Texture is the orientation distribution of the single crystals forming the poly­crystalline aggregate. Due to the anisotropy of the single crystals, particularly the hexagonal alpha phase, texture is of great importance in the deformation behaviour of zirconium alloys. Therefore, understanding texture evolution during thermo­mechanical processing steps and service is a necessary precursor to predicting texture. Texture is affected by temperature [18, 26] and strain rate [27]. In situ diffraction is essential in understanding texture evolution.

Figure 4.9 shows an example of texture evolution in Zr-2.5Nb, with both tem­perature changes and deformation. This measurement was made on the HIPPO diffractometer [28, 29] at the Los Alamos Neutron Scattering Center (LANSCE) using a high-temperature deformation furnace [30]. The texture was measured at room temperature and at a number of steps up to 975 °C[ 31]. At this temperature the sample experienced a compressive strain of 20 %. Texture measurements were made at 975 °C, then at the same temperature steps during cooling to room temperature.

At room temperature the bcc beta phase is meta-stable and increases during heating at locations on the rim of the pole figure where the 0001 alpha pole-figure previously showed maxima, showing that alpha grains transform directly to beta grains [33]. At 975 °C only the bcc beta phase exists, at this point the sample had a strain of 20 %. The beta grains transform to alpha during cooling and (by the Burgers orientation relationship) the {0002} planes of the alpha phase become {110} planes of the beta phase, which is reflected in the texture evolution.

Pre and post treatment conditions of the sample are confirmed in Fig. 4.6a/b and e/f, respectively. The Burger orientation-relationship determines the transition of

(2) hcp/a to maxima in (110) bcc/p during a temperature increase to 975 °C. The resulting textures at high temperature are shown in (c). The (222) bcc/p planes align with the applied compression direction (d). To study the evolution of many properties such as texture, it is necessary to measure at intermediate steps, rather than just the start and end of processing.

Using Bragg-edge transmission [34] and neutron imaging in combination makes simultaneous mapping of the strains and texture (Fig. 4.10) [35] of the crystallites within the entire sample possible.