Magnesium metallic glasses

• Main Author: Xu, Wangsheng.
• Other Authors: Tan Ming Jen
• Format: Final Year Project
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/50961
• Institution: Nanyang Technological University

Metallic glasses are a relatively new class of materials which are characterised by the absence of a crystalline structure. The absence of crystallinity, and thus of their associated features such as absence of grain and phase boundaries, gives rise to many of metallic glasses’ unique properties, such as superior corrosion resistance, higher hardness, higher yield and specific strength, as well as higher fracture toughness. However, they have very poor ductility in tension due to the shear-banding phenomenon. The partially crystallized samples were fabricated in a 2-step process. Amorphous samples were first produced by injection casting. It was found that by varying various parameters such as crucible nozzle size, casting pressure and heating duration, amorphous rods of at least 50 mm in length can be obtained during at least 60% of the castings. The amorphous samples were then annealed to obtain the required percentage of crystallinity. Through this project, it is hoped that Magnesium-based metallic glasses with improved ductility can be obtained by the in situ generation of certain degrees of crystallinity (11%, 24%, 31%) within the amorphous matrix. Characterisations of X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM) confirmed the successful fabrication of partially crystallized samples, and the controllable degree of crystallization was calculated from DSC patterns. Compression testing was conducted on all samples at room temperature and elevated temperature within the supercooled liquid region. It was found that: i) at room temperature, the 24%-crystalline sample exhibited maximum strain at failure that is a 21.6% increase over that for the fully amorphous sample; ii) for testing done within the supercooled liquid region at 120oC, the samples exhibited superplastic deformation behaviour. On the other hand, it was found that the flow stress increases when higher strain rates were used and when the samples had a higher degree of crystallinity. Biocorrosion testing was also carried out on the fully amorphous, 24%-crystalline sample and on the fully crystalline samples. From the ion release tests, it was found that the fully crystalline samples were corroded at a much higher rate than the fully amorphous and partially crystalline sample. Based on the analysis of Energy Dispersive X-ray Spectroscopy (EDS), a possible mechanism for the corrosion of the Mg67Zn28Ca5 samples with different percentage crystallinity was also proposed. An alternative attempt was also applied to improve the mechanical properties of the Mg67Zn28Ca5 metallic glass through the addition of carbon nanotubes (CNT). Two different methods were utilised: i) mixing of CNT into crystalline Mg67Zn28Ca5 powder by ball milling, followed by injection casting to obtain an amorphous structure. However, it was eventually found that the mixture had a melting point that was higher than the maximum operating temperature of the injection caster; ii) directly obtaining the amorphous structure by mechanical alloying, followed by sintering and extrusion to obtain the desired samples. Mechanical alloying was done through the use of prolonged ball milling for up to 42 hours. Samples were removed at regular intervals during the ball milling process and sent for X-Ray Diffraction (XRD) analysis. The XRD analysis indicated that nearly full amorphous alloy powder was obtained by the end of 42 hours. However, due to time constraints, the work was stopped before the sintering and extrusion stages could be carried out.