About the Mechanisms Governing the Hydrogen Effect on Viscoplasticity of Unirradiated Fully Annealed Zircaloy-4 Sheet

Abstract

It has been observed that hydrogen either in solid solution or precipitated under the form of hydrides has an impact on the viscoplasticity of CWSR Zircaloy-4 cladding tubes, increasing significantly the creep resistance. The use of TEM on the structurally complex CWSR material being unlikely to identify the deformation mechanisms, it has been decided to complete this R&D program on recrystallized material. A study has been carried out on fully annealed unirradiated Zircaloy-4 sheet used for the manufacturing of the fuel subassembly grids. Mechanical tests were performed for large ranges of temperatures (300 to 400°C), stresses (120 to 250 MPa), and strain rates (2 ∙ 10-7 to 2 ∙ 10-3 s-1) on as-received and hydrided specimen. The results emphasize: • Hydrogen in solid solution induces a softening of the material. The TEM observations have revealed identical structure of dislocations for both as-received and hydrided specimens. The softening has been particularly observed when dynamic strain aging is activated. It is assumed that atomic hydrogen decreases the dislocation pinning caused by interstitial and/or enhances the intrinsic mobility of the dislocations. With respect to abinitio calculation, atomic hydrogen might be trapped easily by the core of the dislocation, this phenomenon contributing to decrease the lattice friction and to enhance planar glide. • Precipitated hydrides induce a hardening of the material as observed for CWSR Zircaloy 4. The magnitude of the phenomenon depends upon temperature and stress. An analysis of the unload sequences for tension tests and of the secondary strain rates for creep tests leads to the conclusion that hydrides change the kinematics hardening by increasing the internal stress with respect to the as-received material. TEM observation combined with this viscoplasticity approach has revealed that: first, as long as the internal stress is increasing versus plastic strain, hydride are obstacles to the dislocation glide; second, once the internal stress reaches saturation, the hydrides can be jumped over by the dislocations.

Hydrogen-induced microstructure, texture and mechanical property evolutions in a high-pressure torsion processed zirconium alloy

“…4d). Furthermore, hydrogen in solid solution plays an inhibiting role in the static recovery by limiting climb or annihilation of the dislocations, which is linked to the recrystallization phenomenon [24].…”

Recent Advances in Point Defect Studies Driven by Density Functional Theory

“…The effect of hydrogen in metals refers to a great amount of phenomena, the analysis of which is out of the scope of this short review. We will only describe here the effect of H in solid solution in Zr, which has been shown to accelerate the creep rate of Zr alloys under particular experimental conditions [54]. It is experimentally known that H occupies T sites in hcp Zr, in spite of the fact that these sites are smaller than the O ones.…”