Some discussion topics on the representation of the equilibrium Fe–Ni phase diagram and on the nature of Invar effect

This paper presents a thorough analysis of Fe–Ni phase diagrams published over the last 50 years. It is shown that the differences in drawing the low-temperature part of the phase diagram and in the explanation of the nature of Invar alloys are due to a lack of clear experimental evidence for the absence or occurrence of a particular superstructure in the range of compositions with less than 50% Ni content. The experimental results obtained favor a certain particular phase diagram as yet unpublished in the reference literature, and suggest the possible origin of the Invar effect. The phase diagram analysis and the assumption on the nature of the Invar effect are based on results obtained by structural methods.