TRANSMISSION ELECTRON-MICROSCOPY STUDY OF A PRECIPITATE WHICH FORMS IN THE AL-MG-SI SYSTEM

In this work, a detailed study of precipitates formed after ageing for 600 h at 185 degrees C of an alloy containing 1.1 wt% Si, 1 wt% Mg, 0 .01 wt% Cu and 0.7 wt% Mn is presented. The precipitates have a lath s hape with their longer axis aligned with [001](Al) and have average, d imensions of about 300 nm x 12 nm x 5 nm. The unit cell is determined to be hexagonal with a = 1.03 nm and c = 0.4505 nm. The orientation re lationship is (0001)(p)//(001)(Al) and [<2(11)over bar 0>](p) is almos t parallel to [510](Al) The crystallographic information of these prec ipitates are in agreement with that of the known quaternary stable pha se Q (approximate stoichiometry, Al5Cu2Mg8Si6) Since the Cu is not pre sent in the precipitate, it may be replaced by another atom.