Effect Of Microstructure On Hydrogenation Of U - 5.7wt.% Nb Alloy

The microstructures of six aged U-5.7Nb alloys including as-quenched,200℃/6h aged,400℃/3h aged,400℃/9h aged,500℃/2h aged and400℃/9h+500℃/2h aged samples were systematically investigated in this thesis. A series of hydriding experiments for all samples were carried out under the same reaction conditions with the temperature at100℃, while the initial hydrogen pressure of1atm. The experimental results indicated that during the hydriding reaction, the microstructures of the U-5.7Nb alloy were proved to be the determining factor for the expanding of the hydride. This conclusion provides a new scientific understanding of the influence for hydrogen corrosion for uranium-niobium alloys and lays the foundation of the following researches about the hydrogen corrosion for uranium-niobium alloys.During the investigation, the laser scanning confocal microscopy (LSCM), the scanning electron microscopy (SEM), the transmission electron microscopy (TEM) and X-ray diffracmeter (XRD) were applied to analyze the morphologies, the phase compositions and the microstructures before and after the hydriding experiments for each aged alloy sample. The researching results can be briefly summarized at the following aspects. First the analyzing results of the as-quenched and200℃/6h aged samples showed no distinct changes in the LSCM and SEM photographs, the XRD data showed that the phase compositions were both supersaturated α", behaving as the martensite twin inside the matrix. However, the two samples didn’t react with hydrogen under the experimental conditions in this work. As to the400℃aged samples (400℃/3h,400℃/6h), the microstructures changed comparing with the as-quenched sample with the appearance of discontinuous precipitation (DP) on the prior-y boundaries and around the impurity sites. The morphology changed from martensite twin to a lattice-like structure formed by the acicular structure at a relatively larger scale especially caused by the400℃/3h aging treatment. The corresponding XRD results revealed the appearance of the Nb-depleted a phase. After the hydriding experiments under identical conditions, the hydrides formed on the surface of the two aged samples had typical tree-like morphology and treelike-lamellar coexistence morphology, respectively. Here it’s necessary to emphasize that the dimension of the "trees" could match well with the "lattice". At last, the500℃/2h and400℃/9h+500℃/2h aged samples had relatively complete phase decomposition, particularly the later sample almost decomposed entirely. Both samples had fine lamellar DP bi-phase microstructures with the composed phases of α+γ0. The researching results for the samples after hydriding implied that the morphology of the hydrides were all round, within which were the lamellar structures, and the hydriding reaction preferentially occurred in the DP areas on sample surface where the decomposition wasn’t complete enough. The XRD results before and after hydriding indicated that hydrogen reacted with a phase formed during the aging treatment.