Masters Theses

Date of Award

12-2005

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Hahn Choo

Committee Members

Peter K. Liaw, Raymond A. Buchanan

Abstract

Hydride formation is one of the main degradation mechanisms of zirconium alloys in hydrogen-rich environments. When sufficient hydrogen is present, zirconiumhydride precipitates can be formed. Cracking of the brittle hydrides near a crack tip can initiate the growth of a crack leading to the premature failure of the material. Hydride formation is believed to be enhanced by the presence of residual or applied stresses. Therefore, the increase in the stress field ahead of a crack tip may promote precipitation of additional hydrides. In order to verify these phenomena, the effect of internal stresses on the zirconium-hydride-precipitate formation, and in turn, the influence of the hydrides on the subsequent intergranular strain evolution in a hexagonal-close-packed zircaloy-4 alloy were investigated, using neutron and x-ray diffraction. First, the evolution of intergranular strains in a zircaloy-4 was investigated insitu, using neutron diffraction, to understand the deformation behavior at the microscopic length scale. A series of uniaxial tensile loads up to 500 MPa was applied to a round-bar tensile specimen in the as-received condition and the intergranular (hkl-specific) strains, parallel and perpendicular to the loading direction, were studied. The results provide a fundamental understanding of the anisotropic elastic-plastic deformation of the zirconium alloy under applied stresses. Then the hydride formation was examined by conducting qualitative phase mapping across the diameter of two tensile specimens charged with hydrogen gas for ½ hour and 1 hour, respectively. It was observed that the zirconium hydrides (δ-ZrH2) form a layer, in a ring shape, near the surface with a thickness of approximately 400 μm. The hydrogen-charging effects on intergranular strains were investigated and compared to the as-received specimen.

Second, spatially-resolved internal-strain mapping was performed on a fatigue pre-cracked compact-tension (CT) specimen using in-situ neutron diffraction under applied loads of 667 and 4,444 newtons, to determine the in-plane (parallel to the loading direction) and through–thickness (perpendicular to the loading direction) lattice-strain profiles around the crack tip. An increase in elastic lattice strains near the crack tip was observed with the increase in the applied stresses. The effect of hydrogen charging was also investigated on CT specimens electrochemically charged with hydrogen. X-ray diffraction results clearly showed the presence of zirconium hydrides on the surface of the specimen. The internal strain in the hydrogen-charged specimen was measured, using neutron diffraction to provide an understanding of the effect of the surface hydrides on the strain profile near the crack-tip in comparison to the strain data measured from a CT specimen without hydrogen. Future work is planned to correlate the uniaxial behavior with fracture mechanics characteristics of the hydrogen-charged Zircaloy-4 at continuum and mesoscopic length scales using in-situ diffraction and computational modeling.

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