Discontinuous and continuous precipitation in magnesium-aluminium type alloys

The microstructure investigations of the AZ91 alloy and binary Mg-9 wt.% Al alloy after different heat treatments were presented. Solution annealing at 693 K for 26 h (with water quenching), followed by ageing at 423, 473, 543 and 623 K was carried out. After ageing at 423 K only discontinuous precipitates were observed whereas at 623 K only continuous ones were revealed in the microstructure of both alloys. At intermediate ageing temperatures (473 and 543 K) both discontinuous and continuous precipitates occurred competitively. Additional analyses revealed that after cooling a solid solution from 693 K (without quenching) only discontinuous precipitates were formed. On the other hand, after heating a supersaturated solid solution from room temperature to 665 K both discontinuous and continuous precipitates were observed simultaneously. On the basis of the obtained results a model of precipitate types dependent on heat treatments for Mg-9 wt.% Al alloys was proposed. (C) 2008 Elsevier B.V. All rights reserved.