Al-Hf-Ni (Aluminum-Hafnium-Nickel)

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Section II: Phase Diagram Evaluations

Al-Hf-Ni (Aluminum-Hafnium-Nickel) V. Raghavan

The review of this ternary system by [1991Lee] presented a tentative liquidus projection and three isothermal sections at 1200, 1000, and 800 C. The update by [2006Rag] reviewed the partial isothermal section at 1130 C determined by [1997Nag] and the liquidus and solidus surfaces of the Ni-based face-centered cubic (fcc) solid solution from [1999Miu]. Recently, [2008Zha] carried out a thermodynamic assessment of the Ni-rich region of this system using new experimental data as additional input in the optimization and presented a liquidus projection and three isothermal sections at 1160, 1130, and 1000 C.

Computed Ternary Phase Equilibria

The Al-Hf system depicts a number of intermediate phases, see [2006Rag] for a listing. The Al-Ni phase diagram [1993Oka] shows five intermediate phases: NiAl3 (D011, Fe3C-type orthorhombic), Ni2Al3 (D513-type hexagonal), NiAl (B2, CsCl-type cubic, denoted b), Ni5Al3 (Ga3Pt5-type orthorhombic), and Ni3Al (L12, AuCu3-type cubic, denoted c¢). The Hf-Ni phase diagram [2001Wan] depicts the following intermediate phases: Hf2Ni (C16, CuAl2-type tetragonal), aHfNi (Bf, CrB-type orthorhombic), bHfNi, Hf9Ni11 (tetragonal), Hf7Ni10 (orthorhombic), Hf3Ni7 (triclinic), Hf8Ni21 (triclinic), bHfNi3 (hexagonal), aHfNi3 (BaPb3-type rhombohedral), Hf2Ni7 (monoclinic), and HfNi5 (C15b, AuBe5-type cubic).

With starting metals of 99.99% Al, 99.9+% Hf, and 99.995% Ni, [2008Zha] arc-melted 5 Ni-rich alloys. The alloys were annealed at 1160 C for 1 month. The phase equilibria were studied with scanning electron microscopy (in the back-scattered electron imaging mode) and by electron probe microanalysis. The compositions of the co-existing phases in the annealed samples were listed. [2008Zha] modeled the liquid, (Al), (Hf), and (Ni) phases as substitutional solutions. Among the compounds, the focus of study was on the stoichiometric HfNi5 and on Hf2Ni7, and Ni3Al (c¢). The last two have homogeneity ranges and were described by a two sublattice model. HfNi5 and Hf2Ni7 dissolve a negligible amount of Al. The optimized interaction parameters, which include ternary parameters for liquid and (Ni), were listed. For Ni-rich alloys, three isothermal sections at 1160, 1130 and 1000 C and a liquidus projection were computed by [2008Zha]. At 1160 C (Fig. 1), the computed isothermal section shows good agreement with the experimental tie-lines determined by them. At 1130 C (Fig. 2), the computed equilibria are compared with the experimental tie-lines of [1997Nag] and the agreement is satisfactory. At this temperature, a fourphase equilibrium of (c + c¢ + HfNi5 + Hf2Ni7) is seen, as suggested by [1997Nag]. At 1000 C (Fig. 3), the computed diagram is compared with a single experimental tie-line from [1981Nas]. The primary phases of crystallization in the

Fig. 1 Al-Hf-Ni computed isothermal section for Ni-rich alloys at 1160 C [2008Zha]

Fig. 2 Al-Hf-Ni computed isothermal section for Ni-rich alloys at 1130 C [2008Zha]

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