Evolution of microstructure during fabrication of Zr-2.5 Wt pct Nb alloy pressure tubes

Abstract

Microstructural changes occurring during the fabrication of Zr-2.5 pct Nb alloy pressure tubes by a modified route, involving hot extrusion followed by two pilgering operations with an intermediate annealing step, have been examined in detail. In the conventional fabrication route, the hot extrusion step is followed by a single cold drawing operation in which the cold work to the extent of 25 pct is imparted to the material for achieving the required mechanical properties. Tensile properties obtained at each stage of fabrication have been evaluated and compared between the two processes. The main aim of this work has been to produce a microstructure and texture which are known to yield a lower irradiation growth. Additionally, suitable annealing conditions have been optimized for the intermediate annealing which annihilates the cold work introduced by the first cold pilgering operation without disturbing the two-phase elongated microstructure. This elongated α+ β I microstructure is required for obtaining the desired level of strength at 310 °C. The final microstructure and the crystallographic texture of the finished pressure tube have been compared with those reported for the conventionally processed material.