Experimental determination and thermodynamic evaluation of low-temperature phase equilibria in the Fe–Ni binary system

Phase equilibria between αFe and γ(Fe,Ni) in the Fe–Ni binary system at low temperatures above 400 °C were determined experimentally, and thermodynamic descriptions to calculate the Gibbs energy of involved phases were revised with taking into account the interaction effect between the chemical and magnetic orderings. Heat-treatment for equilibration of gas-atomized powder of Fe–Ni alloys without deformation resulted in the formation of a microstructure consisting of directional γ precipitates in α' martensite matrix. Converge-milling processing, a kind of mechanical alloying technique, in advance of heat-treatment, was applied to introduce a large number of lattice defects into powder particles, which enhanced α + γ recrystallization to form an equiaxed dual-phase polycrystalline microstructure. Equilibrium compositions of the equiaxed grains of the α and γ phases were measured by an FE-EPMA, the spatial resolution of quantitative analysis of which is 0.5 μm in diameter with accelerating voltage of 6 kV. Equilibrium compositions of the γ phase coincided with the phase diagram assessed in accordance with experimental results in the literature. On the other hand, the solubility of Ni in the α phase exhibited considerable deviation from “retrograde solubility” determined according to previous data. Thermodynamic analysis with revised parameters suggests that the underestimated solubility of Ni in the αFe phase is caused by the additional Gibbs energy due to lattice defects in the α’ martensitic structure.