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Abstract
Al-based amorphous alloys are an interesting class of material as they often exhibit very high specific strengths and good wear and corrosion resistance. A major recent challenge is the production of these materials free of early transition metals and rare earth elements, without degrading their glass-forming ability (GFA). In this thesis, new multi-component Al-base alloys were designed using an atomic packing efficiency model in conjunction with the knowledge of the liquidus surface of the ternary systems. Two groups of alloys were investigated: (i) solvent-lean (Al < 80 at.%) Al-Cu-Mg-based alloys containing combinations of Ag, Zn and Ni; and (ii) solvent-rich (Al > 80 at.%) Al-Ca-based alloys containing combinations of Ni, Co, Cu, Ga, Ag and In. The designed alloys were subsequently cast into copper moulds or melt spun and investigated by optical and scanning electron microscopy, differential scanning calorimetry and x-ray diffraction. Based on atomic packing efficiency calculations, Al-Cu-Mg ternary alloys with the highest GFA were those compositions associated with 13-atom clusters, and these were located either near a ternary eutectic point or on a liquidus line of the liquidus projection, that is, Al69Cu23Mg8 and Al77Cu15Mg8 alloys, as confirmed by experiment. For the Al-Ca-Ni ternary alloys, atomic packing efficiency calculations indicated that the best glass-forming alloy is Al86.8Ni7.7Ca5.6, also confirmed by experiment. However, the addition of cobalt to the Al-Ca system resulted in poor GFA. The addition of up to three extra elements to the good glass forming Al-Cu-Mg and Al-Ca-Ni systems did not enhance glass formation; this was argued to be due to changes in atomic packing efficiency and enthalpy of mixing due to the various atom-atom combinations. The thermal stability of amorphous ribbons of Al-Cu-Mg-Ni alloys was also carried out. The alloy with the highest stability against crystallization was Al71.25Cu16.15Mg7.6Ni5. The Avrami exponent computed using the Mehl-Avrami-Kolgomorov model was slightly greater than 2.5 over a range of temperatures, thereby indicating that crystallization occurs in three dimensions, and governed by an essentially constant nucleation rate and diffusion-controlled growth. The overall activation energy for crystallization was found to be higher than self-diffusion of aluminium in a crystalline lattice.