfaces were studied through scanning electron microscopy. The results demonstrate that the deformation mechanism in BMGs is affected by two processes: formation and activation of STZs and structural relaxation during deformation. According to the results, there is two distinct ductile to brittle transition (DBT) at very low (0.1 Tg) and intermediate (0.6-0.9 Tg depending on the strain rate) temperatures. The increase of strain rate lowers the ductility and shifts the DBT to higher temperatures. By studying and measuring the fracture surface morphologies such as shear offset, vein patterns, dimple and nano-corrugations, it is found that there is a correlation between the fracture surface morphology and ductility. Also, the equality of the activation energy for ITDM and nearly constant loss (NCL) phenomenon indicates that the activation of fewer STZs takes place during inelastic shuffling of caged atoms by NCL. Indeed, the NCL is the main factor in the occurrence of ITDM phenomenon
  9. Keywords:
  10. Bulk Metallic Glass ; Free Volume ; Ductility ; Shear Band ; Shear Transformation Zone ; Morphology ; Structural Relaxation

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