Research On Microstructure And Orientation Of Hydrides In Zirconium Tubes

Zirconium alloys are widely used in nuclear reactors as cladding materials because of their low neutron absorption, good mechanical properties, high temperature resistance and good corrosion resistance. The cladding materials will absorb hydrogen under high temperature water corrosion during nuclear reactors operation. However, the solubility of hydrogen in zirconium alloy is very limited. Zirconium hydride will precipitated when the hydrogen concentration in the zirconium alloy exceeds the terminal solid solubility. Hydrides are extremely brittle at all temperatures, which reduces the ductility of zirconium alloy and lead to the destruction of the cladding materials ultimately.The thesis studied the hydrides of stress-relieved and recrystallized new Zr-Sn-Nb tubes. The different hydrided-times?4, 6 and 8h? were used to control the different hydrogen content in the tubes under gaseous hydrogen charging experiment. Several characterization and analysis techniques including optical microscopy?OM?, electron backscatter diffraction?EBSD? and transmission electron microscopy?TEM? were jointly employed to investigate the morphology, distribution and orientation of hydrides. Furthermore, the crystallographic orientation relationship between ?-Zr matrix and hydrides was studied in this work. Following conclusions can be obtained from the present study:?1? Metallographic micrographs analysis shows that these hydrides prefer to distribute along circumferential direction of tube in both types of tubes. However, more comparisons clarified the amount of radial hydrides in recrystallized tubes is larger than that of stress-relieved tubes. In addition, the number of radial hydrides increases along the tube wall from inside to outside. These are consistent with the hydrides orientation factor f₄₀ statistics obtained in present work for both stress-relieved and recrystallized tubes.?2? Based on the texture analysis, the stress-relieved and recrystallized tubes have similar C-axis bimodal texture. However, the stress-relieved tube exist <10110>//AD texture, the recrystallized tube exist <11210>//AD texture. In addition, because of the orientation factor are FRD>FTD>FAD, the radial texture is dominant for both tubes. According to radial texture is easy to get circumferential hydrides, the hydrides in both type tubes mostly along circumferential direction of tube. And this is consistent with the conclusion of?1?.?3? EBSD analysis clarified that the hydrides are almost distributed along the circumferential direction of tube in stress-relieved and recrystallized tubes. And the recrystallized tube with gaseous hydrogen charging 8 hours have more radial hydrides than others. Three types of hydrides, i.e. inter-granular, intra-granular and trans-granular hydrides have been confirmed in this work. In addition, much more spot hydrides in stress-relieved tubes than recrystallized tubes have been found and they trend to link together to form hydrides strip.?4? The detailed crystallographic orientation relationships between ?-Zr matrix and hydrides have also investigated through pole figure analysis. It is established that all of the intra-granular hydrides have maintained?0001?_??{111}_?, and the most common crystallographic relationship of inter-granular hydrides is?0001?_??{111}_?. There are two exceptions that {10 117}_? ? {111}_? and {10 110}_? ? {111}_? crystallographic relationship for inter-granular hydrides of stress-relieved and recrystallized tubes respectively.?5? TEM analysis shows that hydrides with long needle-like morphology formed in recrystallized tube with gaseous hydrogen charging 6 and 8 hours. The length of intra-granular hydrides is about 200-500 nm for treating 6h, which is much smaller than that in 8 hours?3-5?m for intra-granular hydrides and 8-10 ?m for trans-granular hydrides?. The hydride phases are identified as face-centered cubic?fcc? structure ?-ZrH1.66?PDF card No. 34-0649? by diffraction pattern.