Application of scanning tunneling microscopy for evaluation of the non-equilibrium state of grain boundaries in nickel subjected to high pressure torsion

The goal of the study is to estimate the relative free energy of grain boundaries in nickel deformed by high-pressure torsion (HPT) using scanning tunneling microscopy. In the process of work, the following studies have been carried out. The technique has been developed for sample preparation of Ni samples subjected to severe plastic deformation (SPD) by the HPT method for subsequent studies by scanning tunneling microscopy, which provides an average value of the root-mean-square surface roughness of approximately 2 nm. Using scanning tunneling microscopy, images of the nickel surface structure have been obtained, which have been processed using the Gwyddion application program. As a result, the grain boundary profiles have been calculated, based on which the values of the dihedral angles at the bottom of the etching grooves have been calculated and the relative energies of the grain boundaries have been calculated. It has been established that after HPT deformation, the average relative energy of grain boundaries significantly exceeds the relative energy of grain boundaries of coarse-grained nickel with grain boundaries of recrystallization origin. Thus, because of HPT, non-equilibrium grain boundaries are formed in the samples. It has been shown that with an increase in the degree of HPT deformation, an increase in the average relative energy of grain boundaries occurs, which indicates an increase in the degree of their non-equilibrium. It has been demonstrated that SPD by the HPT method makes it possible to achieve a higher level of relative energy of grain boundaries than deformation by the equal-channel angular pressing (ECAP) method.