Developing stable fine-grain microstructures by large strain deformation

Methods of deforming metals to large strains are reviewed and the process of equal channel angular extrusion is analysed in detail. The development of microstructure during large strain deformation is discussed, and it is concluded that the main criterion for the formation of a sub-micron grain structure is the generation of a sufficiently large fraction (greater than 0.7) of high-angle grain boundary during the deformation process. For aluminium alloys, it is found that a low-temperature anneal is required to convert the deformed microstructure into an equiaxed grain structure. The material, microstructural and processing factors that influence the formation of such fine-grain microstructures are discussed, and the stability of these microstructures at elevated temperatures is considered.