Investigation On Influencing Factors For Hydride Nucleation And Growth Of U And U-Nb Alloy

Metal uranium and its alloys have been widely used in the field of nuclear and national defence industry.As is known,uranium will be attacked easily by hydrogen in the hydrogen atmosphere for its high chemical activity.The hydriding of uranium behaves special pitting corrosion characteristics,the subsequent growth of the nucleation sites leads to the surface cracking,and then result in the missing of the surface integrity and the reducing of mechanical property.Besides,the reaction product is spontaneous,and is easy to generate radioactive aerosol contaminating the environment.So many related researchers major in the corrosion of uranium and its alloys by hydrogen because the corrosion is regarded as one of the most serious hazard in the nuclear industry.The interaction between uranium and hydrogen has been studied since the late 1940s in several analytical areas including chemical kinetics,metallography and surface chemical analysis.The researching results suggested that there are many factors influencing the hydriding,such as the external environmental factors like temperature,hydrogen pressure and the internal material science factors like surface oxide layer,impurities,grainboundaries,twinning,phase boundaries,inclusions and residual stress.The hydriding of uranium has been universally onberved to be localized,spatially hererogeneous and random.The hydride nucleation and growth have attracted a great deal of investigation work on them and a number of researching resuts have been acquired.However,the hydride nucleation and growth mechanism is still unknow,and the hydriding research of uranium after various processing is insufficient.Such as if the hydriding characteristics will change as uranium undergoes heat treatment under different conditions.If changes,what are the tendency and causation?And besides,until now,hydriding behavious of uranium after deformation processing are not clear.The uranium hydride nucleation and growth mechanism is not understood clearly yet.On the other hand,U-Nb alloy also suffered different degrees of hydride corrosion,whereas relative less study result is involed in the openning literatures.So,in this thesis,the hydriding characteristics of uranium after different heat treatment and deformation have been systematically studied.The differences of hydriding characteristics for uranium suffered different cooling rate(water-quenching(WQ),air-cooling(AC)andfurnace-cooling(FC))from high temperature and deformation processing(1.5%,4%,10%tensile and 1.5%,4%,10%compression)are firstly reported and the impact factors are discussed;In addition,for U-Nb alloy,the hydriding characteristics changed as the microstructure varing from the water-quenched to aging for different time.A micro-scale induction period model has been developed.The main results are embodied in the following aspects:(1)The results revealed that hydride expanding into the metal matrix distinctly varies from each other for uranium after different heat-treatment and transformation.The depth/diameter ratios of hydride craters for each uranium is:heat treatment,WQ>AC>FC,compression,1.5%<4%<10%.The microstructure and residual stress distribution altered as the different heat treatment routs and deformation treatments applied on uranium.Under the same experiment condition,hydriding characteristics including hydride nucleation and growth of above-mentioned uranium behave obvious differences from each other.For the uranium after being heat treatment,the faster the cooling rate,the more the twinning formed and the larger the residual stress accumulated.Accordingly,the depth/diameter values of the hydride craters were larger for faster cooling uranium than that for slow cooling rate.Besides,hydride craters for uranium deformed under high compression strain also have larger depth/diameter values.Therefore,the microstructure and residual stress distribution are important factors impacting the hydride growth into metal matrix.(2)Hydride expansion characteristics around large hydride craters are illuminated.The dense nucleated first nucleation sites on twinning around hydride craters were firstly observed on uranium suffered water quenched,deformed under 1.5%tensile and 1.5%compression strain.As the drformation aggregated,the twninning density increased accompanied by occurring large numbers of low-angle boundaries.Expansion bulge layer appeared around hydride craters.The bulge layer morphologies varied as the deformation strain changed.When uranium deformed under 1.5%tensile strain and 1.5%compression strain,the bulge layer behaved dense nucleated first nucleation sites on twinning,twinning is the main factor.When uranium deformed under 4%and 10%tensile strain and 4%compression strain,the bulge layer behaved a series of amooth raised layer.And when uranium deformed under 10%compression strain,the millions of first nucleation sites integrated,the low-angle boundaries is the main factor.Here,the critical cracking size(CCS)of hydride was proposed.Hydrides formed in uranium under small tensile strain have small CCS,and CCS increased as the tensile strain increased.Uranium under 4%compression strain deformation has the largest CCS in this work.(3)The conclusion that microstructure of U-Nb alloy is important controlling factor on hydride growth has been further clarified.When U-Nb alloy was aged in 400?,there appeared particular ancicular intermediate structure,hydride expanded along the ancicular structure to form special dentritic morphology.The acicular microstructure brought by the treatment played a key role on the proferential hydride growth.As the aging time accelerated,the microstructure evolved,from martensite to ancicular structure to pearlite.Hydride grew along ancicular structure to form dentritic morphology and grew along pearlite to form scaly morphology.The influences from microstructure on the hydride growth were more obvious for U-Nb alloy than that for uranium.(4)A micro-scale induction period model based on the transition region and the previous hydriding models has been developed.Hydriding mechanism of metallic uranium and U-Nb alloy was further explored.The results indicate that,although the component,content,microstructure and hydriding characteristics are different between uranium and U-Nb alloy,the nucleation and growth of hydride are controled by the microstructure.The model suits the hydriding of uranium and U-Nb alloy and maybe other uranium alloys.Within the so-called micro-scale induction period,the H diffuses and accumulates at particular sites before reaching the critical concentration required to form stoichiometric UH3 in the transition regions.Through these experimental studies,the correlation between strcuture together with stress and uranium hydriding mechanism have been verified.The database of hydriding for uranium and its alloys have been enriched,deeper understanding of uranium hydriding factors have been acquired,guidance of significance in hydride corrosion for uranium can be provided on account of the work in this thesis.