L. E. Kar’kina

Dislocation structure of cementite in granular pearlite after cold plastic deformation

Cementite microstructure of the U8 steel with a granular pearlite structure has been investigated by the method of electron microscopy. It has been established that, at the early stages of deformation, carbide particles are deformed through the movement of stacking faults, which are characterized by an α[010] partial shift in the (001) planes of cementite. The Burgers vector, the slip plane [010](001) of the split dislocations forming pileups, and deformation bands have been determined using gb analysis. The stacking fault energy has been estimated in a (001) cementite plane: γsf ∼ 12.8 mJ/m2. With increasing degree of deformation, an additional slip has been shown to occur in cementite by the system [100](011).

Effect of chromium on the electronic structure of the cementite Fe3C

The effect of alloying of the cementite Fe3C with chromium on the band structure, atomic interactions, electric field gradients, and asymmetry parameters for iron nuclei is investigated using the self-consistent full-potential linear muffin-tin orbital (FPLMTO) method. An increase in the cohesive energy for the Fe3C-Cr system indicates an enhancement of the atomic interactions in the lattice of the cementite alloyed with chromium. It is found that the substitution of chromium for iron in the FeII positions containing eight equivalent iron atoms is energetically most favorable.

Structural transformations in Fe-Ni-alloy nanoclusters: Results of molecular-dynamic-simulation

Size effect in Fe-Ni-alloy nanoclusters has been studied by the molecular-dynamics (MD) method using multiparticle interatomic interaction potentials. It is shown that the α-γ transformation in nanoparticles with sizes d>3.5 nm proceeds by the mechanism of nucleation at grain boundaries and propagation of fcc-phase plates. As a result of the transformation, a twinned lamellar domain structure is formed. In particles with sizes 3.0<d<3.5 nm, the α-γ transformation is accompanied by radial-symmetry atomic movements that are close to those characteristic of the Bain scheme. This results in the formation of a single-domain fcc phase. In nanoparticles with sizes 1.5<d<3.0 nm, the α-γ transformation proceeds via an intermediate state that is retained within a temperature range of a few hundreds of kelvins and is characterized by an incomplete phase transformation. It has been found that in Fe-Ni clusters with sizes d≤1.5 nm, the α-γ transformation does not occur. During heating, the initial bcc configuration turns into an icosahedral one through polytetrahedral or amorphous-like configuration.

Effect of cold plastic deformation on the structure of granular pearlite in carbon steels

Carbon steel with a structure of granular pearlite has been investigated by methods of scanning electron microscopy and transmission electron microscopy as a function of the degree of cold deformation. It has been shown that with increasing degree of deformation the microstructure of the ferritic constituent changes regularly from dislocation to cellular and to microbanded. It has been established that the structural state of the granular pearlite is stable upon deformation to 50%. An increase in the carbon concentration in the ferrite occurs. Fragmentation of carbide particles into blocks and their subsequent dissolution hawe been observed upon deformation exceeding 50%. Fine-dispersed globular carbides are formed in the ferrite matrix.

Molecular dynamics simulation of the formation of twin boundaries during agglomeration of nanoparticles

The processes controlling early stages of agglomeration of nanoparticles have been investigated by the molecular dynamics method. It has been established that the formation of boundaries with twin misorientation is the main mechanism of structural relaxation during primary agglomeration of nanoparticles. It has been shown that an increase in the temperature leads to an increase in the number of twin boundaries and that their mutual arrangement depends on the misorientation of the nanoparticles. In the case where twin boundaries are noncoplanar, structure relaxation results in the formation of pentagonal twin boundaries. The role of twinning in the formation of interfaces upon compaction of nanoparticles has been discussed.

Computer simulation of carbon diffusion near b/2[010](001) dislocation in cementite

Partial contributions Ui to the activation energy for the diffusion of carbon atoms in the Fe3C lattice have been calculated. The Ui values have been compared upon the migration of carbon atoms in the ideal lattice, near the stacking-fault plane, and near the core of a partial edge dislocation with a Burgers vector b/2[010]. The most preferable ways of the migration of carbon atoms near the studied structural defects in the (001) cementite plane have been revealed. The values of the stacking-fault energy in this plane have been calculated. The possibility of splitting the dislocation with a Burgers vector b/2[010] into two partial dislocations has been shown.

Atomistic simulation of stacking faults in (001), (010), and (100) planes of cementite

Molecular-dynamics method was used to study γ surfaces for the (001), (010), and (100) planes of cementite. Displacement vectors corresponding to stable stacking faults have been determined. The energy of these stacking faults has been calculated by the molecular-dynamics and ab initio methods. The energy of unstable stacking faults, which characterizes the tendency of a material to plastic relaxation, has been estimated. The reactions of the splitting of perfect dislocations have been suggested; the possibility of the propagation of stacking faults in the planes under consideration is discussed.

Study of the Hardness and Elastic Modulus of Cementite in the Structure of Granular Pearlite by the Nano-Indentation Method

The method of nano-indentation is used to study the microhardness and modulus of elasticity of cementite in the structure of granular pearlite in the initial condition (after spheroidizing annealing) and after cold plastic deformation with ε = 34% and ε = 65%. It is shown that both characteristics of the cementite vary non-monotonically with an increase in strain. The results are compared with electron microscope data on the evolution of the deformed structure of granular pearlite.

Atomistic simulation of stacking faults in cementite: Planes containing vector [010]

The method of molecular dynamics (MD) has been used to study γ surfaces in (103), (101), (102), (201), and (301) planes of cementite, which contain the Burgers vector [010] of the perfect dislocation. Displacement vectors that correspond to stable stacking faults (SFs) and the energies of these SFs have been determined. The energies of unstable SFs, which characterize the tendency of the material toward plastic relaxation, have been estimated. Reactions of dissociation of the [010] dislocation into two partials in planes (101) and (103) have been suggested. It has been established that the planes (103) are characterized by a large number of local minima with a low energy.

Process of faceting in nanoparticles of FCC metals: Results of simulation by the molecular-dynamics method

The process of formation of facets (faceting) in Ni, Al, and Au nanoparticles has been investigated by the molecular-dynamics method. It has been established that the surface of nanoparticles of fcc metals with attainment of a low-energy habit can be transformed via correlated displacements of atomic groups of the facet in the octahedral plane. It has been shown that such a process is similar to the surface diffusion of atomic n-mers with the activation energy depending on the facet size, and for particles with a diameter d < 3.0 nm the correlated displacement of atomic layers proves to be the dominant mechanism of faceting.