Samenvatting
Crystalline materials are found in most components and devices used in the world today. Mechanical reliability, environmental concerns and cost reduction are major factors which require advanced characterization and optimization of the 3D crystalline microstructures. Specifically, the 3D (residual) stress state in the material can be of huge importance, yet, efficient measurement techniques that can capture this on a relatively large scale with high resolution are lacking. Thisthesis focuses on the advancement of two state of the art methodologies, that can potentially, when combined, lead to a novel large volume high resolution 3D (residual) stress measurement technique. First, a novel framework is introduced to perform relative micron-scale stress (or strain) measurements in (poly-)crystalline materials. This is enabled by development of a general, transparent, full-field finite-strain, Integrated Digital Image Correlation (IDIC) framework for High angular Resolution Electron BackScatter Diffraction (HR-EBSD). This results in a direct one-step correlation of the full field-of-view of EBSD patterns, thereby achieving strain and rotation component errors that are, on average, well below 10−5 at strains below Eeq = 0.2%, for large rotations up to 10◦ and 2% image noise. The high accuracy and robustness presented here rivals the most accurate HR-EBSD algorithms currently available, which combine complicated filtering and remapping strategies with an indirect two-step correlation approach of local subset regions of interest.More excitingly, the framework is extended to allow correlation of multiple EBSD patterns, from different grains with high misorientations, in one optimization step, resulting in an intergranular, cross-grain correlation with respect to one reference point in the entire polycrystalline microstructure. Validation on a challenging case-study of 6 highly misoriented, deformed, grains shows that
the use of all 15 unique pattern overlaps in one optimization step results in accuracies well below 10−4 in strain, even at high noise levels of up to 20%. This intergranular HR-EBSD approach has not been attempted, nor even suggested in literature, and will open up new areas in research of e.g. grain and phase boundaries in a variety of materials.
Finally, accurate large volume 3D serial sectioning is proposed by implementation of a Broad Ion Beam gun (BIB) inside a Scanning Electron Microscope (SEM) equipped with EBSD, to repeatedly allow careful removal of thin material layers, after which the pristine surface can be scanned by (HR-)EBSD, eventually allowing accurate 3D (stress) tomography in a static setup. A preliminary setup design is presented and implemented, and supported by additional measurements, to lay the foundations for future finalization of a fully functional, automated, in-situ BIB based static 3D-(HR-)EBSD system.
| Datum prijs | 1 mei 2018 |
|---|---|
| Originele taal | Engels |
| Begeleider | Johan P.M. Hoefnagels (Afstudeerdocent 1) & Marc P.F.H.L. van Maris (Afstudeerdocent 2) |