Phase evolution of Al–Mg metal matrix composites during low temperature annealing at 200 °C and 250 °C

Abstract

In the Al–Mg system, intermetallic compounds can be formed by a solid-state reaction between the elemental components. Metal matrix composites obtained by classical powder metallurgy and room temperature extrusion with the two compositions Al60–Mg40 and Al40–Mg60 (wt. %) were studied. The high fraction of Al–Mg grain boundaries in these materials allows investigation of the early stages of phase evolution. The mixtures are close to the ideal composition of either the intermetallic β-Al3Mg2 phase or the γ-Al12Mg17 phase. Phase analysis was performed using intense synchrotron radiation with an energy of 87 keV and showed that the γ-Al12Mg17 phase was the first to form after 2 h annealing at 200 °C. In situ annealing at 200 °C for 12 h and at 250 °C for 6 h was compared to ex situ annealing at 200 °C and 250 °C for 2 h and 12 h. The γ-phase was the first phase to form, independent of the starting composition. It is suggested that the γ-phase grows via an interface reaction controlled mechanism. After the γ-phase has reached its critical thickness the β-phase starts to form. No other phases were observed to form.