Structure analysis of embedded nano-sized particles by precession electron diffraction. η′-precipitate in an Al-Zn-Mg alloy as example

The Vincent-Midgley precession technique has been used to collect three-dimensional electron diffraction intensity data from a dispersion of coherent precipitates in a matrix. In order to suppress severe effects from multiple diffraction via matrix reflections, a fairly large precession (tilt) angle had to be used. This implied a high background from the surrounding matrix, and limited the number of reflections that could be measured from patterns on image plates. The heavily faulted hexagonal η′-precipitates (a=0.496nm, c=1.405nm) with thickness 3-5 nm occur in four equivalent orientations relative to the aluminium matrix; with frequent overlap of reflections. A model of the average structure in the space group P63/mmc with assumed composition Mg2Zn5−xAl2+x, have been derived by Patterson analysis and intensity comparisons.