The constitution of the ternary system Fe–Ni–Si

The ternary system Fe–Ni–Si was investigated using X-ray diffraction (XRD), metallography (SEM), energy dispersive spectroscopy (SEM/EDS), magnetic susceptibility measurements, and differential thermal analysis (DTA). The existence of the ternary phase τ is corroborated and its crystal structure is confirmed to be isostructural to Cr3Ni5Si2. At 700 °C the homogeneity of this phase extends from 43 to 55 at.% iron at constant 20 at.% Si. Furthermore, for the binary phases NiSi2, Ni2Si and Ni31Si12 extensive solubilities for iron are found, while the solubility of Fe in NiSi, Ni3Si2 or Ni3Si does not exceed 5 at.%. On the other side of the system, the phases FeSi and Fe3Si have large solubilities for Ni, while this is rather limited in FeSi2. The (binary high temperature) phase Fe5Si3 is stabilized by Ni at least down to 700 °C. Tie lines exist between τ and solid solutions based on Fe3Si, Ni31Si12 as well as γ (Ni). DTA measurements for the vertical section Fe2Si–Ni2Si strongly indicate that Fe2Si and θNi2Si form a complete series of solid solutions θ(Fe, Ni)2Si and are thus isostructural. A critical evaluation of the available crystal structure data for Fe2Si supports this assumption. Iron rich θ(Fe, Ni)2Si is a weak ferromagnet. The magnetic moment of ∼1 μB per Fe-atom in θ(Fe, Ni)2Si is only half of the magnetic moment per Fe-atom measured for τ Fe5Ni3Si2. A reaction scheme accounting for all DTA data of the entire system is established and in combination with observations on the first crystallizing phase(s) the liquidus surface of Fe–Ni–Si is revised.