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JPEDAV (2007) 28:464–468 DOI: 10.1007/s11669-007-9142-6 1547-7037 ASM International

Al-Gd-Mg (Aluminum-Gadolinium-Magnesium) V. Raghavan

Several reports on the phase equilibria of this ternary system have appeared in recent years [1996Rok, 1997Rok, 2000Can, 2000Gro, 2001Gro, 2003Den, 2003Cac]. The system is characterized by the presence of a ternary phase Al4GdMg (s) and extensive solubility of the third component in the binary compounds GdMg and GdAl2.

Binary Systems The Al-Gd system [2003Den, 2003Cac] has the following intermediate phases: GdAl3 (D019, Ni3Sn-type hexagonal), GdAl2 (C15, MgCu2-type cubic), GdAl (ErAl-type orthorhombic), Gd3Al2 (Zr3Al2-type tetragonal), and Gd2Al (C23, Co2Si-type orthorhombic). The Al-Mg phase diagram [1998Lia] shows the following intermediate phases: Mg2Al3 (cubic, denoted b), R or e (rhombohedral) and Mg17Al12 (A12, aMn-type cubic, denoted c). The Gd-Mg phase diagram [2003Den, 2003Cac] has four intermediate phases: GdMg5 (GdMg5-type cubic), GdMg3 (D03, BiF3-type cubic), GdMg2 (C15, MgCu2-type cubic), and GdMg (B2, CsCl-type cubic).

Ternary Phase Equilibria With starting metals of 99.99% Al, 99.85% Gd, and 99.96% Mg, [1996Rok] and [1997Rok] melted Gd-lean alloys in a resistance furnace or an induction furnace under Ar atm. For isothermal studies, [1996Rok] annealed alloys at 400
C for 100 h. For determining the thermal arrests, [1997Rok] employed differential thermal analysis (DTA) at a heating/cooling rate of 18
C per min. The phase equilibria were studied with optical and scanning electron microscopy and x-ray diffraction. From the results of about 60 ternary alloys, [1996Rok] constructed an isothermal section at 400
C. The main feature of this section is the presence a ternary compound Al4GdMg (s) with the MgZn2-type hexagonal structure and lattice parameters of a = 0.552 nm and c = 0.888 nm [1996Rok]. From the DTA results and metallographic observations, [1997Rok] constructed a liquidus projection, which depicts nine four-phase invariant reactions. [1997Rok] also constructed three vertical sections at a constant Al content of 50 mass%, and along the Mg-GdAl2 and the Al32Mg-Al25Gd (mass%) joins respectively. [2000Gro] and [2001Gro] carried out thermodynamic calculations of the phase equilibria of this system. The calculations of [2001Gro] indicated that the ternary liquidus temperatures are much higher than what was reported by [1997Rok] and that the thermal arrests below 700
C observed by [1997Rok] are to be attributed to other phase

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equilibria involving the liquid phase (below the liquidus). To check the thermodynamic predictions, [2001Gro] designed key experiments with three alloys. Using starting metals of 99.8% Al, 99.95% Gd and 99.98% Mg, the alloys were made by arc-melting under Ar atm and were subjected to DTA measurements and x-ray diffraction studies. A liquidus projection, an isothermal section at 400
C and four vertical sections at a constant 9 mass % Mg, 50 mass % Al, and 70 mass % Al and along the Mg-GdAl2 join respectively were computed by [2001Gr