For seamless tube production, first a hot extrusion is performed in the temperature range of 600-700 °C. For pressure tube fabrication, this step is followed by a single cold drawing step and a final stress relieving heat treatment. For cladding tubes, the extrusion produces a large extruded tube (‘Trex’ or ‘shell’), of 50-80 mm in diameter and 15-20 mm in thickness, which is further reduced in size by cold rolling on pilger-rolling mills.

After each cold working step of plate or tube material, an annealing treatment is mandatory to restore ductility. It is usually performed in the range of 530-600 °C to obtain the fully recrystallized material (RX). The resultant microstructure is an equiaxed geometry of the Zr grains with the precipi­tates located at the a-grain boundaries or within the grains. The location of the precipitates at the grain boundaries is not due to intergranular precipitation but because they pin the grain boundaries during grain growth (Figure 9). These different heat treat­ments contribute to the control of the cumulative annealing parameter to be described below. For better mechanical properties of the final product, the tem­perature of the last annealing treatment can be reduced to avoid complete recrystallization. This is the stress-relieved (SR) state, obtained with final heat treatment temperature of 475 °C, which is character­ized by elongated grains and a high density of dis­locations, and by relief of the internal stresses, leading to a greater ductility than cold-worked mate­rials. It is mostly applied to the PWR claddings, while for BWRs, a complete recrystallization is performed at 550-570 °C.

2.07.3.4.1 Crystallographic texture development

Two plastic deformation mechanisms are operating during low temperature deformation of the Zr alloys: dislocation slip and twinning. As reviewed by Tenckhoff,14 the most active deformation mechanism depends on the relative orientation of the grain in the stress field.

Dislocation slip occurs mostly on prism plane with an a Burgers vector. It is referred to as the {1010} (1210), or prismatic, system. The total strain imposed during mechanical processing of the Zr alloys cannot, however, be accounted for only with this single type of slip, as the different orientations of the crystal would only give two independent shear systems. At high deformations, and as the temperature is increased, (c + a) type slip is activated on {1121} or {1011} planes. These are the pyramidal slip systems, having higher resolved shear stresses (Figure 2).

Different twinning systems may be activated depending on the stress state: for tensile stress in the c-direction, {1012} (1011) twins are the most frequent, while the {1122} (Ї123) system is observed when compression is applied in the c-direction. The resolved shear stresses of the twin systems have been shown to be higher than the one necessary for slip, but due to the dependence of the Schmid factor on orientation, twinning is activated before slip, for some well-oriented grains. Therefore, there are five independent deformation mechanisms operating in each grain, and thus the von Mises criterion for grain-to-grain strain compatibility is fulfilled.

At the large strains obtained during mechanical processing, steady-state interactions occur between the twin and slip systems that tend to align the basal planes parallel to the direction of the main deformation.15,16 For cold-rolled materials (sheets or tubes), the textures are such that the majority of the

Figure 2 The two Burgers vectors (a and c + a) for strain dislocations in Zr alloys, and the two slip planes (prismatic and pyramidal) in hcp a-Zr.

grains have their c-axis tilted 30-40° away from the normal of the foil or of the tube surface toward the tangential direction, as can be seen in the (0001) pole figure of a cladding tube (Figure 3).

During tube rolling, the spread of the texture can be reduced by action on the ratio of the thickness to diameter reductions (Qfactor): a reduction in thick­ness higher than the reduction in diameter gives a more radial texture, that is, a texture with the c poles closer to the radial direction.16

After cold processing, the (1010) direction is paral­lel to the rolling direction, and during a recrystalliza­tion heat treatment a 30° rotation occurs around the c-direction and the rolling direction is then aligned with the (1120) direction for most of the grains.