Formation of the intermetallic phases Al$_{12}$Mg$_{17}$ and Al$_3$Mg$_2$ during heating of elemental Al-Mg composites studied by high-energy X-ray diffraction

The purpose of the present study is to track the formation of the intermetallic phases that form in the binary Al-Mg system and investigate their orientation relationships to the parent Al and Mg elements. Therefore, two compositions Al60Mg40 and Al40Mg60 (wt%), which are very suitable for studying the diffusion of Al in Mg and Mg in Al were heated from room temperature up to elevated temperatures below the melting point. Powder metallurgy, including cold extrusion was used to create a large interface between the Al and Mg, which facilitates fast reaction kinetics. In order to observe the phase formation during heating and analyze the crystal structures, X-ray diffraction using synchrotron radiation was used. The use of high energy X-ray was extremely helpful, enabling the detection of small phase fractions and information on the orientation relationships between the Al and Mg and the intermetallic phases that formed. The $γ$-Al$_{12}$Mg$_{17}$ phase was the first phase formed in both alloy compositions on annealing. Subsequently, the $β$-Al$_3$Mg$_2$phase was formed. After annealing at 400 ºC for 2 h, the Al40Mg60 composition consisted of a very high amount of Al$_{12}$Mg$_{17}$ and a small amount of Mg phase while the Al60Mg40 composition consisted of mainly Al$_3$Mg$_2$ and a small fraction of Al$_{12}$Mg$_{17}$, indicating that thermodynamic equilibrium has been approximated. On further annealing at 400 ºC for 12 h, both compositions formed only one phase, namely the Al$_{12}$Mg$_{17}$ phase in the Al40Mg60 composition and the Al$_3$Mg$_2$ phase in the Al60Mg40 composition. In this condition, a Pitsch-Schrader orientation relationship was found between the Mg and the γ-Al$_{12}$Mg$_{17}$ phase, the occurrence of which is discussed within the framework of literature models. No orientation relationship between either the Al or the Mg with the Al$_3$Mg$_2$ phase was found