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 intergranular degradation, More specifically, the patented GEE procedure involves the design and control of fee metallic microstructures using thermomechanical treatments and grain boundary characterisation based on the coincidence site lattice model. The phenomenon of multiple twinning is used to create a 'twin limited' microstructure, i.e. a microstructure composed entirely of special grain boundaries and triple junctions that is highly resistant to intergranular degradation. However, the theory behind GEE is not fully developed and therefore further study of the interfacial geometry, including the grain boundary plane and its role in GEE, is required to improve understanding of multiple twinning with the ultimate aim of improving the bulk and intergranular properties of metallic materials, An introduction to GEE is presented, including a number of cases where grain boundary design has improved the properties of fee alloys for industrial applications. The theoretical characterisation of grain boundaries, including interfacial structure and geometry, is reviewed, highlighting the problems associated with microstructural characterisation based on limited knowledge of the grain boundary geometry. The importance of the grain boundary network is discussed: the grain boundary and triple junction character distributions are known to have a significant influence on bulk properties. Finally, the role of the interfacial plane is considered. It is concluded that although GEE has produced significant results, its theoretical basis and the ultimate creation of twin limited microstructures require further development. MST/4722.