Application of scanning tunnel microscopy to assess the degree of non-equilibrium of grain boundaries in niobium obtained by high pressure torsion

The average relative energy of grain boundaries in niobium after high-pressure torsion (HPT) using scanning tunneling microscopy (STM) has been investigated in this work. It was found that defor-mation by HPC leads to the formation of an ultrafine-grained (UFG) structure with an average grain size of ~ 100 nm and a significant increase in hardness (up to 2800 MPa). The energy state of grain boundaries was quantitatively evaluated by STM analysis: in coarse-crystalline niobium, the average relative energy of grain boundaries was 0.27, whereas after 5-turn HPT, it was 0.71. It was found that in UFG-niobium, the boundaries with high relative energy (more than 0,6) prevail, which indicates the formation of non-equilibrium (strain-modified) grain boundaries. The results obtained are consistent with the data for other metals (Ni, Cu, and hafnium bronze) subjected to severe plastic deformation. The work confirms the promising potential of STM microscopy for quantifying the energy of grain boundaries in nanostructured materials, which is important for understanding their mechanical properties and optimizing processing methods.