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ection II: Phase Diagram Evaluations

Al-Mg-Mn (Aluminum-Magnesium-Manganese) V. Raghavan

The early experimental data on this system reviewed by [1995Vil] presented a partial liquidus projection and partial isothermal sections at 750, 710, 670, 500, 450, 425 and 400 °C for compositions near the Al-Mg side. Subsequently, [2005Ohn] developed a thermodynamic description of this system with emphasis on Mg-rich alloys. A liquidus projection and an isothermal section at 710 °C were presented in the update of [2007Rag] from the computed results of [2005Ohn]. Very recently, two detailed thermodynamic assessments of this ternary system by [2007Du] and [2009Shu] were published. Both assessments use descriptions of the Al-Mn system, which yield the phase equilibria in Al-rich alloys different from the currently accepted Al-Mn diagram.

Binary Systems

Temperature, °C

Temperature, °C

The Al-Mn phase diagram [1987Mur, 1996Liu, 2006Pre, 2007Du, 2008Gru] depicts the following intermediate phases: Al12Mn (Al12W-type cubic, denoted G), Al6Mn (Al6Mn-type orthorhombic), kAl4Mn (hexagonal, space group P63/m), lAl4Mn (hexagonal, P63/mmc), Al11Mn4(HT) (Al3Mn-type orthorhombic), Al11Mn4(LT) (Al11Mn4-type triclinic), Al8Mn5 (Al8Cr5-type rhombohedral), c (34.552 at.% Mn; bcc) and e (55-72 at.% Mn; cph). Recent

Al

Al

Atomic Percent Manganese

Fig. 1 Al-Mn binary phase diagram in the Al-rich region [2006Pre]

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versions of the diagram [1996Liu, 2007Du] exclude the intermediate phase c1 (Cu5Zn8-type cubic). Also, there is no unanimity about the nature of the phase relationships at the Al-end. The currently accepted diagram in this region [1987Mur, 2006Pre, 2008Gru] is shown in Fig. 1. Al6Mn forms peritectically at 705 °C from liquid (L) and l. The k phase forms peritectoidally at 695 °C from Al6Mn and l. In their experimental investigation of this region, [2007Du] presented a different result. According to them, the k-phase forms peritectically at 721 °C from L and l. Al6Mn also forms peritectically at 703 °C from L and k. Nucleation difficulties pose problems in identifying the correct equilibria. Opinion about the relative ease or difficulty with which Al6Mn or k nucleates has been divided. [2007Du] reassessed the Al-Mn phase diagram, using their new experimental results along with the literature data. Figure 2 shows their computed diagram in full. [2009Shu] also carried out a new assessment of the Al-Mn system, using the modified quasichemical model for the liquid phase. Their computed diagram is compared with that of [2007Du] in Fig. 2. In both the diagrams, the k-phase forms peritectically, which is at variance with the version in Fig. 1. The Al-Mg phase diagram [1998Lia] has the following intermediate phases: Al3Mg2 (Al3Mg2-type cubic, labeled b), R or e (rhombohedral) and Al12Mg17 (A12, aMn-type cubic, denoted c). There are no intermediate phases in the Mg-Mn system, see [2007Du] for the computed diagram.

Mn

Atomic Percent Manganese

Fig. 2 Al-Mn computed binary phase diagram [2007Du, 2009Shu]

Journal of Phase Equilibria and Dif