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Section II: Phase Diagram Evaluations

Al-Cu-Rh (Aluminum-Copper-Rhodium) V. Raghavan

Partial isothermal sections at 900 and 800 °C were determined by [2000Gru] in the Al-rich region of this ternary system. A decagonal quasicrystalline ternary phase D is stable at these temperatures.

Binary Systems The Al-Cu phase diagram [1998Liu] depicts a number of intermediate phases: CuAl2 (C16-type tetragonal, denoted h), CuAl (g1, orthorhombic) CuAl (g2, monoclinic), Cu5Al4(LT) (f, orthorhombic), e2 (B81, NiAs-type hexagonal), e1(bcc), Cu3Al2 (d, rhombohedral), Cu9Al4(HT) (c0, cubic), Cu9Al4(LT) (c1, D83-type cubic), and Cu3Al (b, bcc). In the above, HT = high-temperature and LT = lowtemperature. The Al-Rh phase diagram [2006Kho] (see Fig. 1 under Al-Pd-Rh) depicts the following intermediate phases: Rh2Al9 (D8d, Co2Al9-type monoclinic), Rh1-xAl3 (orthorhombic, denoted O1 or e16), RhAl3 (orthorhombic, 3, cubic, denoted O2 or e6), Rh2Al5(c) (space group Pm denoted C), Rh2Al5(h) (D811, Co2Al5-type hexagonal, denoted H), Rh3Al7 (monoclinic, denoted V), and RhAl (B2, CsCl-type cubic). The structurally related orthorhombic phases, e6 and e16, have two identical lattice parameters, with a differing third parameter and occur close to the composition RhAl3. Cu and Rh form a continuous facecentered cubic (fcc) solid solution, with a miscibility gap below 1150 °C. In the following text and figures, the symbols O1, O2, C, H, V, and B2 are used to denote the phases.

Fig. 1 Al-Cu-Rh [2000Gru]

Fig. 2

partial

isothermal

section

at

900 °C

Al-Cu-Rh partial isothermal section at 800 °C [2000Gru]

Ternary Isothermal Sections [2000Gru] induction-melted under Ar atm about 20 ternary Al-rich alloys. The alloys were annealed at 900 °C for 70-120 h or at 800 °C for 90-1400 h and quenched in water. The phase equilibria were studied with x-ray powder diffraction, scanning and transmission electron metallography and differential thermal analysis at a heating/cooling rate of 20 °C per min. The local phase compositions were determined by energy dispersive x-ray analysis. The partial isothermal section at 900 °C constructed by [2000Gru] is shown in Fig. 1. The range of the RhAl (B2) phase extends upwards to 55 at.% Al. The binary phases C and O2 dissolve up to 13 and 11 at.% Cu, whereas H dissolves less than 0.5 at.% Cu. At its low Al-end range, C exhibits a superstructure marked C1 in Fig. 1, with a doubling of the cubic lattice parameter. The decagonal phase D has a composition centered around Al64.5Cu16.8Rh18.7. It coexists with the liquid, O2, B2 and C/C1 phases. The partial

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isothermal section at 800 °C constructed by [2000Gru] is shown in Fig. 2. A U-type transition reaction D + L M O2 + B2 occurs between 900 and 800 °C.

References 1998Liu: X.J. Liu, I. Ohnuma, R. Kainuma, and K. Ishida, Phase Equilibria in the Cu-Rich Portion of the Cu-Al Binary System, J. Alloys Compd., 1998, 264, p 201-208 2000Gru: B. Grushko, J. Gwozdz, and M. Yurechko, Investigation of the Al-Cu-Rh Phase Diagram in the Vicinity of the Decagonal Phase, J. Alloys Co