Materials Chemistry and Physics 84 (2004) 112–119 Kinetics and mechanism of formation and transformation of metastable  ′ -phase in Al–Mg alloys D. Hamana ∗ , L. Baziz, M. Bouchear Phase Transformations Laboratory, Faculty of Sciences, University Mentouri of Constantine, Constantine 25000, Algeria Received 27 March 2003; received in revised form 26 September 2003; accepted 7 November 2003 Abstract The analysis of dilatometric and calorimetric curves, completed by optical microscopy observations, permitted to follow the kinetics and the morphology changes of  ′ -phase particles during ageing of Al–12 wt.% Mg alloy. It is now well established that  ′ -phase particles, which are semi-coherent and metastable, appear initially in a globular shape, transform into thin needles, then into particles of the equilibrium -phase after a long ageing time, as predicted by the phase diagram. In addition,  ′ and -phases are formed and dissolved successively and independently one of the other, during non-isothermal ageing. © 2003 Elsevier B.V. All rights reserved. Keywords: Al–Mg alloys; Precipitation; Ageing; Dilatometry; Calorimetry; Metastable phase 1. Introduction A solid solution becomes unstable through supersatura- tion and decomposes partly but not completely into another crystalline phase as the equilibrium one is approached. When this transformation is completed and equilibrium is achieved, the microstructure consists in an original matrix phase, usu- ally with a composition and lattice parameters that are dif- ferent from those of the original matrix, plus a new phase formed as the matrix was consumed. However, a metastable transition phase may appear and is difficult to distinguish optically from a stable phase; such a transition phase either re-dissolves, as a more stable phase forms, or itself transforms into a more stable one. For ex- ample, Yukawa et al. [1] consistently indicated the precip- itates in their Al–9 at.% Mg alloy as being the equilibrium -phase, but Bernole et al. [2], Bernole [3], Starink and Zahra [4] and Hamana et al. [5] thought that the precipitates formed in similar alloys are mainly particles of the  ′ -phase. Moreover, because of the competing effect of surface, vol- ume and strains energies, the number, size and shape of the particles precipitating at various stages of ageing may be quite different. Transmission electron microscopy has in fact demonstrated that in numerous systems of practical interest one precipitate phase has nucleated at the interphase bound- ∗ Corresponding author. Tel.: +213-31-61-48-09; fax: +213-31-61-48-09. E-mail address: d hamana@yahoo.fr (D. Hamana). aries of another one. Russell and Aaronson [6] suggested that the nucleation of the next precipitate should occur pref- erentially at the interphase boundaries of the first one, and that succeeding precipitates ought to follow the same pat- tern. This will occur because of the greater importance of minimising the interfacial free energy. For example, in the Al–Cu system there is evidence for the nucleation of the -phase at  ′ and of  ′ -phase at  ′′ [7]. The situation is quite different for the Al–Mg system [8], though the precipitation sequence is similar to that of the Al–Cu system [8–15]: supersaturated  solid solution → Guinier–Preston (GP) zones →  ′′ →  ′ →  Moreover, the kinetics and mechanism of  ′ particle forma- tion, transformation or dissolution have not sufficiently been studied and are still unknown because the published works are often contradictory. Some authors think that the  ′ par- ticles nucleate on structural defects as dislocations and va- cancy voids/loops [12,16]. However, Starink and Zahra [15] affirmed that there is no evidence of nucleation of  ′ pre- cipitate on structural defects, though an abundant presence of vacancy-type defects (mostly dislocation loops) were ob- served. After 5 years of natural ageing, a grain-boundary precipitation represented by the metastable  ′ particles but, more frequently, by the stable -phase (Al 3 Mg 2 ) was ob- served in Al–11 wt.% Mg alloy by Kaigorodova [17,18]. Nozato and Ishihara [19] thought that the  ′ and -phases 0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.11.001