Phase Diagrams
Iron-Nickel (Fe-Ni) Phase Diagram
In the field of meteoritics, the equilibrium Fe-Ni phase diagram is of great importance. The phase relations between the alpha phase (kamacite) and the gamma phase (taenite) are best described by means of the Fe-Ni diagram.
In pure iron, the A4 (1394 °C) and A3 (912 °C) transformations take place at constant temperatures. If an element enters into solid solution in iron — forming in that way a binary alloy — each of these transformations are required by the Phase Rule to occur over a range of temperature.
Some elements, such as nickel, raise the A4 and lower the A3 transformation temperatures (see Figure 1), expanding and stabilizing the gamma field in the iron-carbon phase diagram.
Above 912 °C, there is a region of complete solid solubility (the gamma field). Below 912 °C, the alpha phase is stable in pure iron. The gamma phase is stabilized when the amount of nickel is increased.
The transformation of the gamma phase into the alpha phase in the Fe-Ni binary system takes place quite sluggishly, due to the slow diffusion of nickel.
Thus, the phases that form when Fe-Ni alloys are ingot-cast, for example, bear little relation to the equilibrium diagram. When cooled to low enough temperatures, the gamma phase transforms into a supersaturated, non-equilibrium, body-centered cubic (BCC) phase called α2, whose chemical composition is the same as the original gamma phase. The temperature at which the α2 phase forms is referred to as the Ms temperature.
Figure 1. Equilibrium Fe-Ni phase diagram shows which phases are to be expected at equilibrium for different combinations of nickel content and temperature. The Fe-Ni phase diagram was calculated with Thermo-Calc, coupled with TCFE6 thermodynamic database. The melting point of iron and nickel at the pressure of 101325 Pa is 1538 °C and 1455 °C, respectively. The Fe-Ni binary system has a peritectic reaction (liquid + delta = gamma) at 1517 °C and 4.6 wt. % Ni.
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• The phase diagram of Iron-Nickel binary system
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