Electron back-scatter diffraction (EBSD) is a scanning electron microscopy (SEM) technique to measure the orientation of grains (texture) with a size down to 0.5 µm. This technique has been widely applied to cubic metals; the application to non-metallic materials such as ceramics and minerals, is a new but rapidly expanding field. With the EBSD technique textures can be measured with reference to particular grains, and it enables one to obtain a so-called microtexture, which is a preferred orientation on a scale which is consistent with a single unit of orientation, i.e. a grain or a subgrain. The measurement of the microtexture enables to be determined of the misorientation relationships between two adjacent grains or subgrains; such relationships are referred to as the mesotexture. In addition, orientation imaging in Back-Scattered-Electron mode is possible by mounting the back-scattered electron detector underneath the EBSD detector.
When a material deforms, all of the microstructure, texture and mesotexture that will develop will depend on the operative deformation and recrystallisation mechanisms. The texture and mesotexture of two different samples have been investigated: one has been deformed in the diffusion-creep regime (deformation accommodated by grain boundary sliding) and one in the dislocation-creep regime. For comparison, some naturally deformed lherzolites have also been studied. The texture of the sample deformed in the diffusion-creep field is fairly uniform for all the major zones axes (Fig. 3.1-8b). The distribution of angles of misorientation resembles that of a statistically random case (Fig. 3.1-8a). The dislocation-creep sample shows a stronger texture (Fig. 3.1-8d). There are more low and intermediate angle boundaries (< 40° ) and fewer intermediate to higher angle (40° - 70° ) boundaries when compared to the random case (Fig. 3.1-8c). The very high angle boundaries occur as often as in the random case. Avé Lallement (1985, Tectonophysics, 119, 89-117) measured the mesotexture in a naturally deformed lherzolite nodule from Dreiser Weiher, Germany. The angular relations have been reproduced in Fig. 3.1-8e. One can clearly see that there are many more low to intermediate angle boundaries than expected in random case. The olivines recrystallized by a subgrain rotation mechanism.
It is therefore concluded that both recrystallisation and deformation
mechanisms have an effect on the texture, the microstructure as well as
on the mesotexture of olivine polycrystals. Both the texture and mesotexture
which develop in olivine aggregates deforming by grain-size-sensitive processes
are similar to those of a statistically random case. Dislocation creep
processes on the other hand commonly result in (strong) preferred orientations
of the mineral grains. The mesotexture developed in such materials deforming
by dislocation-creep processes depends largely on the accompanying recrystallisation
mechanism. In the case of rotation recrystallisation followed by growth,
many boundaries with low to intermediate angle misfits are expected with
the misorientation axes close to low index zone axes. In the case of nucleation
and growth mechanisms, orientation relations are expected, but which remains
the subject of further studies.
Fig. 3.1-8: c-axis texture and grain boundary populations within (a-d) experimentally deformed olivine aggregates and (e) naturally deformed lherzolite. (a, b) Olivine deformed by diffusion-creep showing a weak texture and a weak mesotexture. (c, d) Olivine deformed by dislocation-creep, showing a stronger texture and more low to intermediate angle boundaries. (e) Mesotexture of naturally deformed Dreiser Weiher Lherzolite showing many low to intermediate angle boundaries (after Avé Lallement, 1985).