Nanometer-sized precipitates formed in supersaturated solid-state matrices have been found ~80 years ago[1] and widely used in engineering metallic alloys [2, 3]. However, how solute atoms segregate and transform into a phase in the matrix, usually with another kind of crystal structure, is still an open question and requires investigation using advanced characterisation techniques, such as scanning transmission electron microscopy (STEM) [4].
Here we report a study on precipitation in Al-1.7Cu-0.75In-0.75Sb alloy (at.%) using high-resolution STEM (both in bright field and high-angle annular dark field modes) and energy dispersive X-ray spectroscopy (EDS) in a JEOL JEM 2100F operated at 200 kV. This combined addition is of interest because they form similar structured precipitates as Ge that has been known to enhance precipitation. In this alloy, truncated octahedral InSb (space group, F-43m) particles form first after aging for 2 min at 200oC, followed with the formation of θ’’ always docking at the InSb particles. θ’ begins to form at 10 min, also attaching to InSb particles, but until 30 min it only has local density dominance and shares a similar dominance with θ’’ in the whole sample. 1h-aging makes θ’ become the dominant precipitates both locally and overall. θ’ also shows a slightly preference for magic thicknesses corresponding to minima in volumetric and shear misfit strain (more remarkable than Al-Cu binary alloy but less remarkable than Al-Cu-Sn alloys[5]). Furthermore, given InSb always attach to the edges and never at the centre of θ’, In and Sb addition may enhance nucleation of θ” and θ’ by accelerating Cu diffusion or/and slightly lowering shear strain associated with intermediate thicknesses, rather than reducing θ’ and matrix interface energy.