Mechanisms of void formation during uniaxial tensile testing in a low-temperature-aged U-Nb alloy

The microstructure and damage accumulation of a U-5.5Nb alloy aged at a low temperature were analysed as a function of strain. Four tensile tests were performed and interrupted at different strain levels to investigate the mechanisms of void nucleation, growth and coalescence via optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction and nanoindentation measurements. The results show that voids first nucleate at U(N,C) inclusions at a low strain because they crack due to their low strength. Meanwhile, the higher strength of the Nb2C inclusion than that of matrix does not lead to the fracturing of Nb2C inclusions. However, with the further increasing strain, voids nucleate via the decohesion of the Nb2C-matrix interface because of the mismatch between the strengths of the Nb2C inclusion and matrix. In clusters comprising both Nb2C and U(N,C) inclusions, void formation mechanisms are determined by the Nb2C and U(N,C) inclusions. In addition, in clusters, the weak adhesion between Nb2C and U(N,C) inclusions causes separation. Based on experimental observations, an illustration of failure mechanisms is presented for a low-temperature-aged U-5.5Nb alloy with a single α′′ phase.