Calorimetric Studies of Polymorphic Iron Transformation

Abstract

Iron polymorphism, as a basic phase transformation in the industrial technology of iron-carbon alloys, manifests itself in three forms in iron heating and cooling. An analysis of the literature data revealed a lack of information about the mechanism of pure iron polymorphic transformation. In this work, an experimental verification of the second polymorphic transformation α-Fe ↔ γ-Fe is carried out using high-resolution differential scanning calorimetry (DSC). The object of the study are samples made from wire of technically pure iron (TPI–99.88% Fe) and high-purity zone-purified iron (ZPI–99.995% Fe). Heating and cooling are carried out in an argon atmosphere (99.9995% Ar). Based on the analysis of DSC curves, the following results are obtained: the value of hysteresis of the polymorphic transformation during thermal cycling is ~11°C; the iron polymorphic transformation is not reversible upon heating and cooling and occurs through different mechanisms; existing ideas about the reversible polymorphic phase transformation of α-Fe ↔ γ-Fe upon heating as phase recrystallization within the framework of a deformation (distortion) transition are untenable. It is experimentally proven that the pure iron polymorphic transformation under equilibrium conditions upon heating proceeds diffusion-free with a sequential change of three phase transformations of different origins. Jump in atomic volumes of polymorph phases α-Fe ↔ γ-Fe in the region of phase transformation is explained by indirect shear plastic transformation of the α-Fe polymorph crystal lattice into the γ-Fe polymorph crystal lattice, as is customary to date, but by the sequential destructuring of the α-Fe polymorph into a mixture of “paracluster” and amorphous phases. Within the framework of these studies, the following unsolved problems are identified: firstly, the process during which a significant amount of activation energy G is absorbed, is unclear: for TPI 2300 ± 150 kJ/mol, for ZPI 2400 ± 200 kJ/mol; secondly, there is no explanation for the mechanism of the iron crystal lattice transition to the amorphous state when heated; thirdly, the temperature of the polymorphic transformation range in carbon steel 20 does not coincide with the similar temperature range on the Fe–C diagram.