Modeling of temperature dependence of stress reorientation of hydrides in CWSR Zr alloys

Zr alloy, used as pressure boundary for hot coolant in CANDU, PHWR and RBMK reactors, pickup hydrogen / deuterium during service. Hydrogen present in excess of terminal solid solubility (TSS) precipitates out as hydride phase. Being brittle, the presence of substantial quantities of hydrides can cause embrittlement of the host matrix resulting in loss of ductility, impact and fracture toughness. Also, degree of embrittlement is strongly influenced by the orientation of hydride platelets. During fabrication of cold worked and stress relieved (CWSR) Zr 2.5Nb pressure tube, majority of the α-Zr grains acquire an orientation with their basal poles along the circumferential or radial direction. Since hydride precipitation in α-Zr grains show a habit plane nearly parallel to basal plane, for the texture of the pressure tube material, crystallographically only two orientations are permissible. These are along the circumferential-axial plane and along the radial-axial plane and are called circumferential hydrides and radial hydrides, respectively. Under unstressed condition, only circumferential hydrides form in Zr 2.5Nb pressure tubes. However, due to stress reorientation phenomenon, radial hydrides may precipitate and being oriented normal to hoop stress direction of the pressure tubes, can significantly increase the latter's susceptibility to failure. The radial hydride precipitation in cold worked and stress relieved (CWSR) Zirconium alloys, is associated with a threshold stress, σ'th', below which no reorientation occur. Thus for CWSR Zr alloy pressure tube material, hydride platelets precipitating in the unstressed condition acquire predominantly circumferential orientation whereas those precipitating under stress greater than σ'th'along circumferential direction of tube acquire radial orientation. Ideally speaking there should be a uniquely defined σ'th' but due to the high residual stresses in the CWSR Zr alloy pressure tube material there exist a range of stresses for which both radial and circumferential hydrides co exist. A theoretical model of reorientation based on the packing density difference between the grain boundary and the grain, is proposed to explain the basis of reorientation of hydrides in this alloy. The proposed model takes into consideration the anisotropy in elastic constants of hexagonal zirconium, temperature dependence of elastic constants of hydride and zirconium, misfit strains between hydride and zirconium and their yield strengths. The theoretically predicted values of threshold stresses are in good agreement with the experimentally determined values