Exploring the use of synchrotron X-ray scattering methods for the detection of heterogeneous nucleation
thesisposted on 04.02.2015, 15:39 by Adam Jonathan Brown
The process of solidification is fundamental to a wide range of engineering processes. In most practical cases, such as in grain refinement of aluminium alloys, heterogeneous nucleation is the predominant route by which the transformation of liquid to solid is initiated. The mechanism by which solid crystal grows from heterogeneous particles is widely investigated but not fully understood. This study explores a novel approach to investigating nucleation through the use of synchrotron X-ray scattering techniques. The novel approach provides unprecedented access to the interactions at the solid-liquid interface and permits in-situ data collection as phase transformations proceed. Liquid aluminium was used as a relevant test case, and solid Al2O3 and TiB2 substrates are used to represent, respectively, a model case and industrially relevant scenario for grain refinement. Experimental results reveal the undercooling required for nucleation in the Al/Al[subscript 2]O[subscript 3] system. They also provide detail on thermal expansion in the aluminium prior to melting. Crystal Truncation Rod analysis reveals the existence of a ‘transition layer’ between Al[subscript 2]O[subscript 3] and Al, which is suggested to arise to accommodate residual strain in the solidified aluminium. For the Al/TiB[subscript 2] system, an innovative sample preparation method facilitated investigation of the interface between the materials, providing an experimental model of a TiB[subscript 2] particle within an aluminium melt. Results indicate the proclivity for the formation of Al[subscript 3]Ti in the system, and lead to the proposal that this is likely to occur even in situations in which the assumed Ti content suggests otherwise.