Grain growth and stabilisation of nanostructured aluminium at high temperatures: review

Nanostructured (NS) materials have a large stored energy due to their large grain boundary area and thus tend to be unstable with respect to grain growth during high temperature annealing or deformation. This problem can limit the application of NS materials at high temperatures (>0·5Tm, absolute melting temperature), especially Al alloys owing to their low melting points. Restoration processes and grain growth in NS Al based materials are critically reviewed, with emphasis on nanostructure grain stabilisation at high temperatures. The mechanisms of normal and abnormal grain growth during isothermal annealing are presented, followed by consideration of thermal stabilisation by the addition of solute atoms/impurities and/or dispersion of second phase particles. Grain growth is significantly facilitated by applying deformation at elevated temperatures during preparation or further processing of semifinished NS materials. The dynamic restoration processes, dynamic grain growth and dynamic particle coarsening are addressed in NS Al. Finally, grain growth during consolidation of nanocrystalline powders (one of the principal methods to fabricate bulk NS Al) is presented, and the effects of processing parameters on grain size stabilisation are discussed.