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Title: In situ synchrotron tomography of granular deformation in semi-solid Al-Cu alloys
Author: Kareh, Kristina Maria
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Optimising casting routes involving semi-solid deformation such as semi-solid processing and high-pressure die casting requires a fundamental understanding of the globule-scale mechanisms behind the macroscopic rheological behaviour. This thesis uses time-resolved 3D imaging to directly observe and measure semi-solid alloy deformation from a microstructural perspective. Under isothermal conditions and constant strain rates, deformation mechanisms both at the crystal scale and at the specimen scale were identified during deformation in globular Al-Cu alloys at 64-93% solid. Imaging and quantifying these mechanisms has led to the emergence of a refined understanding of semi-solid deformation based on granular material concepts. It is shown that globularised crystals (above fs > 60%) exclusively rearrange as individual grains during parallel plate compression and backward extrusion at low strain rates. Crystal-crystal interactions are identified for the two loading modes and are not accompanied by plastic deformation of the individual crystals during acquisition. The ubiquitous grain rearrangement is coupled with shear-induced dilation of the solid assembly, whereby the crystals push each other apart in order to accommodate the increasing strain. It is also shown that, for specimens lacking a liquid reservoir, shear-induced dilation causes menisci to be pulled into the specimen from the surface and additionally, at high solid fractions, internal pores to grow. The origins of cracking during semi-solid processing are explored in a granular framework and linked to the shear-induced dilation associated with the solid assembly which increases the initial width of the liquid channels between the grains. Finally, the discrete grain analysis is coupled with the bulk mechanical results to explore the shape of the stress-strain curve and relate it to the imaged and quantified behaviours. All solid fractions tend to the same final stress, hinting at the possibility of a critical state analogous to that in soil mechanics, although fully testing this hypothesis requires varying the confining pressure.
Supervisor: Gourlay, Christopher M. ; Lee, Peter D. Sponsor: Norsk Hydro
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available