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 beta '' coincide or pre-beta ''. 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) over bar 0 3)(ppt), equivalent to [0 0 1](Al)//[0 1 0](ppt), ((1) over bar 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) over bar 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. (C) 2010 Elsevier B.V. All rights reserved.