Abstract
Conditions for the formation of microconcentration inhomogeneities in Fe–Ni alloys, which can decrease their invar characteristics, are determined. The nickel separation can be reached in the course of short-term annealing as a result of deformation-induced nickel segregation, nickel redistribution between martensite and austenite and between ferrite and austenite as well.
Highlights
The retained structural and concentrational stability of austenitic invar Fe–Ni alloys is the important condition for reaching the required low values of the thermal expansion coefficient (TEC) over a given temperature range
The development of the reverse α → γ transformation with the formation of fine γ-phase plates in slow heating at a rate of 0.2– 0.3 deg/min in a temperature range of 300–490°С is accompanied by enrichment of the formed γ phase in nickel and depletion of the initial α phase of nickel in accordance with the equilibrium Fe–Ni phase diagram [4]
In the high-alloyed invar alloys, in particular, the Fe–36Ni alloy, the α martensite does not form after cooling to negative temperatures and no decomposition into ferrite and austenite occurs after quenching and heating to the two-phase (α + γ) region of the Fe– Ni phase diagram
Summary
The retained structural and concentrational stability of austenitic invar Fe–Ni alloys is the important condition for reaching the required low values of the thermal expansion coefficient (TEC) over a given temperature range (below the Curie temperature of austenite). The expected nickel redistribution upon the formation of ferrite in the Fe–36Ni alloys (in accordance with the equilibrium Fe–Ni phase diagram) indicates the possibility of obtaining the concentration inhomogeneity even in relatively stable invars.
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