Polymorphism of Iron. Part 1. Contradictions of Martensitic and Polymorphous Transformations

The current state of iron polymorphism is considered as the most significant phase transformation in the industrial technology of iron-carbon alloys, which manifests itself in four forms in the processes of heating and cooling iron. The analysis of literature data revealed the lack of information about the essence of the mechanism of polymorphic transformation of pure iron in domestic basic textbooks and in popular foreign publications on materials science. The main historical stages of the formation and development of the doctrine of iron polymorphism from the moment of its discovery by D.K. Chernov in 1868 to the introduction of the generally accepted diagram by M. Hansen and K. Anderko in 1949 are considered. It is shown that two questions were at the center of scientific discussions: the question is whether iron has polymorphism, and the question of the nature and degree of carbon solubility in polymorphic modifications of iron. The problem of carbon solubility in austenite and ferrite has been a key issue for steelmaking technology. The question of the mechanism of the occurrence of polymorphic transformation, due to its low significance for theory and practice of iron-carbon alloys, has not been widely raised or considered. Consequently, study of the mechanism of polymorphic transformation of pure iron under equilibrium conditions was not relevant for both science and technology of iron-carbon alloys. At present, many researchers believe that, firstly, polymorphic transformation α-Fe ↔ γ-Fe is less studied than similar transformations in other systems, i.e., it is indirectly stated that direct studies of the polymorphism of pure iron have not been carried out at all. Secondly, mechanism of martensitic transformation in steel, which is currently quite well studied, has been arbitrarily transferred to the α-Fe ↔ γ-Fe polymorphic transformation of pure iron. In fact, it is recognized that the mechanism of transformation of supercooled austenite into martensite and polymorphic transformation of pure iron are identical, which is unacceptable. It is well known that the martensitic transformation proceeds by a diffusionless shear mechanism. There are two generally accepted models for the polymorphic transformation for α-Fe ↔ γ-Fe: the shear martensitic mechanism according to the Bain model (tetragonal lattice distortion) and the Kurdyumov-Sachs model (double lattice shift). However, the paradox lies not in these two models, but in the fact that shear martensitic mechanisms were identified with the mechanism of polymorphic transformation of pure iron under equilibrium conditions. On the basis of the performed studies, a logical conclusion is obvious: the mechanisms of both the metastable martensitic transformation in steels and the polymorphic transformation of pure iron in its equilibrium state are based on a single, common diffusion-free deformation transition, but this transition is “triggered” by completely different physical processes, which are absolutely not identical to each other.