4 IN-SITU TEXTURE MEASUREMENT
4.2 TEXTURE EVOLUTION DURING THE PHASE TRANSFORMATION
The series of pole figures in Figure 4.16 illustrates the most significant changes of crystallographic orientation occurring during a complete transformation. The pole figures shown are the basal
111 for the -phase. These sets of pole figures are representative of the texture for each phase, and can be directly associated with the Burgers orientation relationship. The pole figures shown in Figure 4.16 correspond to thermomechanical cycle T3 of the list shown in Table 4.1, and are typical for most of the tests carried out.4.2.1 RECRYSTALLIZATION
The strong rolling texture observed in the starting material, shown in Figure 4.16(a), remains practically constant from room temperature to approximately 620C. At this temperature, the diffraction images start to change: the intensities along each ring start to scatter, evidencing alterations in the microstructure. The texture begins to change and stabilises again at approximately 730C, remaining unchanged until the onset of the
phase transformation at ~810C. Figure 4.16(b) illustrates the texture at 795C.
Figure 4.16 and pole figures obtained in-situ using SXRD during a complete phase transformation: (a) cold-rolled, (b) recrystallized, before the start of the phase transformation, (c) 46% and 54% , (d) at the end of the phase transformation ~98% , (e) 100% before grain
growth, (f) 15% and 85% , (g) 85% and 15% , (h) after -quenching
Texture Evolution during -quenching of a Zirconium Alloy 159
2004, Lonardelli et al. 2007, Tenckhoff 1988, Wagner et al. 2002). This characteristic crystallographic rotation has been attributed to primary recrystallization and subsequent grain growth. Some differences have been observed between the present study and previous work, the most important being the behaviour of the basal poles. It has been reported that the
0002
maxima move towards ND during annealing (Tenckhoff 1970), but these results show that the splitting towards TD occurs during annealing.4.2.2 PHASE TRANSFORMATION
The phase transformation starts at 810C and finishes at approximately 940C.
Figure 4.16(c) shows pole figures corresponding to an intermediate step of the phase transformation at 845C, where the volume fraction of and is similar. At 935C, the transformation is almost complete, illustrated by the pole figures in Figure 4.16(d). At this point, the Rietveld analysis estimates 2% volume fraction of -phase remaining.
Previous work on CP titanium (Lonardelli et al. 2007) has shown that, during transformation on heating, the
112 0
pole density parallel to RD is significantly reduced, which was attributed to preferential transformation of the recrystallizationtexture component. The justification given by Lonardelli et al. is that the grains with this orientation are more prone to twinning, and nuclei are formed mainly in these highly deformed regions. In the synchrotron experiment presented here, however, the texture does not change considerably at least during first half of the transformation, Figure 4.16(b-d). The pole intensity of the recrystallization texture component appears to increase slightly only in Figure 4.16(c), when the fraction of -phase has been reduced to ~15%. This would suggest that if there is preferential transformation, it occurs in grains that do not belong to the principal texture component. Figure 4.17 illustrates how the texture index does not vary significantly during most of the
transformation.
Figure 4.17 Evolution of texture indices during the transformation (Romero et al. 2009).
The succession of pole figures in Figure 4.16(c-e) displays slightly increasing pole intensities with temperature. In Figure 4.17 the texture index increases when the
Texture Evolution during -quenching of a Zirconium Alloy 161 volume fraction exceeds 50%, before dramatically increasing when the phase transformation is close to completion. This evidence indicates strengthening of the texture during heating. Strengthening of the texture was seen previously in CP titanium (Lonardelli et al. 2007), being associated to competitive growth between grains. In-situ SEM and EBSD studies in CP titanium rolled sheets (Seward et al. 2004) have shown that in the very early stages of the transformation there is simultaneous nucleation of phase not only at the grain boundaries as allotriomorphs, but also within the grains as plates. Seward et al. reported that grain boundary tends to maintain the Burgers orientation relationship with one of the parent grains, whereas the orientation distribution of intragranular is more random. They suggest that the final stage of the transformation consists of a competitive growth between these two types of nuclei, being dominated by the grain boundary , since it has faster moving interfaces. If one assumes that the transformation in cold-rolled sheets of Zircaloy-2 occurs in a similar manner, the slight strengthening of the texture on heating can be explained by preferential growth of variants that have nucleated in the grain boundaries.
4.2.3 TEXTURE
The phase transformation is complete at 945C, when only reflections are detected in the diffraction image and a significant microstructural change is evidenced, probably the first stages of grain growth. The scattered intensities along the diffraction rings at 945C and 950C (see Figure 4.8), evidence extremely rapid grain
growth once the -transus has been exceeded. The texture observed at the end of the
transformation is not particularly strong, with the pole intensities ranging from 0.5 to 2 times random and a texture index of only 1.85. The texture measured at 950C shows sharper pole intensities, but since it comes from a highly scattered diffraction image, it is difficult to differentiate between genuine texture strengthening and an insufficient number of sampled grains. The most relevant feature of the high four maxima surrounding ND, two tilted ~60 towards RD and two tilted ~30 towards TD, the latter resembling the split maxima of the
0002
pole figure of the recrystallization texture before the transformation.The texture observed here is similar to that found during in-situ texture measurements in zirconium and CP titanium reported in the literature. Wenk et al. (Wenk et al. 2004) carried out in-situ texture measurements during the phase transformation of 6 x 6 x 6 mm3 cubes of hot-rolled Zircaloy-4, while Lonardelli et al. (Lonardelli et al. 2007) studied 5 x 5 x 5 mm3 cubes of cold-rolled CP titanium. These experiments were carried out in a vacuum furnace with heating and cooling rates about 20C/min, using neutron diffraction. Their samples were kept at different steps of temperature for about one hour, while the temperature stabilised and all the rotations for texture measurement were performed. Figure 4.18 compares the pole figures obtained in the SXRD experiment in this work, with those obtained by Wenk et al. using neutron TOF diffraction in Zircaloy-4. The -transus temperature for Zircaloy-4 is approximately
Texture Evolution during -quenching of a Zirconium Alloy 163 970C, so in Figure 4.18(a) the transformation is incomplete, while in Figure 4.18(b) the transformation is certainly complete, even with some evidence of grain growth.
The origin of the texture reported by Wenk et al. and Lonardelli et al. was attributed to preferential transformation of the recrystallization orientations. As mentioned in the previous section, no evidence of preferential transformation was observed during the
transformation in this work.
Figure 4.18 Comparison between pole figures: (a) Zircaloy-4 at 950C obtained using neutron diffraction (Wenk et al. 2004), (b) Zircaloy-2 at 950C obtained using SXRD in this work. Note the
logarithmic scale used.
4.2.4 PHASE TRANSFORMATION
In most of the tests carried out during the synchrotron experiment, the transformation upon cooling occurred very quickly. However, it is clear that even the initial fractions of
-phase produce pole figures with all the features of the final texture. The succession of
pole figures in Figure 4.16(f-h) does not show significant changes from the initial volume fraction (~15%), to the final texture at room temperature when the
transformation is complete. The initial rolling texture is significantly weakened by the complete heat treatment. The starting texture index is 4.09. It drops to 2.89 after annealing and to 1.65 once the material has cooled down from above the -transus.
The main features seen in the pole figures after -quenching are still the
significantly decreased intensities. New pole intensities are observed, that were not present before the transformation. These new texture components and the presence of variant selection will be discussed in detail in Chapter 7. The pole figures in the transformed condition are very similar to those found by Gey et al. from cold-rolled and fully recrystallized CP titanium sheets after a complete phase transformation (Gey and Humbert 2002).