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Title: In-situ synchrotron X-ray imaging and tomography studies of the evolution of solidification microstructures under pulse electromagnetic fields
Author: Du, Wenjia
ISNI:       0000 0004 8499 2237
Awarding Body: University of Hull
Current Institution: University of Hull
Date of Award: 2018
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This research studies the dynamic evolution of dendritic structures and intermetallic phases of four Al based alloys during the solidification under pulse electromagnetic fields (PMFs). An advanced PMF solidification device was upgraded, built, commissioned for the research. The alloys used were Al-15Cu, Al-35Cu, Al-15Ni and Al-5Cu-1.5Fe-1Si. Systematic in-situ and real-time observation and studies were carried out at the TOMCAT beamline of Swiss Light Source, I13-2 beamline of Diamond Light Source and ID19 beamline of European Synchrotron Radiation Facility in the duration of this project. Synchrotron X-ray radiography and tomography were used primarily to observe and study the influence of PMFs on the nucleation and growth of primary dendritic structures and intermetallic phases under different magnetic flux and solidification conditions for the four alloys. More than 20 TB images and tomography datasets have been obtained throughout this research. Much effort and time was spent on segmenting, visualising and analysing these huge datasets using the Hull University supercomputer cluster, Viper, and the software, Avizo, ImageJ (Fiji), etc to explore and extract new insights and new science from those datasets. In particular, the skeletonisation function available from Avizo was customised and used to quantify the complex 3D microstructures and interconnected networks of different phases for the alloys. The important new findings of the research are: (1) Fragmentation of primary Al dendrites in the Al-15%Cu alloy was found when the magnetic flux of PMF applied is above 0.75 T; similarly, the fragmentation of Al3Ni intermetallic phases in the Al-15%Ni alloy was also observed when the magnetic flux of PMF applied is above 0.8 T. The clear and real-time observation of the fragmentation events in both dendritic and intermetallic phases provide unambiguous evidence to demonstrate that PMFs play a dominant role in structure fragmentation and multiplication, which is one important mechanism for structure (grain) refinement. (2) PMFs also produces pinch pressure gradient inside the semi-solid melt. Due to the different magnetic anisotropic properties between the liquid and solid phases, shear stresses due to the pinch pressure gradient may be produced. In the case of Al-15%Ni alloy, shear stresses of up to 30 MPa is created, which is sufficient to fracture Al3Ni phases. For the first time, such fragmentation mechanism for the Al3Ni phases in the Al-15%Ni alloy was revealed in this research. (3) The transition (or change of growth modes) of Al columnar dendrites to seaweed type dendrites in Al-15Cu alloy; and the facet growth to dendritic growth of the Al3Ni phases in the Al-15%Ni alloy were also observed in real-time when the magnetic flux is in the range of 0.75~0.8 T. Again, such dynamic changes in structure growth under PMFs are due to the enhanced melt flow caused by the applied fields. (4) In-situ tomography observation of PMF processing of the Al-5Cu-1.5Fe-1Si alloy also shows the effect of PMF on the refinement of the Chinese script type Fe intermetallic phases. In addition, the true 3D morphologies of three different types of Fe intermetallic phases in this alloy were clarified, again for the first time, in this research.
Supervisor: Mi, Jiawei Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering