Al-Mg-Mn (Aluminum-Magnesium-Manganese)

The recent thermodynamic assessments of this ternary system by [2007Du] and [2009Shu] were reviewed in the update by [2009Rag]. Subsequently, [2009Ren] reported an isothermal section at 400 °C by studying Mg-rich alloys. These results supplement those given in the above references and are reviewed here briefly.

Binary Systems

The Al-Mn phase diagram depicts the following intermediate phases: Al₁₂Mn (Al₁₂W-type cubic), Al₆Mn (Al₆Mn-type orthorhombic), λAl₄Mn (hexagonal, space group P6₃/m), μAl₄Mn (hexagonal, P6₃/mmc), Al₁₁Mn₄(HT) (Al₃Mn-type orthorhombic), Al₁₁Mn₄(LT) (Al₁₁Mn₄-type triclinic), Al₈Mn₅ (Al₈Cr₅-type rhombohedral), γ (34.5-52 at.% Mn; bcc), and ɛ (55-72 at.% Mn; cph). The Al-Mg phase diagram has the following intermediate phases: Al₃Mg₂ (Al₃Mg₂-type cubic, labeled β), R or ɛ (rhombohedral) and Al₁₂Mg₁₇ (A12, αMn-type cubic, denoted γ). There are no intermediate phases in the Mg-Mn system.

Ternary Isothermal Section

With starting metals of 99.99% purity, [2009Ren] induction-melted four ternary alloys with Mg contents in the range of 65 to 88 at.%. A diffusion couple of Mn with a 55Mg-45Al (at.%) master alloy and the four ternary alloys were annealed at 400 °C for 446 or 672 h and quenched in water. The annealing experiments revealed tie-lines between (Mg) and (βMn) and between (Mg) and Al₁₁Mn₄(LT), but not between (Mg) and the intervening Al-Mn phase Al₈Mn₅. [2009Ren] attributed this to the very slow formation of Al₈Mn₅ and tentatively included this phase in the isothermal section constructed by them. This view is in line with the thermodynamic calculations of [2007Du], which show that the tie-line between (Mg) and Al₈Mn₅ is stable at 400 °C. However, there is the other possibility that a transition reaction in the solid-state (not revealed in the thermodynamic description) (Mg) + Al₈Mn₅ ↔ (βMn) + Al₁₁Mn₄(LT) could have occurred above 400 °C. A narrow tie-triangle of Al₈Mn₅ + (βMn) + Al₁₁Mn₄(LT) along the Al-Mn side produced by this reaction would then preclude the equilibrium between (Mg) and Al₈Mn₅. The isothermal section constructed by [2009Ren] at 400 °C is shown unmodified in Fig. 1, in combination with the computed results of [2007Du] for Al-rich alloys.

Fig. 1

Al-Mg-Mn isothermal section at 400 °C [2009Ren, 2007Du]