EFFECTS OF EXCESS MAGNESIUM OR SILICON ON THE TWO-STEP AGING BEHAVIOR OF Al-Mg2Si ALLOYS.

Al-0. 7 and 1. 2% Mg//2Si alloys and those containing excess Mg or Si were investigated by hardness, electrical resistivity and specific heat measurements and high-resolution transmission electron microscopy. The alloys were solution-treated at 560 degree C, quenched, preaged at 20 similar 100 degree C, and aged at 175 degree C. If preaged at low temperatures, the maximum hardness of the alloys two-step aged is generally inferior to alloys aged at 175 degree C. In the excess alloys, clusters formed during preaging increase, but most of them do not transform into G. P. zones. Two-step aging is effective in the low Mg//2Si alloy in which precipitates are uniformly distributed.