Abstract
Faceting of 40·0° (∊ = 9), 109·5° (∊ = 3), and 129·5° (∊ = 1) 〈110〉 coincidence boundaries, where ∊ denotes reciprocal volumetric density of coincidence sites, has been observed during grain boundary migration in 99·998 a/o aluminium bicrystals. Facet inclinations generally correspond to low-to-moderate density coincidence planes, implying that structure of the migrating boundaries can be described in terms of an ensemble of non-coherent twin interfaces, many of which have not heretofore been observed. Possible causes of faceting and its effect on grain boundary migration are suggested.