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Title: Micro- and macro-mechanical testing of grain boundary sliding in a Sn-Bi alloy
Author: Jiang, Junnan
ISNI:       0000 0004 7230 2585
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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This project explores the fundamental mechanisms of grain boundary sliding (GBS) with an emphasis on its role in superplasticity, using both micro- and macro-mechanical testing methods. GBS plays an important role in the deformation of polycrystalline materials, especially at high homologous temperatures (above half of the melting point). Classical models for GBS (Rachinger sliding and Lifshitz sliding) assume that all grains and grain boundaries undergo the same process, but recent research has shown this is not true. Individual grain boundaries differ in their ability to participate in sliding and diffusion. Therefore, it is important to investigate the response of individual grain boundaries to stress. This project uses microcantilevers, loaded using a nanoindenter, to investigate the response to stress of individual grain boundaries in Sn-1%Bi, which is expected to exhibit GBS at room temperature. The response of individual grain boundaries are correlated with grain boundary characters determined using electron backscattered diffraction (EBSD). On the macroscopic scale, both in-situ and ex-situ shear tests are conducted to investigate the superplastic behaviour of this material. The strain rate sensitivity index of the material with a grain size of 8.5 μm is found to be around 0.45. Surface marker lines have quantitatively revealed grain boundary sliding. The investigation from surface studies is expanded to the interior of bulk material in 3D by conducting an in-situ tensile test coupled with diffraction contrast tomography (DCT) at a synchrotron facility. The microcantilever tests enable grain boundary sliding and diffusion creep to be investigated separately by varying the normal and shear stresses on the grain boundary plane. GBS is dependent on grain boundary structure (misorientation angle, rotation axis and grain boundary plane orientation). The microcantilever size is similar to the grain size used in the macro-mechanical tests. It is demonstrated that the shear stress for steady-state GBS is comparable in micro- and macro-tests. Grain neighbour switching events have been identified in the interior of bulk material in 3D for the first time.
Supervisor: Wilkinson, Angus ; Todd, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Materials science ; Materials ; Grain boundary sliding ; Superplasticity ; Micromechanics