I. N. Kar’kin

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.

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.

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.

Interaction of dislocations with nanoprecipitates of the metastable phase and dispersion strengthening of the Fe-Cu alloy

The interaction of dislocations with copper-enriched precipitates in the matrix of body-centered cubic iron has been investigated by the molecular dynamics method. It has been shown that dislocations stimulate the development of a phase instability of body-centered cubic copper precipitates in a specific range of their sizes. This process is accompanied by the pinning of dislocations by precipitates and makes a significant contribution to strengthening. The results obtained provide an adequate explanation for the observed dependence of the strengthening in the Fe-Cu system on the precipitate size.

Monte Carlo simulation of the kinetics of decomposition and the formation of precipitates at grain boundaries of the general type in dilute BCC Fe–Cu alloys

The kinetics of decomposition of a polycrystalline Fe–Cu alloy and the formation of precipitates at the grain boundaries of the material have been investigated theoretically using the atomistic simulation on different time scales by (i) the Monte Carlo method implementing the diffusion redistribution of Cu atoms and (ii) the molecular dynamics method providing the atomic relaxation of the crystal lattice. It has been shown that, for a small grain size (D ~ 10 nm), the decomposition in the bulk of the grain is suppressed, whereas the copper-enriched precipitates coherently bound to the matrix are predominantly formed at the grain boundaries of the material. The size and composition of the precipitates depend significantly on the type of grain boundaries: small precipitates (1.2–1.4 nm) have the average composition of Fe–40 at % Cu and arise in the vicinity of low-angle grain boundaries, while larger precipitates that have sizes of up to 4 nm and the average composition of Fe–60 at % Cu are formed near grain boundaries of the general type and triple junctions.

Effect of chemical interaction on the stability of metal clusters in FCC metals

Structural stability of small Ni, Al, and Au metal clusters with a number of atoms close to N = 55 and 147 has been studied by the method of molecular dynamics with the use of realistic potentials of interatomic interaction. It has been shown that in Ni, in which the icosahedral configuration is most stable, the mass spectrum predominantly contains peaks, which correspond to N = 55 and 147. At the same time, for Au and Al clusters, the consequence of magic numbers differs from that specified by the close packing of atoms, and its realization depends on experimental conditions. The results obtained allow concluding that the position of peaks in the mass spectrometric experiments with small clusters is determined by morphological features of the structural state, which depend on the character of interatomic interaction.

Size effect during two-particle agglomeration: Results of molecular dynamics simulation

The microscopic mechanisms controlling the atomic rearrangements during agglomeration of Ni particles 3 to 7 nm in size at temperatures T = 0.6–0.95 Tm have been studied by the molecular dynamics method. Pentagonally twinned crystals were obtained as a result of coalescence for the disorientations corresponding to special Σ11 and Σ27 large-angle boundaries.