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Section II: Phase Diagram Evaluations

Al-Ni-Rh (Aluminum-Nickel-Rhodium) V. Raghavan

Very recently, [2007Prz] investigated the phase relationships in the Al-rich region of this ternary system and presented partial isothermal sections at 1080, 1000, 900, and 800
C. Besides the quasicrystalline decagonal phase D, a ternary phase of hexagonal symmetry labeled v and another phase labeled x of unknown structure were found.

Binary Systems The Al-Ni phase diagram [1993Oka] shows five intermediate phases: NiAl3 (D011, Fe3C-type orthorhombic), Ni2Al3 (D513-type hexagonal, denoted d), NiAl (B2, CsCl-type cubic, denoted b), Ni5Al3 (Ga3Pt5-type orthorhombic), and Ni3Al (L12, AuCu3-type cubic, denoted c¢). The Al-Rh phase diagram [2006Kho] depicts the following intermediate phases: Rh2Al9 (D8d, Co2Al9type monoclinic), Rh1-xAl3 (orthorhombic, denoted O1 or e16), RhAl3 (orthorhombic, denoted O2 or e6), Rh2Al5(c) (space group Pm 3, cubic, denoted C), Rh2Al5(h) (D811, Co2Al5-type hexagonal, denoted H), Rh7Al3 (monoclinic, denoted V) and RhAl (B2, CsCl-type cubic). Ni and Rh form a continuous face-centered cubic solid solution.

Fig. 1 Al-Ni-Rh isothermal section at 1080
C for Al-rich alloys [2007Prz]

Ternary Phase Equilibria The decagonal D phase has a basic periodicity of 0.4 nm. The v phase forms between Al76Ni4Rh20 and Al76Ni13Rh11 and has the hexagonal lattice parameters of a = 1.2229 nm and c = 2.7158 nm at the composition Al76Ni8Rh16. The phase x of unknown structure forms around Al70Ni11Rh19 [2007Prz]. With starting metals of 99.999% Al, 99.99% Ni, and 99.95% Rh, [2007Prz] induction-melted under Ar atm a number of alloys. The alloys were annealed at 1080-800
C for 24-3168 h and quenched in water. The phase equilibria were studied with scanning and transmission electron microscopy, x-ray powder diffraction, energy dispersive x-ray spectroscopy, and differential thermal analysis at heating/cooling rates of 10-50
C per min. For each alloy, the phases identified and their compositions were listed. All the structurally related e phases were clubbed together and labeled e. The isothermal sections for Al-rich alloys constructed by [2007Prz] at 1080, 1000, 900, and 800
C are redrawn in Fig. 1-4. At 1080
C (Fig. 1), the decagonal D phase is stable around the composition Al71Ni18Rh11 and forms tie-lines with e, Ni2Al3, and liquid. The x phase is present around Al70Ni11Rh19. Ni2Al3 dissolves up to 4 at.% Rh. The e, C, and V phases dissolve up to 12.5, 10.5, and 3 at.% Ni, respectively. The B2 phases NiAl and RhAl probably form a

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Fig. 2 Al-Ni-Rh isothermal section at 1000
C for Al-rich alloys [2007Prz]

continuous solid solution (denoted B2), which extends up to 60 at.% Al [2007Prz]. At 1000
C (Fig. 2), the D phase is not stable. The third component solubility in e and Ni2A3 has slightly increased, whereas it remains almost the same in C, V and x phases.

Journal of Phase Equilibria and Diffusion Vol. 29 No. 3 2008

Phase Diagram Evaluations: Section II

Fig. 3 Al-Ni-Rh isothermal section at 900
C for Al-rich alloys [2