HRTEM and ADF-STEM of precipitates at peak-ageing in cast A356 aluminium alloy

Precipitates at peak-ageing in an A356 Al–Mg–Si alloy cast by a semi-solid process have been studied by high-resolution transmission electron microscopy (HRTEM) and annular dark-field scanning transmission electron microscopy (ADF-STEM). The major precipitate (ppt) at peak-ageing is the monoclinic β″ or pre-β″. Its orientation relationship with the fcc-Al matrix is [0 0 1] Al//[0 1 0] ppt, ( 0 2 0 ) Al / / coincide ( 6 0 1 ) ppt and ( 2 0 0 ) Al / / coincide ( 4 ¯ 0 3 ) ppt , equivalent to [0 0 1] Al//[0 1 0] ppt, ( 1 ¯ 3 0 ) Al / / ( 1 0 0 ) ppt and (3 2 0) Al//(0 0 1) ppt. The habit direction of this precipitate is [0 0 1] Al//[0 1 0] ppt forming facet planes on ( 1 ¯ 3 0 ) Al / / ( 1 0 0 ) ppt and (3 2 0) Al//(0 0 1) ppt, in which very good atomic matching was found. The contrast of the precipitate in ADF-STEM is reversed to the typical atomic number contrast. To understand such contrast, a dynamical simulation based on the multi-slice method was therefore performed using different atomic stacking models and successfully explain the reverse contrast of the experimental images of the precipitate.