The crystallography and morphology of the S'-phase precipitate in an Al(CuMg) alloy

The aluminium alloy considered in this paper has a Cu/Mg ratio of 1·94 with 1·53 wt% Cu and 0·79 wt% Mg. It has been observed that on ageing at 190°C, S' precipitates form and grow as rods parallel to <100>M cubic matrix directions with a slight tendency to cluster in groups on {110}M f.c.c. matrix planes. These groups of precipitates did not however coalesce to form laths on the {110}M or {210}M planes as has been reported in alloys with similar Cu/Mg ratios. The accepted orientation relationship has been confirmed and used to simulate a detailed and complete S'-phase diffraction pattern and morphological structure which agrees with electron microscope observations. It is suggested that lath formation is dependent upon the (Cu + Mg) concentration, which is low enough in this alloy to prevent it. Based on S'-phase lattice-parameter measurements, it is suggested that the S' precipitates are only a slightly strained version of the Al2CuMg-S phase and need not be distinguished from it.