Al-Fe-Ni (Aluminum-Iron-Nickel)
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Section II: Phase Diagram Evaluations
Al-Fe-Ni (Aluminum-Iron-Nickel) V. Raghavan
The previous review of this system by [1988Ray] presented liquidus and solidus projections for Al-poor Fe-Ni alloys and for compositions near the Al corner, a reaction sequence for the solidification reactions, isothermal sections at 1250, 950, 850, and 750 °C for Al-poor compositions, and a section at 600 °C near the Al corner. An update by [1994Rag] presented isothermal sections at 1050 and 950 °C from [1982Kha] and a vertical section along the Ni3Al-Ni3Fe join from [1987Mas]. A number of new reports on the phase equilibria of this system have since appeared.
Binary Systems The Al-Fe phase diagram [1993Kat] shows that the facecentered cubic (fcc) solid solution based on Fe is restricted by a ␥ loop. The body-centered cubic (bcc) solid solution exists in the disordered A2 form (␣), as well as the ordered B2 and D03 forms. Apart from the high-temperature phase , there are three other intermediate phases in this system: FeAl2 (triclinic), Fe2Al5 (70-73 at.% Al, orthorhombic), and FeAl3 or Fe4Al13 (74.5-76.6 at.% Al, monoclinic). The AlNi phase diagram [1993Oka] depicts five intermediate phases: Ni3Al (23-28 at.% Al, AuCu3-type cubic, denoted ␥⬘), Ni5Al3 (32-36 at.% Al, Ga3Pt5-type orthorhombic), NiAl (31-58 at.% Al, CsCl-type cubic), Ni2Al3 (58-63 at.% Al, D513 type hexagonal); and NiAl3 (75 at.% Al, Fe3C-type orthorhombic). Recently, [2001Miu] reinvestigated the liquidus in the Ni-rich region of the Al-Ni diagram. [2002Bit] reassessed the solidus of the AlNi (B2) phase in this binary system. The Fe-Ni phase diagram [1993Swa] is characterized by a very narrow solidification range with a peritectic reaction at 1514 °C, between bcc ␦ and liquid, that yields the Fe-based fcc solid solution. A continuous solid solution denoted ␥ between fcc Fe and Ni is stable over a wide range of temperatures. At 517 °C, an ordered phase FeNi3 forms congruently from ␥.
According to [1996Lem], this phase forms at 930 °C through a ternary peritectic reaction (FeAl3 + Ni2Al3 + L ↔ Q) and decomposes through a ternary eutectoid reaction at 847 °C (Q ↔ Ni2Al3 + NiAl3 + FeAl3). It may be noted that [1982Kha] reported the peritectic formation of Q at 925 °C and did not present an isothermal section at 900 °C, pending confirmation of the stable presence of Q.
Isothermal Sections [1994Jia] studied the partitioning of alloying elements among the ␥, ␥⬘, and B2 phases in Ni-Al base systems. Diffusion couples were prepared from induction-melted alloys and were annealed between 1300 and 800 °C for 10 to 1000 h. The composition of the coexisting phases was measured by electron probe microanalysis and listed. In the equilibrium of ␥⬘/␥, Fe segregates preferentially in the ␥ phase. The partition coefficient KFe for ␥⬘/␥ equilibrium decreases from 0.75 at 1300 °C to ∼0.33 at 800 °C. In the equilibrium of ␥⬘/B2, Fe segregates preferentially in the B2 phase. Here, KFe decreases from ∼0.9 at 1300 °C to ∼0.75 at 900 °C. [1994Jia] constructed partial isothermal sections at
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