Towards a True Fe-Ni Phase Diagram
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TOWARDS A TRUE FE-NI PHASE DIAGRAM
P L. Rossiter and R. A Jago Department of Materials Engineering, Monash 3168, Australia
University, Clayton,
Victoria
ABSTRACT A modification to the existing Fe-Ni phase equilibrium diagram is proposed that takes account of the low-temperature ordering reaction to FeNi. It is shown that true equilibrium is never attained during slow cooling of Fe-Ni alloys, even for iron meteorites (which cool extremely slowly). In all real cases, a metastable phase diagram applies, in which the depressed Y/-i.'r solvus produces a more extensive r+ FeNi phase field than for the equilibrium case. This enlarged phase field is used to explain the decomposition of supersaturated Fe-Ni toYr+ FeNi, which is observed only in iron meteorites.
SOLID STATE FE-NI PHASE EQUILIBRIA Despite a period of study stretching back to ca. 1800, only the most rudientary details of phase equilibria in the Fe-Ni system below 500C are as yet available. The difficulty in obtaining precise data is basically due to the extremely low diffusivities of the species below this temperature, but is exacerbated by the similarity of the atomic scattering factors of Ni and Fe, which impose limitations on the usefulness of diffraction experiments. Goldstein and coworkers have continued to refine the positions of phase boundaries at the Fe-rich end of the diagram, and a recent diagram [I], incorporating other results [2], is shown in Fig. I. The diagram shows a large oC+ Yphase field, although the oS. phase does not form in laboratory specimens prepared by cooling from the Y phase [3]. Unless special methods are employed [2-6] to facilitate diffusion, therefore, unequilibrated single-phase Yalloys are usually obtained.
900 700 °c 500 300
0
20
40
60
80
wt. % Ni
Fig. 1. Fe-rich portion of a recent Fe-Ni incorporating additional data from Ref. 2.
phase
100
equilibrium
diagram
[1],
Once supersaturated alloys reach the temperature denoted "M " in Fig. 1, they transform easily to a distorted bcc phase by a diffusionless, shear transformation. The product phase, "martensite", is metastable and easily decomposed into the equilibrium phases (0(+ V) by heating to relatively low Mat. Res. Soc.
Symp. Proc. Vol.
21 (1984) QElsevier Science Publishing Co.,
Inc.
408
temperatures. This reaction path has been utilised in at least two studies [3,4] to determine the extent of the a-+ Y'region of the phase diagram. An equilibrium diagram incorporating a eutectoid reaction at 345eC and 52 at.% Ni was advanced by Heumann and Karsten [6]. The proposal was based on an examination of relatively few specimens equilibrated within a very restricted temperature range, and has not met with general acceptance. Further, recent results of the present authors [7] indicate that Fe-Ni alloys prepared by techniques involving heating at low vapour pressures are likely to contain the spinel (NiFe) 3 0,0 which can lead to spurious results. The techniques employed by the former authors do, however, permit examination of a relatively inaccessible region
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