U-2.5wt% Nb Alloy, Hydrogen Erosion And Its Impact On The Mechanical Properties

U-Nb alloys have been widely concerned in the field of nuclear engineering as an important material. However, because of its reactiveness to entironment atmosphere and the metastable structure, the aging behavior of U-Nb alloys has been investigated in recent years, and the study of surface corrosion and phase transformation in aging process is hot problem. As a structural material, the mechanical properties of U-Nb alloys are very important. In the present work, surface hydrogen corrosion and its effect on the mechanical properties has been systemically studied.The hydrogenation kinetic of U-2.5wt%Nb has been studied systemically by a in-situ hot-stage microscope(HSM) and a P-V-T method. The nucleation and growth processes of hydride were continuously monitored and recorded on a computer. The results showed that the temperature dependence of the hydride front velocity at sufficently low temperature obeys the Arrhenius law and activation energy is 24.34kJ/mol. At hige temperatures, a maximum velocity is reached beyond which the velocity decreases sharply. At pressure much higher than the equilibrium pressure, the this velocity becomes practically independent of hydrogen pressure . The density of nucleation hydride spots increases with reaction temperature up to a certain value, and then decreases with temperature increase. The density of nucleation hydride spots increases with hydrogen pressure increase. For the bigining of hydriding of U-2.5Nb alloy, grain boundary is not the preferential spot and the nucleation of hydide is apt to happening on harsh surface. Hydiding induction time decreases with the temperature increases, which follow the arrhenius law under a temperature of about 125℃, but beyond 125℃, induction time increases sharply with temperature increasing. Induction time varies as the inverse of the hydrogen pressure, which follow the reciprocal relation. The oxide thickness on U-2.5Nb surface affect the induction time clealy, and induction time increases with the increasing of oxide thickness. U-2.5Nb alloy is susceptible to hydrogen corrosion, the hydriding rate of U-2.5Nb alloy is higher than that of U. The effect of Pre-heat treatment, the presence of CO impurities and Plasma based nitrogen ion implantation on induction time were also studied.To evaluate the effect of hydrogen corrosion on tensile properties of U-2.5Nb, tensile specimens were hydrided quantificationally, and then tested on tensile test machine to measure the tensile properties. The morphologies of hydrogen corrosion and fracture surface were observed by SEM and OM. The results show that, ductility parameter such as elongation and reduction in area decrease distinctly due to hydrogen attack, and ultimate tensile strength decreases slightly with the increasing of hydrogenation corrosion. According to morphologies of hydrogen corrosion and fracture surface, it is concluded that the pits, micro-cracks and diffuse hydrogen in solid induced by hydrogen attack result in the loss of ductility of U-2.5% Nb. According to strain energy density theory, base on the model of a crack on surface, the effect of corrosion crack on mechnical properties was calculated by finite element method. The calculated value accords with the experiment result.Dynamic mechanical properties and microstructure evolvement of U-2.5Nb under different strain rates(10^-3-10³/s) were studied by Split Hopkinson Pressure Bar (SHPB) apparatus, and effect of hydrogen corrosion on dynamic mechanical properties was also studied. The stress-strain curves show a usual strain hardening behavior, flow stress increased with the increasing strain rate. A constitutive relations based on Johnson-Cook model was constructed to describe the stress-strain relationship of U-2.5Nb at the investigated strain rates. The correlations between the simulated curves and the experimental results are in good agreement. Hydrogen corrosion resulted in difference in strain hardening behavior and increase of flow stress. There is no obvious grain microstructure change, and slippage is the main deformation method at the investigated strain rates.