Development of Al-Si based alloys

Abstract

The development of Al-based metallic glasses capable of easily generating an amorphous structure on cooling from the melt is recognized as one of the major challenges in this field of research. In the past, the Al-based amorphous alloys could be categorized into four types: (1) Al-LTM-RE; (2) Al- LTM- ETM; (3) Al-LTMAEM and (4) Al-TM-Metalloid. Recent findings in Al-rich Al-Ni-Co-La-Y alloys, where a critical casting diameter of ~ 1mm has been reported, has rekindled interest in the possibility of producing Al-based bulk metallic glasses (BMGs). The research outlined in this thesis is focused on the design of Al-Si-based metallic glasses without rare earth elemental additions but having good glass-forming ability (GFA). Based on the underlying thermodynamic and kinetic factors that dictate the retention of a glassy structure in an alloy melt upon cooling, three fundamental concepts were simultaneously adopted in order to identify alloy compositions with the highest propensity for glass-formation in a range of Al-Si-based alloy systems. These incorporate the heat of mixing between alloying elements, the thermodynamic driving force for crystallization within an alloy system and the efficient packing of atoms for maximizing alloy melt viscosity, thereby slow down the kinetics of crystallisation. With this in mind, transition metals Cr, Mo, Mn, Fe, Co, and Ni, were selected as viable additions to the binary Al-Si alloy system.

Compositions that satisfy these criteria were produced by arc-melting and vacuum casting into wedge-shaped copper moulds in order to examine their GFA. The microstructure and morphology of the as-cast alloys were investigated mainly using electron microscopy and x-ray diffraction techniques. Results showed that glass formation is possible within the systems studied, with alloys in the Al-Ni-Si and Al-Co-Si systems forming glasses with a critical casting size of up to 400 um and 195 um, respectively. The GFA of the alloy systems studied in the thesis were found to follow the sequence: Al-Ni-Si > Al-Co-Si > Al-Cr-Si > Al-Mo-Si > Al-Mn-Si > Al-Fe-Si. The results indicate that the nature of the electronic bonding between Al and the transition metal elements strongly affects GFA of a given alloy system and that atomic packing efficiency also contributes to maximizing glass formation. The optimized compositions in the Al-Ni-Si alloys contained three types of efficiently packed clusters, which suggested a self-consistence principle that the compositions with all elements efficiently packed by their first-shell atoms are crystallization resistant and more likely to form a glass. Wider featureless regions were observed in the Al-rich lower liquidus temperature regions, indicating a lower crystallization driving force, which is a positive factor in potential glass formation. By carefully design and synthesis, the GFA in the Al-Si based alloys was explored, providing an insight towards the development of Al-based alloys free of rare-earth elements.