Re-determination of γ/(γ + α-Mg) phase boundary and experimental evidence of R intermetallic compound existing at lower temperatures in the Mg–Al binary system

Equilibrated phase compositions at 250–400 °C in the Mg–Al binary system have been determined by means of both diffusion couple technique and alloy method. The results show that the solubility of Al in the Mg17Al12 (γ) intermetallic compound is ∼36.6 at.% when the Mg17Al12 phase is in equilibrium with the α-Mg solid solution, i.e., γ/(γ + α-Mg) phase boundary remarkably shifts to the Mg-rich side. The R intermetallic compound (R3¯) does stably exist at 250 °C, and its composition range is 51.2–54.5 at.% Al, which is completely different from the presently accepted Mg–Al phase diagram. Moreover, the composition region of β phase (Fd3¯) is determined in the range of 57.5–60.5 at.% Al below 400 °C.

► The γ/(γ + α-Mg) phase boundary shifts to the α-Mg side by about 3 at.% Al. ► The R compound should have a wider temperature composition range more than 250–400 °C. ► The R phase is non-stoichiometric with a maximum range of 51.2–54.5 at.% Al. ► The composition region of β phase is Mg-(57.5–60.5) at.% Al below 400 °C.