Materials Chemistry and Physics 72 (2001) 401–404 A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy D. Bradai a , P. Zieba b,* , E. Bischoff c , W. Gust c a Institut de Physique, Université des Sciences et de la Technologie Houari-Boumedienne, BP 32 El-Alia, Alger, Algeria b Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta St. 25, 30-059 Cracow, Poland c Max-Planck-Institut für Metallforschung and Institut für Metallkunde, Universität Stuttgart, Seestr. 92, 70174 Stuttgart, Germany Received 5 February 2001; received in revised form 26 February 2001; accepted 28 February 2001 Abstract The occurrence of discontinuous precipitation (DP) and dissolution reactions has been investigated in Mg-10wt.% Al alloy. The application of a special cyclic heat treatment allowed to observe a new mode of the discontinuous dissolution reaction. The reaction starts preferably and occurs faster at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction occurs, in the early stage, primarily at the reaction front of the former DP colonies. The proposed explanation is attributed to a characteristic movement of the reaction front of the DP colony away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mg–Al alloys; Discontinuous precipitation; Discontinuous dissolution; Grain boundary migration 1. Introduction Discontinuous dissolution (DD) is a solid-state phase transformation occurring by the backward migration of a former reaction front of a discontinuous precipitation (DP) colony [1–3]. The reaction starts at temperatures close to the solvus temperature and results in the formation of an inhomogeneous α ∼ solid solution according to the scheme: α + β → α ∼ where α is the solute-depleted lamella and β is the solute-rich lamella. The DD reaction can occur at three different sites [4–6], namely the former reaction front of the DP colony, the im- pingement of two colonies of discontinuous precipitates and the original location of a grain boundary (Fig. 1). The Mg–Al alloys are a good example showing the common behaviour of the DD reaction [7,8]. In the case of a single seam of the DP, the reaction occurred only at the reaction front of a for- mer DP colony. In the case of a double seam, the reaction started also at the original locations of the grain boundary * Corresponding author. Present address: Institut für Metallkunde, Uni- versität Stuttgart, Seestr. 75, D-70174 Stuttgart, Germany. Fax: +49-711-1211280. E-mail address: nmzieba@imim-pan.krakow.pl (P. Zieba). when the DD process at the reaction front of the former DP colony was in a very advanced stage. It will be shown in the present paper that after a special heat treatment the original locations of the grain boundaries are the most favourable sites for the start and development of the DD reaction in the case of a single seam morphology. Moreover, in the case of a double seam the reaction starts at the original locations of the grain boundary and is not observed at the reaction front of former DP colony. This has not been observed so far. 2. Experimental A Mg alloy containing 10 wt.% Al was prepared for the investigation by vacuum induction melting of the elements of high purity (3N5) and casting into 11 mm diameter rods. They were then homogenized at 703 K for 48 days in Duran glass capsules and water quenched. Samples of 6 mm thick- ness were cut off from the rods by spark erosion, and a final homogenization was applied at 703 K for 48 h, followed by a water quenching. The following procedure was applied in order to obtain the post-dissolution structures. The samples were aged at 495 K for 30 min and then the solution annealed at 695 K for 30 min, three times in succession. Then, the final ageing at 495 K for 30 min was performed, followed by annealing at 645 K for 7 min. 0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0254-0584(01)00334-0