87 research outputs found
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
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. Β© Pleiades Publishing, Ltd., 2013
Solid solution decomposition and Guinier-Preston zone formation in Al-Cu alloys: A kinetic theory with anisotropic interactions
Using methods of statistical kinetic theory parametrized with
first-principles interatomic interactions that include chemical and strain
contributions, we investigated the kinetics of decomposition and microstructure
formation in Al-Cu alloys as a function of temperature and alloy concentration.
We show that the decomposition of the solid solution forming platelets of
copper, known as Guinier-Preston (GP) zones, includes several stages and that
the transition from GP1 to GP2 zones is determined mainly by kinetic factors.
With increasing temperature, the model predicts a gradual transition from
platelet-like precipitates to equiaxial ones and at intermediate temperatures
both precipitate morphologies may coexist.Comment: 9 pages, 8 figure
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
Intrinsic nanoscale inhomogeneity in ordering systems due to elastic-mediated interactions
Phase diagram and pattern formation in two-dimensional Ising model with
coupling between order parameter and lattice vibrations is investigated by
Monte-Carlo simulations. It is shown that if the coupling is strong enough (or
phonons are soft enough) a short-range order exists in disordered phase for a
broader temperature interval. Different types of this short-range order
(stripe-like, checkboard-like, etc.) depending on the temperature and model
parameters are investigated. With further increase of the coupling, a
reconstruction of the ground state happens and new ordered phases appear at low
enough temperatures.Comment: final version, Europhys. Lett., accepte
Topological defects, pattern evolution, and hysteresis in thin magnetic films
Nature of the magnetic hysteresis for thin films is studied by the
Monte-Carlo simulations. It is shown that a reconstruction of the magnetization
pattern with external field occurs via the creation of vortex-antivortex pairs
of a special kind at the boundaries of stripe domains. It is demonstrated that
the symmetry of order parameter is of primary importance for this problem, in
particular, the in-plane magnetic anisotropy is necessary for the hysteresis.Comment: Accepted to EPL; 7 pages, 3 color figure
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