Towards the ab initio based theory of the phase transformations in iron and steel

Despite of the appearance of numerous new materials, the iron based alloys and steels continue to play an essential role in modern technology. The properties of a steel are determined by its structural state (ferrite, cementite, pearlite, bainite, martensite, and their combination) that is formed under thermal treatment as a result of the shear lattice reconstruction "gamma" (fcc) -> "alpha" (bcc) and carbon diffusion redistribution. We present a review on a recent progress in the development of a quantitative theory of the phase transformations and microstructure formation in steel that is based on an ab initio parameterization of the Ginzburg-Landau free energy functional. The results of computer modeling describe the regular change of transformation scenario under cooling from ferritic (nucleation and diffusion-controlled growth of the "alpha" phase to martensitic (the shear lattice instability "gamma" -> "alpha"). It has been shown that the increase in short-range magnetic order with decreasing the temperature plays a key role in the change of transformation scenarios. Phase-field modeling in the framework of a discussed approach demonstrates the typical transformation patterns

Local deformations and chemical bonding in Fe-X (X = Si, Al, Ga, Ge) soft magnetic alloys

Dilute alloys based on ferromagnetic bcc iron modified by 3p (Al, Si) and 4p (Ga, Ge) elements are studied using the methods of the density-functional theory. It is shown that the local deformations and solution energies depend on the position of an alloying element in the periodic system. The nature of Fe-X chemical bonding varies from weak metallic in Fe-Ga to strong quasi-covalent in Fe-Si, which determines the values of local deformations in these alloys. The formation of pairs of impurity atoms in the position of the next-nearest neighbors leads to tetragonal lattice deformations, the value of which is highest for Si and Ge. The role of local deformations in the formation of unusual magnetic properties of Fe-X alloys is discussed. © 2013 Pleiades Publishing, Ltd

Alloying element segregation and grain boundary reconstruction, atomistic modeling

Grain boundary (GB) segregation is an important phenomenon that affects many physical properties, as well as microstructure of polycrystals. The segregation of solute atoms on GBs and its effect on GB structure in Al were investigated using two approaches: First principles total energy calculations and the finite temperature large-scale atomistic modeling within hybrid MD/MC approach comprising molecular dynamics and Monte Carlo simulations. We show that the character of chemical bonding is essential in the solute–GB interaction, and that formation of directed quasi-covalent bonds between Si and Zn solutes and neighboring Al atoms causes a significant reconstruction of the GB structure involving a GB shear-migration coupling. For the solutes that are acceptors of electrons in the Al matrix and have a bigger atomic size (such as Mg), the preferred position is determined by the presence of extra volume at the GB and/or reduced number of the nearest neighbors; in this case, the symmetric GB keeps its structure. By using MD/MC approach, we found that GBs undergo significant structural reconstruction during segregation, which can involve the formation of single-or double-layer segregations, GB splitting, and coupled shear-migration, depending on the details of interatomic interactions. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.Ministry of Education and Science of the Russian Federation, MinobrnaukaFunding: The research has been performed in the framework of the state assignment of the Ministry of Education and Science of the Russian Federation (topic “Structure”, No. А18-118020190116-6 and “Pressure”, No. А18-118020190104-3)

Effect of impurities on growth and morphology of cementite nanowires

Effects of doping on morphology of iron carbide (cementite) nanowires have been explored by first principle electronic structure calculations. We examined the role of several realistic impurities (Si, Mn, V, P and S) in the formation energies of the cementite nanowires with different sizes and morphologies. It is shown that the presence of the impurities decreases the formation energy and can switch the preferable axis of the cementite nanowire growth. The conditions of the formation and decomposition of cementite nanowires in steels are also discussed.Comment: 12 pages, 3 figures, few refs added, minor correction, final version accepted in J. Phys. Condens. Matte

Effect of magnetism on the solution energy of 3p (Al, Si) and 4p (Ga, Ge) elements in iron

The method based on the density-functional theory has been used to study the solubility of 3p (Al, Si) and 4p (Ga, Ge) elements in ferromagnetic and paramagnetic states of bcc iron. To simulate the paramagnetic state, two different approaches have been employed, which were implemented using the SIESTA and LSGF packages. It has been established that the solution energy of all these impurities decreases upon the transition into the paramagnetic state. The solution energies obtained by averaging over the ensemble of unpolarized magnetic configurations agree well with the values obtained in the coherentpotential approximation. At the same time, the allowance for the magnetic polarization in the vicinity of an impurity leads to a decrease in the solution energy, which is most clearly pronounced at temperatures close to TC. The temperature dependence of the solution energies of the impurities in the paramagnetic state is discussed. © Pleiades Publishing, Ltd., 2013

Molecular dynamic simulation of a homogeneous bcc -> hcp transition

We have performed molecular dynamic simulations of a Martensitic bcc->hcp transformation in a homogeneous system. The system evolves into three Martensitic variants, sharing a common nearest neighbor vector along a bcc direction, plus an fcc region. Nucleation occurs locally, followed by subsequent growth. We monitor the time-dependent scattering S(q,t) during the transformation, and find anomalous, Brillouin zone-dependent scattering similar to that observed experimentally in a number of systems above the transformation temperature. This scattering is shown to be related to the elastic strain associated with the transformation, and is not directly related to the phonon response.Comment: 11 pages plus 8 figures (GIF format); to appear in Phys. Rev.

Autocatalytic mechanism of pearlite transformation in steel

A model of pearlite colony formation in carbon steels with ab-initio parameterization is proposed. The model describes the process of decomposition of austenite and cementite formation through a metastable intermediate structure by taking into account the increase of the magnetic order under the cooling. Autocatalytic mechanism of pearlite colony formation and the conditions for its implementation have been analyzed. We demonstrate that pearlite with lamellar structure is formed by autocatalytic mechanism when thermodynamic equilibrium between the initial phase (austenite) and the products of its decomposition (cementite and ferrite) does not take place. By using model expression for free energy with first-principles parameterization we find conditions of formation of both lamellar and globular structures, in agreement with experiment. The transformation diagram is suggested and different scenarios in the kinetics of decomposition are investigated by phase field simulations.Comment: This work was submitted to Phys.Rev.Applie

Magnetism and local distortions near carbon impurity in γ\gamma-iron

Local perturbations of crystal and magnetic structure of $\gamma$-iron near carbon interstitial impurity is investigated by {\it ab initio} electronic structure calculations. It is shown that the carbon impurity creates locally a region of ferromagnetic ordering with substantial tetragonal distortions. Exchange integrals and solution enthalpy are calculated, the latter being in a very good agreement with experimental data. Effect of the local distortions on the carbon-carbon interactions in $\gamma$-iron is discussed.Comment: 4 pages 3 figures. Final version, accepted to Phys.Rev. Let