5 MICROTEXTURE CHARACTERISATION
5.2 LOAD-FREE TRANSFORMATIONS. THE EFFECT OF THE PEAK TEMPERATURE ON THE MICROTEXTURE
The first scenario studied during the in-situ experiment was -quenching without applied stress. Figure 5.4 shows results of EBSD mapping in sample T1, -quenched at 1000C during 10 seconds. This map was taken at the location of the thermocouple, which is, approximately, where the synchrotron X-ray beam irradiated the sample during the in-situ experiment. It is important to notice the directions chosen for each IPF map. In Figure 5.4(a), the colouring of the -phase corresponds to an inverse pole figure of ND, which is aimed to illustrate the deviation of the inherited texture with respect to the initial strong rolling texture. In a map of this type, the initial rolling texture is dominated by red colour since the basal planes are very close to ND. In the map in Figure 5.4(a), the distribution of the colouring does not show any particular trend, indicating that the rolling texture has been weakened.
Texture Evolution during -quenching of a Zirconium Alloy 185 Figure 5.2 Pole figures corresponding to EBSD maps of samples -quenched in the SXRD experiment:
(a) T1, (b) T3, (c) S1, (d) S2, (e) S3 at the fracture tip and (f) S3 ~4mm away from the fracture
Figure 5.3 Pole figures corresponding to reconstruction maps calculated from EBSD data of samples
-quenched in the SXRD experiment: (a) T1, (b) T3, (c) S1, (d) S2, (e) S3 at the fracture tip and (f) S3
~4mm away from the fracture
Texture Evolution during -quenching of a Zirconium Alloy 187 Despite the fact that the global texture has been weakened by -quenching, in the EBSD maps there is evidence of variant selection and local texture memory. The regions encircled in white in Figure 5.4 show prior / grain boundaries that produced variants with low misorientation between them (<8), and with crystallographic orientations very close to that before -quenching, i.e. with their
0002
poles near ND.The grains corresponding to these boundaries share a common
110 pole, and the variants produced have their
0002
pole parallel to that common
110 pole. This mechanism of variant selection has been observed previously (Stanford and Bate 2004, Bhattacharyya et al. 2007), and will be discussed in detail in Chapter 7.The special / grain boundaries are related not only to the local replication of texture components present before -quenching, but also to the local strengthening of others, as illustrated by the yellow circle towards the left hand side in the maps in Figure 5.4. In this particular example a new texture component (close to
g3), dominates the region.
This evidence indicates that mechanisms of local variant selection at special / grain boundaries could, at least partially, explain the appearance and strength of the new texture components observed in the -quenched condition.
The IPF colouring of the -phase map in Figure 5.4(c) corresponds to RD, aiming to illustrate the strong alignment of the
111 poles parallel to RD by a predominant blue colour. RD is used as the reference direction for all of the -phase EBSD maps presented in this chapter. The reconstruction algorithm worked relatively well,achieving just below 70% of area reconstructed. In order to produce the map shown in Figure 5.4(c) the missing areas could be extrapolated easily.
The band contrast map and the reconstruction map of sample T1, shown in Figure 5.4(b-c), show a prior grain size of ~150-160 m, and in the area scanned there were approximately 140 grains. According to the literature (Bozzolo et al. 2007) this would be enough to determine the features of the texture with some uncertainty in the densities of the orientation distribution.
The results of EBSD mapping in sample T3, heat treated at 950C for 3 seconds, are shown in Figure 5.5. The inherited texture in this sample is weaker than that found in sample T1, evidenced by lower pole intensities in Figure 5.2(b) than in Figure 5.2(a).
Due to the resolution of the maps, evidence of local variant selection at special /
grain boundaries cannot be pointed as clearly as in sample T1.
In the maps obtained from sample T3 shown in Figure 5.5(b-c), the prior grain size observed was approximately ~70-80 m, and the -reconstruction map does not appear as dominated by the blue colour as that from sample T1. All this evidence suggests that the limited -grain growth in sample T3 is related to a weaker -texture, confirmed by comparing the pole figures in Figure 5.3(a) and Figure 5.3 (b), which in turn is related to a weaker inherited texture. The differences in texture with respect to sample T1 lie in the intensity of the two pole maxima corresponding to texture components
g1 and g2.
Texture Evolution during -quenching of a Zirconium Alloy 189 Figure 5.4 EBSD maps from sample T1: (a) IPF map of -phase (ND), (b) band contrast map showing
probable prior grain boundaries, (c) IPF map of -phase reconstruction (RD).
Figure 5.5 EBSD maps from sample T3: (a) IPF map of α-phase (ND), (b) band contrast map showing probable prior β grain boundaries, (c) IPF map of β-phase reconstruction (RD)
Texture Evolution during -quenching of a Zirconium Alloy 191 5.2.1 RECONSTRUCTED BETA TEXTURE IN LOAD-FREE SAMPLES
One of the limitations of texture measurement using SXRD was that, due to accelerated
grain growth after the phase transformation is complete, it was not possible to follow the evolution of the texture beyond this point, i.e. during grain growth. In order to investigate the evolution of the -phase texture once the phase transformation is complete, the texture just before the start of the phase transformation was reconstructed using the variant-based reconstruction technique (Davies et al. 2007) as explained in section 2.2.4, and compared to the texture obtained at the end of the phase transformation.
Figure 5.6 shows three different sets of pole figures illustrating the evolution of the texture in sample T1. Figure 5.6(a-b) were obtained in-situ using SXRD at the end of the phase transformation, while Figure 5.6(c) was obtained reconstructing the texture from the map shown in Figure 5.4. All the features are similar, but the intensities are significantly higher in the pole figures from reconstructed EBSD data. If one assumes that these changes in intensity are not caused by poor grain statistics, then this result suggests that the texture of the phase changes after the transformation is complete. This is probably due to some kind of preferential growth that strengthens the main texture components.
Figure 5.6 Comparison between pole figures obtained using reconstructed EBSD maps and SXRD