Grain boundary engineering and the role of the interfacial plane

Grain boundary engineering (GBE) involves the use of microstructural design to improve bulk material properties and enhance resistance to intergranula r degradation, More specifically, the patented GEE procedure involves the d esign and control of fee metallic microstructures using thermomechanical tr eatments and grain boundary characterisation based on the coincidence site lattice model. The phenomenon of multiple twinning is used to create a 'twi n limited' microstructure, i.e. a microstructure composed entirely of speci al grain boundaries and triple junctions that is highly resistant to interg ranular degradation. However, the theory behind GEE is not fully developed and therefore further study of the interfacial geometry, including the grai n boundary plane and its role in GEE, is required to improve understanding of multiple twinning with the ultimate aim of improving the bulk and interg ranular properties of metallic materials, An introduction to GEE is present ed, including a number of cases where grain boundary design has improved th e properties of fee alloys for industrial applications. The theoretical cha racterisation of grain boundaries, including interfacial structure and geom etry, is reviewed, highlighting the problems associated with microstructura l characterisation based on limited knowledge of the grain boundary geometr y. The importance of the grain boundary network is discussed: the grain bou ndary and triple junction character distributions are known to have a signi ficant influence on bulk properties. Finally, the role of the interfacial p lane is considered. It is concluded that although GEE has produced signific ant results, its theoretical basis and the ultimate creation of twin limite d microstructures require further development. MST/4722.