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Abstract
There is a technological drive for lightweight, tough and high strength materials for engineering applications. Bulk Metallic Glasses (BMGs), in particular Magnesium-based BMGs, may prove viable candidates based on their high specific-strength when compared to their crystalline counterparts.
To improve on the inherent brittle nature of these amorphous alloys and enhance their ability to accommodate plastic strain, secondary phases can be engineered within their structure, essentially creating BMG composites. Many types of composite fabrication methods have previously been attempted with BMGs, which typically fall under the two broad categories of in situ and ex situ processing. Ex situ techniques in particular, allow for a wide variety of materials that can be used as reinforcement and for mechanical control over the reinforcement material in the amorphous matrix.
One category of ex situ processing which has received very little investigation is that of thermoplastic forming (TPF) within the supercooled liquid region (SCLR) of BMGs. Pressing secondary phases into BMGs whilst they are in a supercooled liquid state is possible, however, results have shown limited infiltration in Mg-based BMGs because they still retain a high viscosity (~106 Pa.s), even at optimal processing temperatures in the SCLR.
This thesis makes use of a layered/laminate method of matrix and reinforcing phases to enable an adequate distribution of secondary phases throughout the final composite sample. The consolidation theory for infiltrating secondary phases into a BMG layer in the SCLR utilises the Hagen-Poiseille equation, whereby the infiltration depth can be calculated by consideration of the glass viscosity, time to crystallisation of the glass, and the diameter of the reinforcement. The interfacial strength is estimated from the difference in thermal expansion coefficient, which determines the strength of the mechanical interlock between the matrix and reinforcement.
Using Mg65Cu25Y10 BMG as the matrix material, carbon fibre reinforced BMG composites were successfully fabricated by hot pressing the metallic glass layers and 5-7 um diameter carbon fibres within the SCLR (5 minutes at 180 °C). X-ray diffraction confirmed that the amorphous nature of the matrix is retained after processing, and that the process does not cause formation of any unwanted interface products. The mechanical properties of these composites was evaluated using 4-point bending where a maximum flexure strain of 328 MPa prior to failure was observed for a fibre volume fraction of 10%. Analysis of the fractured composites showed that carbon fibres underwent brittle fracture close to the fracture surface and that the fibres were useful in preventing shear band propagation through the amorphous matrix.
Additionally, some extensions of this method have led to laminate composites using carbon fibre pre-preg , which allows for simultaneous curing of the carbon fibres and forming of the final composite piece. Composite fabrication within the SCLR shows promise for using a wide variety of reinforcing materials within a BMG matrix for processing at low temperatures and within short time frames.