Precipitation kinetics and transformation of metastable phases in Al–Mg–Si alloys

In situ synchrotron small-angle X-ray scattering (SAXS) and transmission electronic microscopy were used to study the precipitation and transformation behaviors of metastable β″ and β′ phases of naturally aged Al–Mg–Si alloys, AA 6022, during continuous heating, and subsequent isothermal aging. A new SAXS analysis approach for the complex SAXS profiles enabled a quantitative analysis of the structural evolution and the corresponding kinetics of the concurrent needle-like β″ and rod-like β′ phase transformations, of different size characteristics. The stable cross-section and rapid length growth along the needles of β″ phase, and the radius growth of β′ phase during the temporal evolution were characterized well by the concomitant nucleation, growth and coarsening stages. For an alloy aged at 260 °C, the β″ precipitates (∼3.5 nm in radius) and β′ precipitates (∼10.5 nm in radius) reached the coarsening stages at t = 30 and 210 min, respectively. During the coarsening stage, the β″ precipitates dissolved partially into the matrix and transformed partially to the β′ phase, and then stabilized with the residual ∼65% precipitates. The same methodology was also applied to an Al–Mg–Si alloy, AA 6111, containing Cu. The β″ precipitates of AA 6111 alloy aged at 180 °C showed an inert radius (∼1.8 nm) and a growing length with aging time, and the longer incubation period of ∼60 min observed in the early stage of precipitation, relative to AA 6022, was attributed to the more natural clusters.