CHARACTERISING MICROTEXTURE USING EBSD

Different FEGSEMs were employed to collect the results presented in this chapter: a Zeiss SUPRA 40, a CamScan 2040S, a Philips Sirion and a Philips XL30. All of them were equipped with HKL EBSD acquisition systems. The Channel 5 software suite (HKL Technology 2006) was used to post-process and analyse the data.

Selected samples were prepared as described in section 3.5. The typical EBSD acquisition parameters are listed Table 2.1. With this sample preparation and acquisition parameters the indexing rate obtained was in most cases above 80%, with some specific maps with indexing between 70% and 80%. The acquisition rate achieved ranged between 0.1 and 0.5 seconds per measuring point. Unless stated otherwise, all the maps were acquired using a relatively small step size, between 0.5 and 2m, enough to be able to identify the  variants (colonies) in the microstructure. In some cases, the step size was small enough to see individual  lamellas in band contrast maps.

Table 5.1 Typical EBSD acquisition parameters

All the maps were subjected to a systematic post-processing procedure, aimed to reduce the noise and eliminate effects such as pseudosymmetry (described in section 1.2.4).

The post-processing procedure is outlined below, and illustrated in Figure 5.1(a-c). In Figure 5.1(a-c), the non-indexed points are white, and the misorientations corresponding to systematic misindexing of hexagonal phases, i.e. 30 rotations about the 



c axis, are 

delineated in black.

Texture Evolution during -quenching of a Zirconium Alloy 181 Figure 5.1 EBSD data post-processing: (a) raw data (IPF colouring), (b) after wild spikes correction and

extrapolation, (c) after systematic misindexing correction, (d) band contrast map after  data correction showing probable prior  grain boundaries, (e) result of  reconstruction algorithm (IPF colouring), (f)

extrapolation of  reconstruction

1. Correction of wild spikes: isolated points are removed.

2. Extrapolation: non-indexed points are assigned a value of orientation, extrapolated from the values of adjacent points. The extrapolation is carried out in steps, using data from 8, 7, 6 and 5 adjacent points successively. After this step for most of the maps, the percentage of non-indexed points was below 2%. A final extrapolation using 4 neighbours was applied if necessary. Extrapolation was used with caution, since large portions of non-indexed points can indicate the presence of unknown phases or contamination on the surface.

3. Remove systematic misindexing: misindexed points caused by pseudosymmetry are corrected, i.e. rotated 30 around the 



c axis. 

4. Remove extremely small clusters: clusters of points with areas equal or smaller than four pixels and that are detected as grains, i.e. having a misorientation above 3, are removed and then assigned a value of orientation by extrapolation.

After reducing the noise, it is possible to have an idea of the prior  grain size and shape by displaying certain misorientations in the  orientation maps. As mentioned in section 2.2.4,  variants belonging to the same parent  grain have characteristic misorientations. Any other misorientation between identified  variants would constitute a prior  grain boundary. In Figure 5.1(d), the misorientations between  colonies that probably constitute prior  grain boundaries are delineated in black, while boundaries between  variants having a misorientation below 3 are delineated in yellow.

In order to know the actual orientation of the prior  grains, the  reconstruction algorithm developed by Davies et al. (Davies et al. 2007) (described in detail in section 2.2.4) was used in some  maps. The resultant  orientation maps were corrected

Texture Evolution during -quenching of a Zirconium Alloy 183 following the steps 1 and 2 of the procedure above. This is exemplified in Figure 5.1(e-f). It is important to notice that the amount of unsolved points in the raw  reconstructed maps sometimes reached 40%, thus in some cases even after extrapolation there were still areas without precise  orientation information.

The results of EBSD mapping and  reconstruction on selected samples will be presented in the following sections. The presentation of the results will consist of several orientation maps and pole figures, described as follows:

1. Inverse pole figure (IPF) map. In this map, the colouring corresponds to the orientation of each measurement point in an inverse pole figure of a selected sample direction. The sample directions selected are shown in the legend of each map.

2. Band-contrast map including probable prior  grain boundaries and misindexing boundaries. In this map, the greyscale represents the quality of the electron backscatter pattern (EBSP) for each point. The lighter the shade of grey, the higher the band contrast and hence the EBSP quality. Band contrast maps reveal microstructural features such as grain boundaries and surface imperfections. The prior  grain boundaries (delineated in black) correspond to boundaries between  variants whose misorientation are above 3, and are not within 2.5 of the characteristic misorientation of variants belonging to the same parent  grain.

Boundaries between  variants with misorientation between 3 and 8 , are delineated in bright blue. Probable misindexing boundaries are delineated in dark blue.

3.  reconstruction map using IPF colouring and including  grain boundaries. This map is the result of applying the  reconstruction algorithm. The colouring corresponds to an inverse pole figure for a cubic crystal in the selected sample direction. High-angle (black) and low-angle (grey) grain boundaries are included, with limit values of 10 and 1 respectively.

4. Pole figures obtained from the  and  orientation maps. All the EBSD pole figures presented in this chapter correspond to recalculated pole figures, from ODFs calculated using the harmonic method as implemented in TEXTAN III (Bate 1990).

Orthotropic sample symmetry was applied. The  pole figures are those obtained directly from reconstructed data, before applying any extrapolation.

For comparison purposes all the EBSD pole figures obtained from the samples studied in this chapter are compiled in Figure 5.2 () and Figure 5.3 (). These figures will be discussed later.

5.2 LOAD-FREE TRANSFORMATIONS. THE EFFECT OF THE