Peculiarities of phase diagram of Fe-Ni system at Ni concentrations 0 – 20 at.%.

The phase boundaries of martensitic transformation in Fe-Ni alloys with the composition in the range of 0 - 20 Ni at.% have been determined by X-ray diffraction, differential scanning calorimetry, measurements of magnetic susceptibility and magnetization. It is shown that the alloys quenched from 1100 °C at room temperature are in a single-phase state of α ₂ martensite, except for Fe ₈₀Ni ₂₀, which additionally contains a small amount of γ phase. It is found that in the Fe ₉₇Ni ₃ alloy, the temperature-induced magnetic and structural transformations occur independently of each other. At a nickel concentration from 5 to 20%, the structural α → γ transformation upon heating is accompanied by the magnetic transition from the ferromagnetic to the paramagnetic phase, which is a first-order phase transition. When cooling the alloys with x ≤ 10, the paramagnet - ferromagnet transition starts several degrees ahead of the structural γ → α transformation. The lattice volume changes and thermal effects upon the martensitic transformation are determined. It is shown that the phase diagram including metastable martensitic transformation can be used together with the equilibrium phase diagram to determine the structural states of the Fe-Ni system.