Precipitation of reoriented hydrides and textural change of α-zirconium grains during delayed hydride cracking of Zr–2.5%Nb pressure tube

Cantilever beam (CB) specimens referred to as L90 and L60 with the notch directions tilted normal to, and at an angle of 60° to, the transverse direction of a cold-worked and annealed Zr–2.5%Nb pressure tube, respectively, were subjected to delayed hydride cracking (DHC) testing at 250°C. L60 specimen showed slanted growth of the DHC crack compared to L90 without tilting. An X-ray diffractometric study was carried out on the fractured surfaces of the two CB specimens after DHC testing. The δ-hydride phase was confirmed to sit on the fracture surface, demonstrating the growth of the DHC crack through fracturing of the reoriented hydrides. Furthermore, the texture of the reoriented hydrides was determined for the first time. Comparing the pole figures of the {1 1 1}δ-hydride and the (0 0 0 1)α-zirconium, it is concluded that the reoriented hydrides nucleate first of all at the α-zirconium grains. A change in the orientation of the α-zirconium grains, mainly by twinning on the {1 0 1 2} planes, was demonstrated to occur during the propagation of the DHC crack. On the basis of the above findings, a mechanism of the DHC was discussed.