On “macroscopic” characterization of mixed grain boundaries

Grain boundaries having no special geometric features are referred to as mixed. Mixed boundaries constitute a large class, and the need for a finer differentiation arises. A mixed boundary can be characterized by linking it to the nearest boundaries with special geometric attributes, in particular, by its decomposition into twist and tilt components. We investigate and compare the quantitative methods and parameters used for description of mixed boundaries. The considered methods are based either on decomposition or on some other measures of deviations from pure-twist or pure-tilt boundaries. Furthermore, the methods are either limited to boundary representations with the smallest misorientation angles or take into account all symmetrically equivalent boundary representations, and these two approaches differ in their meaning, physical significance and conclusions. The relationships between the methods of mixed boundary characterization are clarified and the suitability of particular parameters for analysis of experimental data is assessed. These issues are important for a thorough understanding of grain boundary distributions in the space of macroscopic boundary parameters. The considered techniques and parameters are used to examine mixed boundaries in the Ni-based superalloy IN100.