Twinning-related grain boundary engineering

This overview reports the fundamental concepts of grain boundary engineering (GBE) accompanied by a critical appraisal of GBE investigations. The main conclusions are as follows. There are many variants on processing route schedules to produce successfully a GBE microstructure with improved properties. The role of twinning is indirect and twins have several functions at different stages of the GBE process, which are necessary for the development of microstructure and properties: firstly to retain strain, then to generate non-coherent Σ3s (and other `special' boundaries), and finally to break up the random boundary network. Connectivity of boundary types is more important than the absolute fraction of special boundaries. The `strain recrystallisation' description of GBE processing is in fact a recovery process. Accordingly, strain-recrystallisation and strain-anneal GBE do not involve separate mechanisms. The CSL model has ambiguities with regard to recognition of special boundaries. Finally, near-future challenges for GBE are identified.