Mats Dahlbäck

Detailed Analysis of the Microstructure of the Metal/Oxide Interface Region in Zircaloy-2 after Autoclave Corrosion Testing

Two varieties of Zircaloy-2, with different second phase particle (SPP) size distributions and different corrosion resistance, were oxidized in a steam autoclave. Transmission electron microscopy (TEM) of large thin-foil cross-sections of the oxide and the adjacent metal shows an undulating metal/oxide interface in both materials with a periodicity of slightly less than 1 μm and an amplitude of around 100 nm. The SPPs oxidize slower than the surrounding metal, and the absence of volume increase leads to void and crack formation as the SPPs become embedded in the oxide. On SPP oxidation, iron diffuses out of the particles into the surrounding oxide. A sub-oxide with an oxygen content of approximately 50 at. % and a layer thickness of about 200 nm was observed close to the metal/oxide interface. There is a 200 nm oxygen concentration gradient into the metal, from the level close to the sub-oxide of about 30 at. % down to a few atomic percent. All tin in the matrix is incorporated in the sub-oxide, and no segregation to the metal/oxide interface was found.

Oxidation Mechanism in Zircaloy- 2—The Effect of SPP Size Distribution

The metal/oxide interface region in Zircaloy-2 oxidized in autoclave was studied with transmission electron microscopy (TEM) and atom probe tomography. In addition to waviness on the micrometer scale the metal/oxide interface was found to have irregularities on a finer scale, and metal islands were found especially at metal hills (delayed parts of the oxidation front). The thickness of the sub-oxide layer varies considerably along the interface in the same sample, from 100 to virtually 0 nm. The sub-oxide composition may vary on a very fine scale (down to 5nm), and it can sometimes be a mixture of sub-oxides with different oxygen content. The metal matrix in contact with the sub-oxide is saturated with up to 32 at. % oxygen, and the oxygen diffusion profile in the metal is in approximate agreement with literature data for pure Zr. However, the diffusion length appears to be somewhat larger at interface metal hills than under valleys, probably for both geometrical and stress state reasons. Hydride precipitates, hardly visible in conventional TEM, give a good image contrast when employing high angle annular dark field imaging. A model for the oxidation process is presented, where the creep deformation of the metal close to the interface and the formation of lateral cracks in the oxide are of highest importance. The effect of second phase particle (SPP) size is suggested to be twofold: Small and numerous SPPs give a stronger metal and therefore higher stress in the oxide. Small SPPs also nucleate many more lateral cracks in the oxide, which gives a weaker oxide. Together this leads to formation of large cracks associated with transition in the oxidation rate at an earlier time than for a material with larger and fewer SPPs, and thereby a higher oxidation rate.

Texture Evolution of Zircaloy-2 During Beta-Quenching: Effect of Process Variables

The nuclear industry is interested in developing thermomechanical processes to produce random crystallographic orientation (texture) from cold-rolled Zircaloy-2 sheets used to manufacture boiling water reactor (BWR) channels. Randomized textures are beneficial because they minimize anisotropic irradiation-assisted growth, which in turn reduces bowing and uncontrolled deformation of BWR channels during service. The texture evolution of cold-rolled Zircaloy-2 sheets during the allotropic α→β→α phase transformation was characterized by using synchrotron X-ray diffraction in situ and electron backscatter diffraction. The initial strong rolling texture is weakened only if the α→β→α phase transformation is complete. Plastic deformation and grain growth in the β-phase lead to changes in the β texture and modify the inherited α texture. The global texture evolution is not sensitive to levels of stress that do not cause β plastic deformation. These findings demonstrate that accurate temperature control of the β-quenching process is of utmost importance in order to minimize undesirable irradiation growth of BWR channels during service, and that plastic deformation in the β phase can be employed to modify the inherited α texture. BWR channels with β-quenched textures will exhibit minimum irradiation growth caused by texture.