Kinetics and mechanism of formation and transformation of metastable β′-phase in Al–Mg alloys

The analysis of dilatometric and calorimetric curves, completed by optical microscopy observations, permitted to follow the kinetics and the morphology changes of β′-phase particles during ageing of Al–12 wt.% Mg alloy. It is now well established that β′-phase particles, which are semi-coherent and metastable, appear initially in a globular shape, transform into thin needles, then into particles of the equilibrium β-phase after a long ageing time, as predicted by the phase diagram. In addition, β′ and β-phases are formed and dissolved successively and independently one of the other, during non-isothermal ageing.