On the precipitation of magnesium silicide in irradiated aluminium-magnesium alloys

Thermal neutron irradiation of aluminium or its alloys causes the productio n of silicon by transmutation. In aluminium-magnesium alloys, the transmuta tion-produced silicon reacts with magnesium and forms small precipitates. T he precipitation in irradiated Al-Mg alloys is similar to the early stage o f aging in thermally treated Al-Mg-Si alloys. This study evidences the simu ltaneous generation of two crystallographically different precipitate types . On the basis of electron diffraction patterns, unit cell parameters are d erived and compared with structures found in thermally aged alloys. One of the two precipitate types has an Mg,Si composition, while the other is an A l-Si-Mg intermetallic compound with high aluminium and silicon but low magn esium content. The formation of magnesium poor precipitates is important si nce it indicates that the threshold neutron fluence for grain boundary prec ipitation of silicon may be much higher than estimated in the past. (C) 200 0 Acta Metallurgica Inc. Published by Elsevier Science Lid. All rights rese rved.