The Effect Of Alloying Element Nb On The Hydrogenation Behavior Of Uranium

In order to slow down or even eliminate hydrogen corrosion of uranium and its alloys,the characteristics and mechanism of hydrogen corrosion must be deeply understand.Based on the method of PVT the nucleation and growth of hydride on uranium-niobium alloy was observed in situ by a Sievert’s-type apparatus with an optical microscope.And the influence of niobium content on the hydrogenation behavior of quenched uranium-niobium alloy was discussed by using X-ray diffraction(XRD),laser scanning confocal microscope(LSCM)and scanning electron microscope(SEM).The results show that,the niobium content has a significant effect on the hydrogenation behavior of the quenched uranium-niobium alloy,and the initial nucleation of hydrogenation is related to the microstructure of uranium-niobium alloy.Detailed conclusions are as follows:(1)The effect of alloying element niobium on the microstructure of quenched uranium-niobium alloy was systematically studied.The results show that U-lwt.%Nb and U-2wt.%Nb alloys were mainly acicular α’ martensitic after quenching.The microstructure of the quenched U-2.5wt.%Nb alloy was a transition from an acicular to a banded martensite of the α’ phase.The quenched state of U-4wt.%Nb,U-5wt.%Nb and U-5.7 wt.%Nb alloy was the α" phase of band martensite.While the quenched U-7 wt.%Nb alloy mainly showed the γ0 phase of the lath martensite.(2)The influence of alloying element niobium on the hydrogen corrosion behavior of quenched uranium-niobium alloy was preliminarily revealed.The results show that the addition of element Nb would increase the corrosion resistance of uranium-niobium alloy in terms of overall trend,while not the same in terms of different stages of niobium content.There were four stages:the first stage was dilute alloy,such as U-lwt.%Nb,U-2wt.%Nb,the Nb content was very low and the impact of Nb content was very significant.The second stage was U-2.5wt.%Nb,which was a mixture of acicular martensite and lath martensite,so the interface defects were relatively high.In order to reduce the interface energy,a large number of mismatch dislocations were formed at the interface,Which leads to the formation of a large number of preferential nucleation sites in the initial stage of the hydrogenation behavior.The third stage is U-4wt.%Nb and U-5wt.%Nb,the content of Nb in this stage was higher and the reaction was obviously slowed down.Until the fourth stage U-5.7wt.%Nb and U-7wt.%Nb,the Nb contents were very high that the solubility of H increased greatly,and the hydrogenation corrosion was difficult to occur.(3)The effects of microstructure on the hydrogenation behavior of slow-cooled biphasic U-2.5 wt.%Nb alloys were discussed.The results show that hydrogenation of the U-2.5wt.%Nb alloy was very fast,and only the "big family" nucleation points were formed.The hydrogen corrosion rate of Nb-poor a phase was greater than the Nb-rich γ1-2 phase and priority hydrogen corrosion was occurred at the α phase,then the γ1-2 alternated between the hydrogenated a phase started to be attacked,and the hydrogen damage point grows up quickly once nucleated.Which mean the Nb-poor a phase was a preferential nucleation site of biphase U-2.5wt.%Nb alloy.It was found that the internal hydride of biphase U-2.5wt.%Nb alloy was diffused with the hydrogen point as the center of the network to the internal.And the internal hydrogen diffusion region which had lower hardness and elastic modulus than the matrix.(4)The effects of surface roughness on uranium hydrogenation were investigated.The samples of different surface roughness had the preferential orientation behavior in the process of hydrogenation corrosion,that was all along the direction of scratches.And the hydrocracking was preferentially nucleated at the valley position formed by sandpaper grinding.In addition,the lower the surface roughness of the sample was,the slower the hydrogen occuring.