| Uranium and uranium alloys are very sensitive to the enviroment and readily react with many kinds of gases, which causes corrosion and decrease in material nature and may lead to failure. Hydriding is characterized with fast reaction rate, big volume expansion and pitting corrosion, which could destroy the mother material very seriously and have drawn much research attension in the last few decades. The study on the hydriding behavior of uranium and uranium alloys was one of the frontiers in nuclear materials science due to the great scientifical and engineering value. However, this reaction is very complex and influenced by many factors, including hydrogen pressure, temperature, impurity gases, characters of the oxidation layer, microstructures of the metal and so on. What is the most important, the hydide corrosion of uranium has been observed to be localized, spatially hetorogeneous, and seemingly random, the hydride seems to nucleate at some special locations. Until now, with virtually no reports to the contrary, a strong correlation of hydride corrosion to misorientation boundaries on uranium has been repeatedly observed. Despite the near unanimity in associating hydride corrosion with at least certain types of defects, there is a considerable divergence of opinion on causation. Researchers at AWE and NRC-Negev regard the properties of the superficial oxide layer as the determinants of initiation site. While other institutions such as LANL and LLNL hold the opposite opinion, relegating the oxide layer to a secondary role while implicating the metal properties. A thorough review of uranium hydride corrosion reports reveals that, the dissension on the issue of’essential’causation in the community of uranium hydride research continues, and the chief goal is to identify the root causes and the mechanism for hydride corrosion, especially for hydride nucleation sites. The later could used to define protection strategies of uranium and uranium alloys. Besides, although uranium has been extensively studied, in contrast, the more widely used uranium alloys have been less concerned in the literatures.Firstly, U-0.79wt.%Ti alloy——one kind of the most important uranium alloys, is widely used in the nuclear industry for its good mechanical and anti-corrosion nature. Hence, it is of practical value to study the the hydride corrosion of U-0.79Ti alloy. Secondly, samples of various microstructure and chemical elements distribution could be prepared through different thermal treatment, which provides an idea model system for studies of fundamental problems in the issue of root causes for hydride nucleation. In this thesis, by applying and combining different kinds of analysis method to U-0.79wt.%Ti (U-0.79Ti) as a model system, we studied the reaction behaviror with hydrogen. We firstly identified the three forms of titanium elements in U-0.79Ti alloy, and then studied the hydride nucleation and growth in the early stage of the specially-prepared samples of different microstructures. We have oberved a strong correlation of hydride sites with the microstructures of the underlying metals. The results provided new evidences for the dispute on the issue of root causation for uranium nucleation sites. And finally we discussed the reaction mechanisms of hydride corrosion for U-0.79Ti alloy.The main research work and contents are listed as follows:(1) We have systematically studied the variety in the microstructures of U-0.79Ti alloy and revealed the effect of the introduction of alloying element Ti, thermal aging on the microstructure of U-Ti alloy. For the first time, we identified that titanium exists mainly in three forms, solution (a’), intermetallic (U2Ti) and titanium inclusions. And we pointed out that, by using the U-0.79Ti microstructure’s thermal treatment sensitivity, the effects of the difference existing forms of titanium on hydride corrosion could be studied by specially-prepared samples.(2) The method of pre-treatment for U-0.79Ti was determined by in-situ XPS monitoring the character on the very surface of uranium and U-Ti alloy during vacuum thermal heating from room temperature to700℃. The XPS spectrum suggested that, during the low temperature heat treatment (<200℃), the outmost UO2+x oxidation layer was transformed into UO2or UOxCy, which accouts for the decrease in the induction time. As we know, by comparing with uranium metal, it is the first time in the reports that, a novel decomposition of UOxCy and segeration of titanium on the very surface was obsevered. Which show the influence of titanium on the surface physical characters of U-Ti alloy. The results provide an intesting model for the study of diffusion, segeragation, and interaction with minor elements.(3) We firstly observed the strip-like hydride corrosion in U-0.79Ti alloy with the so-called "p+U2Ti" microstructure. The hydride nucleates in and grows along α-U phase and is constrained by α-U/U2Ti boundary. The samples are composed of alternative α-U and U2Ti planelets. The Scanning Kelvin Probe (SKP) results shew that the work function of these two phases are quite different. The reactivity of a-U is higher. In our experiments, the a-U phase developed a thicker oxide layer. If the view-point of AWE and NRC-Negev is suitable for all kinds of uranium alloys, the hydride should nucleate in the U2Ti phase, the results did not support such a deduction. This means that the oxidation layer is of secondary importance in determing the nucleation sites. In contrast, the correlation between the hydride nucleation and the microstructure indicates that the metal properties are the determinant factor.(4) By using the state-of-the-art hot stage microscopy and laser confocal microscope, we studied the hydride corrosion of500℃/2h aged U-0.79Ti alloy and observed the preferred hydride nucleation in the phase boundary of the mother y phase. It is well known that, the thermal annealing at500℃cause the decompostion of martensite into α+U2Ti along the phase boundary. By composition of the hydride corrosion in the decomposed area and the martensite, it is concluded that the solution titanium could decrease the reactivity toward hydride corrosion. The results supported the viewpoint of "metal properties is the key factor that determine where the hydride corrosion starts".(5) The relationship between titanium inclusions and hydride nucleation in the early stage was studied by in-situ hot stage microscope. We found that inclusions are always connected with hydride nucleation sites, most of them developed into growth centers and are more aggeresive for the metal. The effects of inclusions on hydriding in U-Ti alloy is similar to that of the inclusions (most are UC) in uranium. Until now, the root causation of this behavior is not clear. Which need further researches. However, the results shew the influence of chemical elements’distribution on hydriding corrosion.Through these systematic experimental studies, the effects of the triple forms of titanium on hydride nucleation have been identified. Besides, the variety in the microstructure of U-0.79Ti alloy provides an ideal model for the study of the root causation. The experimentally observed link between the microstructure and chemical elements distribution and the hydriding nucleation sites provided further evidence to support the opinion of "metal properties is the determinants for the hydride nucleation", which put forward the study on the root causation of hydride corrosion. It can help us understand the pitting corrosion character of uranium hydriding, and provide some beneficial guidance in the corrosion protection for uranium and uranium alloys. |
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