Annealing induced an uneven development of the microstructure in single-direction rolled Nb sheets

The study explored the annealing-induced evolution of microstructure and texture inhomogeneity in single-direction rolled Nb sheets, and samples were taken from the surface and the center of the sheets for isothermal annealing. Subsequently, these samples were analyzed using X-ray diffraction, electron backscatter diffraction, and nanoindentation techniques to assess their microstructure, texture, and mechanical properties. The findings revealed that the deformation in the center of the single-direction rolled Nb sheets was significantly larger than at the edges, which resulted in a notable texture deviation. Specifically, the raw surface of the rolled sheet exhibited a fiber structure characterized by alternating orientations, whereas the center displayed a fiber structure with a prominent (001) orientation texture band. Furthermore, the regions within the center texture band that were difficult to slip were susceptible to stress concentration. After the annealing, the surface had abnormal grain growth due to the higher strain energy and gave an unclear preferred orientation. Conversely, the center of the sheet utilized stress concentration points as nucleation sites, inheriting the orientation of the original texture band and evolving into a uniform equiaxed recrystallized structure. Nanoindentation revealed uniform micro-properties post-annealing. Surface hardness showed discontinuous clustering due to large and non-oriented grains, while the center showed arithmetic hardness progression due to similar orientations. These findings guide the texture control and process design for Nb targets, enabling tailored properties.