Understanding the plastic deformation mechanisms in polycrystals is a long-standing fundamental problem and its improvement has significant potential impact on the increase in materials resistance to typical failure modes such as fatigue cracking and stress corrosion cracking and hence the increase in the materials strength. However many deformation models are yet to be validated as quantitative experimental results at mesoscale to correlate dislocations and microstructure features are limited. This thesis furthers the High Resolution EBSD (HR-EBSD) technique in Geometrically Necessary Dislocation (GND) density measurement from qualitative analysis with a typical map size of 100 μm x100 μm to quantitative analysis with a map of 500 μm x500 μm by determining the optimised scanning step size (0.5 μm) and detector binning level (4x4 binning). This allows a statistically large number of grains to be sampled. Combining with obtained crystallographical information from a conventional EBSD system, systematic studies on GNDs behaviours with respect to a range of microstructure features such as grain boundaries and triple junctions were conducted on monotonically deformed polycrystal copper samples under tension. Relatively high GND density points were found near triple junctions and some grain boundaries whereas the low GND density points tend to appear near the grains’ interiors. These tendencies are particularly profound in low and moderately deformed samples. Hence more detailed analyses were performed to investigate the relations of GND density and the properties of grain boundaries and triple junctions. These quantitative analyses were complemented with direct visual assessment. The visual inspection provides interesting findings such as the strong GND structure dependence on grain orientations and GND structure development through increasing deformation; grain-grain interaction influences on GND structure development and GND structures near triple junctions. These GND density studies provide experimental results to validate some of the existing plastic deformation models for instance Ashby’s model of hardening and Hall-Petch relation. However, some of the new observations on GND structures at mesoscale cannot be fully rationalised by existing proposed mechanisms. Hence new models have been proposed that these GND structures might be generated from the intersections of different slip systems which occurred in various parts of a grain, or by the dislocation piling-up at some microstructural features e.g. triple junctions and twin boundaries.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:644893 |
| Date | January 2013 |
| Creators | Jiang, Jun |
| Contributors | Wilkinson, Angus |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:31926294-d734-42f1-8b26-cbbb56438219 |
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