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Content archived on 2024-05-29

Advancing orientation and strain determination with high spatial resolution

Final Activity Report Summary - LOCALATT (Advancing orientation and strain determination with high spatial resolution)

The project deals with the determination of local crystalline lattice orientations and lattice distortions (strains). Both are important in characterisation of many (poly) crystalline materials. The arrangement of orientations of crystallites is an essential characteristic of a polycrystalline material. Determining this arrangement (or orientation topography in the form of orientation maps, or crystallographic texture) is essential for understanding material's properties. However, in many engineering materials, the size of the crystallites is small, and the determination of crystallite orientations requires application of special techniques. Most of them are based on electron diffraction because this technique allows for high spatial resolution. An important breakthrough in measurement of local orientations occurred in early nineties with the development of automatic orientation mapping systems based on scanning electron microscopy (SEM) and the so-called electron backscattered diffraction (EBSD).

Our work concerned a similar mapping system but based on transmission electron microscopy (TEM). Kikuchi diffraction patterns obtainable by TEM belong to the same family as EBSD patterns (K-line patterns), and it is natural to expand orientation mapping to TEM. Moreover, lateral resolution in TEM is considerably better than that of SEM, and transmission Kikuchi patterns allow for good accuracy in orientation determination. TEM based system has the advantage of being applicable to materials with ultra-fine grains. But even this approach fails in the case of highly deformed metals with orientation gradients because the in such ceases K-line diffraction patterns become diffuse. The problem can be partly solved if the so-called as microdiffraction spot patterns are used because their dependence on crystal orientation is weaker. Electron diffraction has been used for determination of crystal orientations for a long time, and the novelty comes with the automation of the analysis of diffraction patterns. With this project, a new automatic system has been implemented. It allows for using both Kikuchi or spot patterns. Automatically acquired patterns are solved by dedicated software, orientations are calculated, and then orientation based images of microstructures (orientation maps) are created.

Another important aspect of the project has to do with elastic deformations of crystalline lattices (strains). Understanding local strains locked in (poly)crystalline materials is essential for explaining and preventing failure of components. Strains are of interest for many branches of materials science. To give an example, let us mention that the most prominent area concerned with small sub-micrometer strains is microelectronics: on the one hand, strains in microelectronic devices cause formation of defects that in the end lead to malfunctioning of the devices but on the other hand, controlled strain can be used to affect the electronic band structure to increase carrier mobility (strained silicon technology). There are a number of methods of strain determination. We worked on a method utilizing TEM convergent beam electron diffraction (CBED). This technique has a very good spatial resolution but the application of the technique is difficult.

One of the obstacles is the ambiguity in strain determination. It can be partly overcome by using multiple patterns originating for the same location. Managing all aspects of the simultaneous analysis of multiple patterns is complicated, and we developed software facilitating such investigation. Another method of local strain determination which has been advanced within this project utilizes the so-called divergent beam X-ray diffraction (Kossel) patterns. This classical technique is being advanced using scanning electron microscopy (SEM) and digital image acquisition. Such an experimental set-up is under development at Paul-Verlaine Université in Metz. We worked on the computational aspects of strain determination. With the software developed within this project, more reliable strain results can be calculated from the CBED and Kossel diffraction patterns, and they are obtained faster and with less effort.