Solidification: Real-time investigation of grain nucleation and growth during liquid to solid phase transformation of aluminum alloys

Aluminum, the most common metallic element in the earth's crust, offers structural materials with high strength and light weight. It is widely used throughout the world economy, particularly in the transportation, packaging, and construction industries. Once extracted from ore, solidification is the most important phase transformation in the development of aluminum products, especially for critical applications. All of these applications rely on the ability of aluminum alloys to form a homogenous granular structure with a relatively small grain size, known as grain refinement. This thesis involves the use of in-situ experimental techniques like neutron scattering, synchrotron radiation, and differential thermal analysis to investigate the mechanism of nucleation and subsequent growth of individual grains during solidification. The aluminium alloys contained grain refiners like TiB2 particles along with solute titanium. The results indicate that the grain refiners have little influence on the overall short-range order in the liquid aluminum alloy prior to the solidification. Grain refinement is only effective when both solute titanium and TiB2 particles are present in the melt during solidification. The results shows that the TiB2 substrates stabilise a TiAl3 phase in a limited temperature range above the solidification temperature of aluminum. This observation pinpoints the highly debated mechanism that is responsible for enhanced grain nucleation during solidification of grain refined aluminum alloys. The results further demonstrate that the nucleation process is limited to the initial stage of the solidification and is complete at a solid phase fraction of about 20% for all the investigated aluminum alloys. The growth curves of the individual aluminum grains exhibit three different stages during solidification. The first stage of grain growth curve is consistent with the model prediction for diffusion-controlled growth of non-interacting grains. The later stages can be explained in terms of a release of latent heat and to a minor extend by the indirect interaction of the growing grains via overlapping diffusion fields