In this study, severe plastic straining through equal-channel angular pressing (ECAP) is imposed on age-hardenable Al-Mg-Si alloys having two different compositions of excess Si or Mg in the Al-Mg₂Si pusedo-binary system. Thereafter, the alloys are subjected to aging and the microstructures are examined using transmission electron microscopy. It is shown that the precipitation behavior is significantly changed through application of ECAP. Figure 1 shows (a) a TEM bright field image and (b) a dark field image including (c) an SAED pattern of the sample containing excess Si after ECAP and annealing for 1 hour at 473 K. The dark field image was taken using a diffraction spot indicated by the arrow. Many rod-like precipitates are visible which are elongated along the <100>_m direction of the matrix. A lattice image of a precipitate is shown in Fig.2 (a) and a diffractogram taken from the precipitate is in Fig.2(b). Close inspection of the image and diffractogram reveals that this particle is a type A precipitate. It was reported that the formation of type A precipitates occurred in a solution-treated sample after prolonged aging such as for 1000 hour at 473 K. Thus, the aging for 1 hour for the present ECAP sample is very short and it is considered that the aging was accelerated by severe plastic strain introduced by the ECAP process. This result is consistent with an earlier observation that precipitation process proceeds at a faster rate than that in the normal strain-free condition.

[Published in Materials Science Forum, 475-479, (2004), pp.4047-4050]