N. P. Lazarev

Temperature-controlled martensitic phase transformations in a model NiAl alloy

Reversible martensitic phase transformations in a partially disordered Ni–Al alloy within the composition range from 60to65at.% of Ni are investigated using molecular dynamics simulation. During a complete temperature cycle a wide hysteresis in enthalpy, volume, and shape of the simulated crystals is observed. The temperature T0 of the phase transformation is found from the calculated free energy evolution. To investigate the atomic-scale development during the phase transformation a local order parameter is defined which is based on a combined method of Voronoy tessellation [J. Reine Angew. Math. 134, 198 (1908)] with common-neighbor analysis. This local order parameter allows us to get a detailed localized picture of nucleation and growth of the new phases. Both homogeneous formation of the new phase and heterogeneous nucleation are observed. The velocity of new phase growth front is estimated.

Evolution of spatial heterogeneity in a Zr-based metallic glass

Abstract We report an investigation of the evolution of spatial heterogeneities in bulk amorphous Zr 54.5 Ti 7.5 Al 10 Cu 20 Ni 8 . The glassy state is characterized by fluctuations on a 1 nm scale. Metastable non-crystalline clusters are activated to grow at temperatures near the glass transition. Impinging clusters form boundaries where additional free volume is localized. At high volume fraction of impinging clusters enhanced diffusion occurs on the cluster boundaries resulting in a crystallization of the large clusters having a mean size of 2.5 nm.

Theory of spontaneous amorphization of metastable crystalline phases

Abstract Some metastable crystalline alloys show spontaneous amorphization and subsequent re-crystallization during transformation. This means that the amorphization is the result of competitive transformation kinetics in which both the metastable amorphous and stable crystalline phases are formed in parallel. However, the amorphous phase formation rate is much larger than that of the crystalline phase, so that in the first stage of transformation the resulting phase is amorphous. In the paper a theoretical description of the kinetics of these two processes is presented, taking into account the competitive formation of crystalline and amorphous phases on the boundaries of grains of the initial phase. The equations of nucleation and growth of the competitive phases are solved. The general approach is applied to the amorphization transformation kinetics in Cd–Sb alloys. The numerical analysis of equations of kinetics and the comparison with experimental results are carried out for Cd 43 Sb 57 .

Dislocationless sliding in a polycluster glass

Abstract At low temperature, T → 0, the yield stress of a perfect crystal is equal to its so called theoretical strength. The yield stress of imperfect crystals is controlled by the stress threshold of dislocation mobility. A non-crystalline solid has neither the ideal structure nor gliding dislocations. Its yield stress depends on the distribution of local critical stresses attributed to each atomic site at which the local inelastic deformation occurs. We describe the exactly solvable model of planar layer strength and sliding with an arbitrary homogeneous distribution of local critical stresses. The kinetics of thermally-activated creep of the sliding layer is described. The sliding activation volume scales with the applied external stress as ∼ωe-β, where β < 1. The proposed model accounts for mechanisms of the low temperature deformation of polycluster metallic glasses, since intercluster boundaries of a polycluster metallic glass are natural sliding layers of the described type. We also discuss applicability of the model to the low temperature plastic deformation of nano-crystalline materials.

Theoretical strength and homogeneous sliding in metallic glass: exactly solvable model

Abstract At low temperature, T→0, the yield stress of a perfect crystal is equal to its so-called theoretical strength. The yield stress of nonperfect crystals is controlled by the stress threshold of dislocation mobility. A noncrystalline solid has neither an ideal structure nor gliding dislocations. Its yield stress, that is, the stress at which the macroscopic inelastic deformation starts, depends on distribution of local, attributed to each atomic site, critical stresses at which the local inelastic deformation occurs. We describe exactly solvable model of planar layer strength and sliding with an arbitrary homogeneous distribution of local critical stresses. The rate of the thermally activated sliding is closely related to parameters of the low-temperature strength. The sliding activation volume scales with the applied external stress as where β<1. The proposed model accounts for mechanisms and the yield stress of the low-temperature deformation of polycluster metallic glasses, because intercluster boundaries of a polycluster metallic glass are natural sliding layers of the described type.

Cooperativity, cage effect and hopping diffusion in supercooled liquids and glasses

Molecular dynamic simulations of structure, thermodynamic and kinetic properties of model metallic Ag–Cu alloy are performed to elucidate its behavior at glass transition. In spite of small variations of inherent structure of the alloy the relaxation kinetics undergo dramatic changes at the glass transition. The time dependences of the mean square displacements and the non-Gaussianity parameter show the signs of anomalous diffusion in an intermediate time region. The analysis of time evolution of van Hove correlation function indicates the existence of both jump displacements and short-range cooperative atomic rearrangements. Below Tg these cooperative rearrangements do not contribute to a long-range diffusion but they still dominate the relaxation at short time.

Energy functional derivative with respect to the length of a curvilinear oblique cut in the equilibrium problem for a Timoshenko plate

This paper describes the dependence of the solution of the equilibrium problem for a Timoshenko plate and the total energy functional of the plate on the perturbation of an oblique crack. The nonlinearity of the problem is caused by the boundary conditions in the form of inequalities (conditions such as the Signorini conditions), which describe mutual nonpenetration of the opposite crack faces. The continuous dependence of the solution of the problem on the perturbation of the crack length is established. A formula for the energy functional derivative of the perturbation of the crack length is obtained.

Simulation of synthesis of cluster-assembled nanostructured materials

A model is described to simulate the formation of nanostructured materials by cluster beam deposition. Clusters are modelled by spherical balls with a given size distribution function, which fall to the substrate and stick to the growing structure. The mobility of clusters along the film surface is modelled by introduction of a critical angle at which a falling ball meets a ball that belongs to the structure. When the falling ball touches one of the balls in the structure at an angle smaller than the critical one, it sticks to the film; otherwise the ball rolls along the surface till it meets other balls. It is shown that a variety of structures similar to those observed experimentally can be produced. The density of the model structures, percolation thresholds and the surface roughness are analyzed.

On Relaxation Kinetics in Liquid and Glassy Ag-Cu Metallic Alloy

Molecular dynamics simulations of structure, thermodynamic and kinetic properties of model metallic AgCu alloy are performed to elucidate its behavior at glass transition. In spite of small variations of inherent structure of the alloy the relaxation kinetics undergo dramatic changes at the glass transition. The time dependences of the mean square displacements and the non-Gaussianity parameter show the signatures of anomalous diffusion in an intermediate time region. Analysis of time evolution of van Hove correlation function indicates the existence both jump displacements and short-range cooperative atomic rearrangements. Below T g these cooperative rearrangements do not contribute to long-range diffusion but they still dominate the relaxation at short time.