Disclinations at grain boundary triple junctions: Between Bollmann disclinations and Volterra disclinations

We discuss the question whether Bollmann's classification of triple junctions into I- and U-lines corresponds to a physical reality. We describe a virtual multiple junction of only small angle boundaries in two ways: First, by using Bollmann's I/U-line scheme (Bollmann disclinations) and second, by an elasticity approach (rotational Volterra cuts, Volterra disclinations). Depending on the number of boundaries, the multiple junction is found to be an I-line or a U-line. We conclude that the Bollmann disclination is a real crystallographic defect that is essentially different from the Volterra disclination.