A study of microstructures responsible for the emergence of the invar and permalloy effects in Fe–Ni alloys

• Phase transition ‘ordering-phase separation’ occurs in Fe–Ni alloys at about 600 °C.
• A modulated structure is the main reason of the invar properties of Fe–Ni alloys.
• A two-phase fully coherent γ + L12 structure is the main cause of permalloy properties.

The paper presents experimental results obtained by transmission electron microscopy and X-ray photoelectron spectroscopy on the Fe68Ni32 and Fe23Ni77 alloys, which indicate that in these alloys, the phase transition ordering-phase separation occurs in the temperature range near 600 °C. Above the transition temperature, the sign of the ordering energy is positive; clusters enriched in atoms of one of the components are observed in the structure of the alloys. A modulated microstructure is formed after heat treatment at temperatures, at which the invar effect in the Fe68Ni32 alloy is the maximum. Below the transition temperature, the sign of the ordering energy is negative, which determines a tendency to form chemical compounds. After aging at these temperatures, when the Fe23Ni77 alloy has high permalloy properties, highly-dispersed fully coherent particles of the L12 (γ′) phase are found to precipitate in the solid solution.