On Unusually Low Thermal Expansion Found in the Irreversible Iron-Nickel Alloys

Unusually low thermal expansion was found out from some of the irreversible iron-nickel alloys, and its origin was investigated as a link of the Invar-problem. As the internal stresses were accumulated in these alloys by cold-work or, especially, by repeated thermal cycles between the martensitic transformations \(α{}γ\), the quantity of transformation from the γ phase to the α phase decreased till the alloys could not transform to the α phase but wholly retained the distorted γ' state. And it was noticeable that this distorted state exhibited the Invar-like almost zero thermal expansion at a wide temperature range. For looking into this phenomenon further, the X-ray investigation as well as the thermal dilatation measurements were carried out. Therefrom it was concluded that the distorted γ' state, which is composed of various intermediate structures such as produced on the way to the martensitic transformation γ→α, gives rise to various regions with different Curie points, especially, more regions with lower Curie points; it is such an inhomogeneous state of magnetization to cause the anomalous properties in the Invar alloys.