Faceting of the Σ 3 coincidence tilt boundary in Nb

Due to their unusual properties, the grain boundaries (GBs) with the lowest possible inverse density of coincidence sites Σ = 3 play a special role in the GB engineering. The as-grown shape of the cylindric tilt grain boundary (GB) in Nb bicrystal grown by the floating zone method has been studied with the electron back-scattering diffraction method. Both grains form the superlattice called coincidence site lattice (CSL) with the lowest possible inverse density of coincidence sites Σ = 3. Four different CSL facets (100) Σ3CSL , 110 Σ 3CSL , 120 Σ 3CSL and 210 Σ3CSL were observed simultaneously. Flat facets (100) Σ3CSL , 110 Σ3CSL , 120 Σ3CSL and 210 Σ3CSL form smooth edges (no slope discontinuity) with rounded rough GB portions. Rough surface curves away from the plane of the (–1, 2, 0) Σ3CSL facet at the edge with (–1, 1, 0) Σ3CSL facet as x β with β = 1.61 ± 0.09. At the edge between (210) Σ3CSL and (–1, 2, 0) Σ3CSL facets β = 1.46 ± 0.09. Both values reveal the GB roughening belonging to the Pokrovsky-Talapov universality class. It has been shown for Pb surfaces [K. Arenhold, S. Surnev, P. Coenen, H.P. Bonzel and P. Wynblatt, Surf. Sci. 417 (1998) L1160] that the β value depend on the details of the steps interaction at the vicinal surface. In our case the difference between measured β for two different facet edges can be due to the similar details of GB steps.