Observations of grain boundary impurities in nanocrystalline Al and their influence on microstructural stability and mechanical behaviour

The exceptional properties of nanocrystalline materials lend themselves to a wide range of structural and functional applications. There is recent evidence to suggest that grain boundary impurities may have a dramatic effect on the stability, strength and ductility of nanocrystalline metals and alloys. In this study, transmission electron microscopy and atom probe tomography were used to characterize specimens deposited at different base pressures, thus providing a direct comparison of impurity content with microstructural stability and mechanical behaviour. Atom probe measurements provide clear experimental evidence of grain boundary segregation of oxygen in samples deposited at higher base pressures. It is proposed that these oxygen atoms pin the boundaries, preventing stress-assisted grain growth and resulting in increased strength and loss in ductility. This study provides the first direct experimental evidence that boundary impurities play a critical role in determining the microstructural stability and deformation behaviour of nanocrystalline metals.