The nucleation and growth of the γ′ phase in aluminium-silver(-copper) alloys

This thesis examined the nucleation and early stages of growth of the γ′ (AlAg2) intermetallic phase in Al-Ag and Al-Ag-Cu alloys. This hexagonal close-packed phase forms as high aspect ratio, plate-shaped precipitates with {111} habit in aluminium alloys containing silver. Although the γ′ phase is not used to strengthen commercial alloys, the Al-Ag alloy system has been used as a model to better understand precipitate nucleation and growth. As such it is value in understanding industrial wrought aluminium alloys whose mechanical properties depend on the ability to manipulate the nucleation and growth of the intermetallic phases. This project was undertaken to investigate the reasons behind the apparent difficulty confronting the nucleation of the γ′ phase, which appears inconsistent with what would seem to be a modest energy barrier to nucleation. The precipitate is coherent with the matrix and forms with negligible volumetric strain and yet precipitates very gradually and only in the presence of dislocations or other defects. High resolution TEM was used to characterise γ ′ precipitates at the earliest possible stage of their existence in Al-Ag and Al-Ag-Cu alloys. It was possible to image γ′ precipitates with thicknesses of 0.92–1.38 nm, and with diameters of 5–6 nm. These precipitated on stacking faults associated with vacancy dislocation loops. Close examination of precipitates showed that a) the nucleating volume was too small for the phase to self-accommodate the strains associated with precipitation and b) that growth occurred via single unit cell ledges. It was shown that this will result in an addition shear strain energy barrier for both growth and nucleation. It was argued that this additional strain energy term will considerably increase the activation energy barrier to nucleation and may explain the poor nucleation potential of the γ′ phase. This work clarifies the reasons behind the difficulty in nucleating γ ′ phase. Similar poor nucleation behaviour is also found in other aluminium-based alloys, including commercial systems, and this work may offer some insight into such alloys.