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Title: Effects of a minor addition of Si, Sn and In on formation and mechanical properties of Cu-Zr-Al bulk metallic glass
Authors: Lau, Chung Yam (劉宗欽)
Department: Department of Physics and Materials Science
Issue Date: 2008
Course: AP4116 Dissertation
Programme: BEng (Hons) in Materials Engineering
Instructor: Dr C H Shek
Subjects: Metallic glasses -- Mechanical properties
Abstract: To develop bulk metallic glasses (BMGs) for structural applications, prevention of the catastrophic failure caused by the formation and propagation of a single dominant shear band is a must. As the bulk metallic glass formers showing stability with respect to crystallization, many BMG matrix composites have been developed by making use of various crystalline reinforcement phases to inhibit shear band propagation. However, very few studies on the optimization of the microstructure and mechanical behavior of these BMG matrix composites were done in the past. In this thesis, Cu-Zr-Al metallic glass was selected to study the influence of 1 % atomic ratio addition of Sn, Si, In, on the mechanical behaviors. The effect was studied at room temperature (RT) using various techniques including differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electronic microscopy (SEM) etc. All glassy alloys were prepared by arc melting and suction casting. Meanwhile, the effect of small alloying addition on the thermal stability of Cu46Zr46Al8 was investigated. XRD examination shows amorphous structures on all formulated glassy alloys. The absent of sharp crystalline peak in diffraction pattern give evidence on the glassy forming ability of Cu-Zr-Al-BMG with addition of chosen elements. In the finding of stiffness dependence, Cu-Zr-Al-Sn BMG gave a remarkable increase in hardness in the Vickers hardness test.; Cu-Zr-Al-Sn BMG with collapsed appearances were shown on the indented edges. The occurrence of the collapsed edges can be explained by embrittlement of amorphous matrix with nano-crystals. Both addition of Si, Sn to Cu-Zr-Al system enhanced the stiffness of the referencing Cu-base BMG. Study on the effect of silicon, tin, indium addition on the crystallization behavior of Cu46Zr46Al8 was also performed. From the DSC curves obtained, an enlargement of the supercooling region can be observed by alloying silicon together with the base Cu-Zr-Al BMG, this implied that the thermal stability has been enhanced. The glass-forming ability of Cu-Zr-Al metallic glass-forming alloys has been improved with 1% atomic addition of silicon, which has been attributed to silicon destabilizing oxide nucleation sites and increasing atomic size difference of the BMG. Moreover, room temperature compression test revealed a significant strain hardening and plastic strain elongation of 8.1% before failure for the 3-mm diameter (Cu46Zr46Al8)99Sn1 sample. The microstructure of the fracture surfaces of each parametric sample was studied and compared under SEM. The remarkable enhancement on plastic deformation can be explained in terms of shear band pattern and shear band density. They all give evidences on the resistance of catastrophic failure of Cu-Zr-Al-Sn BMG.
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