Solid-State Equilibria in the Iron-Nickel System

Abstract

Critical examination of the literature on the iron-nickel system indicates that the available phase diagram for the solid-state region is inadequate. The reason for this is that solid-state transformations in this system are mostly sluggish (particularly below 500°C) and classical equilibration-anneal methods using specimens from massive alloys are laborious and generally fall short of yielding equilibrium. Equilibria can, however, be readily achieved by preparing the alloys in the form of fine-particle powders by the decomposition-reduction of mixed-crystal iron-nickel formates and/or by distilling off the mercury from mixed iron and nickel amalgams. Room-temperature and high-temperature x-ray diffraction studies of such alloy powders were made to obtain equilibrium data in compositions containing up to ∼65 at. % Ni, at temperatures between 1000° and 120°C. The γ/γ+α boundary as relocated from the results exhibits the following new features: (1) The boundary is concave downwards instead of upwards as in the older diagrams; (2) Three discontinuities are found at (a) ∼3 at. % Ni and ∼760°C, (b) ∼35–43 at.% Ni and 525°C, and (c) ∼330°C and ∼50 at. % Ni. These discontinuities have been interpreted to be indicative of three isobaric isothermal three-phase equilibria hithertofore unsuspected to exist in the Fe–Ni system. The boundary data below 300°C are being reported for the first time for this system. The need for a closer reexamination of the iron-nickel system is strongly indicated from the present results.