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The anisotropic behavior of elastic properties of the low-temperature phase of uranium was studied by theoretical and experimental methods in this work. Firstly, based on the density functional theory, the elastic constants and compliance constants of α-U were obtained. Secondly, Young's modulus, bulk modulus, shear modulus, and Poisson's ratio of polycrystalline α-U were calculated using the Voigt-Reuss-Hill model. Then, according to the crystal symmetry of α-U, the spatial distribution characteristics of its Young's modulus were calculated and displayed. The Young's modulus of α-U in some specific crystal directions (including the typical deformation-induced twinning {1 3 0}, {1 7 2}) were also given. Nanoindentation method combined with electron backscatter diffraction (EBSD) technology were used to measure and characterize the grains with different crystal orientations, to obtain experimental values of Young's modulus of α-U in different crystal orientations. Also, the inverse pole figure of Young's modulus of α-U was given.

Equal-channel angular pressing (ECAP) and mechanical machining were employed to introduce large plastic shear strain into bulk and the surface layer of a U-5.5wt.%Nb (U-5.5Nb) alloy, respectively. Applying ECAP, the U-5.5Nb alloy exhibited a single yield feature on the stress-strain curve in comparison to the “double yield” feature in the water quenched (WQ) counterpart. The difference is attributed to the characteristics of severe plastic deformation (SPD) structures obtained by ECAP. Mechanical machining resulted in a further severe microstructure refinement of the SPD surface layer with nano-grains/subgrains, which is accompanied with a dramatic increase of both the hardness and the elastic modulus of the surface layer. It is further proposed that the suppression of the shape memory effect (SME) in the nanostructured U-5.5Nb alloy may be responsible for the enhanced elastic modulus. The results of corrosion rate and pitting susceptibility showed a microstructure dependency, where the ECAP sample exhibited a better pitting resistance than that of the WQ counterpart.

The microstructure and hydriding properties of as-quenched and 500 °C/2 h aged U-13at.%Nb alloys were investigated. After suffering 500 °C/2 h process, the as-quenched alloy with single metastable α″ phase partially decomposed into lamellar pearlite along prior-γ grain boundaries and around impurities accompanied by the redistribution of niobium. The as-quenched U-13at.%Nb alloy performed perfect corrosion resistance to hydrogen while the aged poor owing to their different microstructure. The hydriding reaction order for the aged alloy in this work was determined to be 0.62. The decomposed areas had relatively lower Volta potential than the non-decomposed areas, which decreased the hydrogen corrosion resistance. In addition, the conclusion that Nb-poor α-like-U reacted with hydrogen preferentially than Nb-rich phase was confirmed by KFM results that Nb-poor phase had relatively lower Volta potential.

Niobium as alloying element significantly improves physical and chemical properties of metallic uranium, exhibiting great application potential in uranium alloy materials. The corrosion resistance performance as well as the internal alloy phase structure of uranium–niobium alloy is closely related to aging processes. Microstructure and hydriding characteristics of the 400 °C/9 h + 500 °C/2 h aged uranium–13 at.% niobium alloys (U–13 at.%Nb) were investigated from the point of view of relationship between the microstructure and growth of the hydriding areas. The microstructure, morphology and composition of the alloy phases before and after the hydriding were well characterized by the laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Experimental results indicated that the hydrogen preferentially reacted with the Nb-depleted phase α-like-U to form monolithic β-UH 3 Nb x , and the alloy microstructure played an important role in hydride growth.

To study the behavior of hydrogen corrosion at the surface of U, U–2.5 wt%Nb alloy and U–5.7 wt%Nb, a gas–solid reaction system with an in situ microscope was designed. The nucleation and growth of the hydride of the alloy were continuously observed and recorded by a computer. The different characteristics of the hydrides on U metal and U–2.5 wt%Nb showed that the later alloy is more susceptible to hydrogen corrosion than the former. The growth rate of hydride of U–2.5 wt%Nb, calculated by measuring the perimeter of the hydride spots recorded by the in situ microscope, exhibited a reaction temperature dependency in the range of 40–160 °C, for pressure of 0.8 × 105 Pa. An Arrhenius plot for growth rate versus temperature yielded activation energy of 24.34 kJ/mol for the hydriding of U–2.5 wt%Nb alloy. The maximum hydriding rate was obtained at 125 °C, whose thermodynamics reason was discussed.