Crystallographic measurement of the β to α phase transformation and δ-hydride precipitation in a laser-welded Zircaloy-2 tube by electron backscattering diffraction

Crystallographic measurement of the β to α phase transformation and δ-hydride precipitation in a laser-welded Zircaloy-2 ferrule tube were carried out using an electron backscattering diffraction pattern (EBSP). A basket-weave structure with sub-micron lath width caused by quenching from the β to α phase was observed in the heat-affected and fusion zones, and mainly showed a grain boundary misorientation angle of 60° with an <1 1 2 ¯ 0> rotation axis. This result is consistent with the Burgers orientation relationship of {1 1 0}β//(0 0 0 1)α and <1 1 1>β//<1 1 2 ¯ 0>α for the β to α phase transformation. The texture of the quenched α′ phase was strongly inherited from the original α phase, having a radial (0 0 0 1) basal pole and axial {1 1 2 ¯ 0} textures, even in the fusion zone. The primary hydride habit plane in the welded Zircaloy-2 was (0 0 0 1)α//{1 1 1}δ, matching previously obtained results for recrystallized cladding tubes. In addition to the primary habit plane, secondary habit planes were observed for the other low-index planes {1 0 1 ¯ 0} and {1 0 1 ¯ 1} in the fusion zone. The heterogeneous accumulation of hydrides in the transition zone between heat-affected and unaffected zones was mainly due to the residual stress distribution in the narrow region.