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Phase Diagram Evaluations: Section II

Al-Ir-Nb (Aluminum-Iridium-Niobium) V. Raghavan

The previous work on this system by [1976Hor] reported several ternary compounds in the isothermal section at 1100 °C of the Nb-rich region. Recently, [2005Miu] determined a partial liquidus projection and an isothermal section at 1650 °C for Ir-rich alloys of this system.

Binary Systems The Al-Ir phase diagram in the Ir-rich region determined by [2005Miu] depicts a eutectic reaction between (Ir) and IrAl (B2, CsCl-type cubic) at 30.5 at.% Al and 2020 °C. The other Al-Ir phases are: Ir2Al5 (cubic, space group Pm 3n), IrAl3 (D018, Na3As-type hexagonal), Ir4Al13 (monoclinic) and Ir2Al9 (D8d, Co2Al9-type monoclinic). The Ir-Nb phase diagram [Massalski2] has the following intermediate phases: Ir3Nb (L12, AuCu3-type cubic), a2 (39-45.5 at.% Nb; orthorhombic), a1 (47-48 at.% Nb; L10, AuCu-type tetragonal), r (58.5-67 at.% Nb; D8b, rCrFe-type tetragonal), and IrNb3 (A15, Cr3Si-type cubic).

alloy: 70Ir-20Al-10Nb (atomic percent). The sample was annealed at 1650 °C for 24 h. The phase equilibria were studied with scanning electron metallography, wavelength dispersive x-ray spectroscopy and differential thermal analysis. The partial liquidus projection constructed by [2005Miu] is shown in Fig. 1. In the Ir-rich region, a U-type transition reaction L + Ir3Nb M (Ir) + IrAl is postulated by [2005Miu]. Figure 2 shows the partial isothermal section at 1650 °C. In the solid solution based on Ir3Nb, Al substitutes for Nb up to the solubility limit of 7.3 at.%. No ternary phases were found in this region. Figures 1 and 2 may be considered tentative, in view of the very limited experimental results.

References

With starting metals of 99.99% Al, 99.9% Ir, and 99.9% Nb, [2005Miu] arc-melted under Ar atm an Ir-rich ternary

1976Hor: R. Horyn, The Niobium-Aluminum-Iridium Ternary System. Part II. X-ray Study of the Intermediate Phases and Phase Equilibria at 1100 °C, J. Less-Common Met., 1976, 45, p 315-321 2005Miu: S. Miura, K. Ohkubo, Y. Terada, Y. Kimura, Y. Mishima, Y. Yamabe-Mitarai, H. Harada, and T. Mohri, Phase Equilibria in the Ir-Rich Portion of the Ir-Al-X (X: V, Nb and Ta) Ternary Systems, J. Alloys Compd., 2005, 395, p 263-271

Fig. 1 Al-Ir-Nb partial liquidus projection for Ir-rich alloys [2005Miu]

Fig. 2 Al-Ir-Nb partial isothermal section at 1650 °C for Ir-rich alloys [2005Miu]

Ternary Phase Equilibria

Journal of Phase Equilibria and Diffusion Vol. 29 No. 4 2008

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