Graduate Thesis Or Dissertation
 

Synthesis and characterization of Cu-reinforced Zr₄₁.₂Ti₁₃.₈Cu₄₁.₂Ni₁₀Be ₂₂.₅ bulk metallic glass forming alloy

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vd66w467t

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  • The development of amorphous metal alloys with high glass forming ability allows the fabrication of amorphous samples with thickness greater than 1mm, known as bulk metallic glasses(BMGs). Outstanding mechanical properties and ease in processing due to low melting points, low solidification shrinkage and higher supercooled liquid region make BMG a good candidate as a matrix for processing the composites. The work done in the thesis focuses on the synthesis and characterization of bulk metallic glass composites. Highly conductive Cu fibers were introduced into Zr₄₁.₂Ti₁₃.₈Cu₄₁.₂Ni₁₀Be ₂₂.₅ (Vitreloyl™) BMG of poor conductivity to produce a composite with a thermal anisotropy The main challenge in processing was to avoid the dissolution of Cu fibers into the matrix because Cu has a negative heat of mixing with most of the matrix components. The dissolution of the Cu ,changes the parent composition of matrix and hence deteriorates its glass forming ability which leads to the crystallization of the matrix. In this work, the composites with circular and square cross-sections with the volume fraction ranging from 7% to 60% were processed successfully by pressure-gravity infiltration. The microstructure, thermal stability and structure of the composites were investigated by optical microscopy, differential scanning calorimetry(DSC) and X-ray diffraction(XRD). The metallic glass remained predominantly amorphous after adding up to 60% volume fractions of Cu fibers. Optical microscopy shows the clear distinction between the reinforcement and matrix with no macroscopic dissolution of reinforcement into the matrix. However, limited crystallization at the Cu/Vitreloyl™ interface occurred during the processing. This crystallization is attributed to the heterogeneous nucleation starting from the Cu/Vitreloyl™ interface due to the presence of Cu fibers, which act as nucleation sites, followed by the growth in the areas surrounding the fibers with enhanced Cu concentration. The concentration profile of the composite was later simulated and related with the crystallization near the fibers. From these simulations and observations, the success in processing these composites is attributed to the low shear rates, small processing times and temperatures involved in the processing. A combination of 2-dimensional triangular and square ordering of the fibers was observed in the composite samples. X-ray diffraction patterns of the composites show the peaks from the reinforced fibers and crystals around the Cu/Vitreloyl™ interface superimposed on the broad diffuse maxima from the amorphous phase. Later, the percent crystallinity in the sample was quantified by thermal analysis using the DSC. The DSC thermograms confirmed that the samples reinforced with 60 volume% Cu fibers with only 20% crystallinity can be obtained. The mechanical properties of the composites were studied in compression. Despite of presence of weak Cu fibers, compressive strengths of lO6OMpa, 699Mpa and 857Mpa for the composites reinforced with 40, 50 and 60 volume percent, which is higher than the high strength steel was found. Finally, the best processing parameters in terms of processing time and temperature are determined for the composite reinforced with 60 volume percent fibers. This volume fraction was chosen due to its uniform fiber distribution and hence distinct anisotropy. One promising application of this metallic glass matrix composite could be in the meso or microscale heat exchanger, where high heat transfer in one direction is required.
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