Advanced Materials Research Vol. 1013

Abstract: Molecular Dynamics (MD) simulations have been carried out on ultrathin Ni₃Fe alloy with face-centered cubic (FCC) lattice upon application of uniaxial tension at nanolevel with a speed of 20 m/s. the deformation corresponds to the direction <001>. To the calculated block of crystal - free boundary conditions are applied in the directions <100>, <010>. Morse potential was employed to carry out three dimensional molecular dynamics simulations. A computer experiment is performed at a temperature corresponding to 300 K. MD simulation used to investigate the effect of long of ultrathin Ni₃Fe alloy on the nature of deformation and fracture. The engineering stress–time diagrams obtained by the MD simulations of the tensile specimens of these ultrathin Ni₃Fe alloy show a rapid increase in stress up to a maximum followed by a gradual drop to zero when the specimen fails by ductile fracture. The feature of deformation energy can be divided into four regions: quasi-elastic, plastic, flow and failure. The yield strength decreased with increasing long of alloy, but increases with increasing the cross sectional area. Plasticity disappear when the length of the allays is too large. The results showed that breaking position depended on the alloy length.

Abstract: The general structure of the multilevel models of inelastic deformation of materials including the internal structure evolution description was considered in the present paper. It also provides variants to address the key issues of similar type models, which are important for further development of such models. A new approach for constitutive relations coupling between constitutive relations of different scale levels was described. The suggested approach establishes connection of similar characteristics in different scale levels. The proposed coupling method also provides an unambiguous determination of material frame indifferent derivative of the Cauchy stress tensor at the macro scale, which is necessary for the formulation of constitutive relations for large deformations. In order to make it clearer an example of a two-level model of polycrystalline metals is considered; based on the physical sense analysis the hardening laws and lattices rotations of crystallites are suggested.

Abstract: The paper deals with the results of studying the flow structure and heat transfer in a cylindrical channel with a porous insert. While formulating the problem the space-averaged model of interpenetrating continuums was used. The modelled system is described by the equations of continuity, motion and thermal conductivity of a gas phase, as well as the equations of thermal conductivity for a porous backfill. The results of the research show that, due to the displacement of the gas flow from the wall region to the central part of the channel, in the initial flow section a low-temperature area is formed at the inner boundary of the porous layer. The temperature of the structural frame in the initial flow section also decreases because of its cooling by the flow displaced from the wall. In the downward flow direction heating-up of the structural frame occurs due to heat transfer from the gas flow moving in the axial zone. In addition, the structural frame performs the role of a thermal reservoir and, in turn, heats the gas flow in the porous layer.

Abstract: The proposed dislocation model describes the orientation dependence of the critical resolved shear stress (CRSS) and deformation mechanisms on the yield point in single crystals of austenitic stainless steel with nitrogen impurities. The model takes into account the following: the change of the interstitial atom position in the lattice from octahedral interstice to tetrahedral site owing to passage of a leading Shockley’s partial dislocation; the change in the separation width between two partial dislocation in external stress field; the relationship between the width of the extended dislocation and the elastic interaction of the extended dislocation with the impurity atoms.

Abstract: Dislocation junctions, formed as a result of dislocation reactions, affect the plastic strain process, at least, for two reasons. First of all, junctions serve as barriers to shear-forming dislocations and restrict their path, therefore, the size of the shear zone. Sizes of the shear zone are determined by the density of reacting dislocations in non-coplanar slip systems, forming long enough barriers in the form of dislocation junctions. Secondly, non-breaking dislocation junctions are accumulated inside the shear zone, which leads to an increase in the intensity of the dislocation density accumulation.The present work is devoted to the study of the influence of dislocation junctions on accumulation of the density of dislocation debris (debris junctions) due to formation of stable junctions. For this purpose, the probability density function of lengths in non-breaking junctions is calculated. The model of dislocation interactions, built by the authors of the paper for FCC single crystals, is used.

Abstract: A mathematical model of elementary crystallographic slip, limited by a closed piecewise-continuous dislocation loop is proposed. A study of the evolution of the first dislocation loop emitted by a dislocation source in copper is carried out. It has been shown that on the dislocation loop on screw orientation and orientations close to it, about half a free path length before the stopping point there arises a concavity, which grows in size up to the stopping of the dislocation loop. In the final configuration the radius of the dislocation loop on screw orientation is practically by an order of magnitude less than the radius on edge orientation.

Abstract: A mathematical model of plastic deformation of dispersion-hardened materials with FCC matrix and strengthening particles having various coupling with the matrix is presented. The model is based on the equations of balance of deformational defects of various types with allowance for their transformation in the process of plastic deformation. The influence of scale characteristics of the strengthening phase, temperature, and strain rate on the evolution of the dislocation subsystem and on strain hardening of an alloy with FCC matrix is investigated.

Abstract: A study is made of the process of ignition of reactive channel walls by a laminar flow of hot gases, including the stages of heating of a substance and of reacting in the surface layer with self-acceleration of the chemical reaction. The process is determined by the heat exchange between the gas and the wall, the strength of the heat source in the chemical-reaction zone, and the sink of heat due to conduction in the radial and axial directions. In the stage of self-heating, we can have heat sink not only deep into the wall and/or through its external boundary but into the gas flow as well. The problem has been solved in a conjugate formulation. The influence of the temperature, the velocity of the gas at the entrance to the channel, and the wall thickness on ignition characteristics has been studied.In spreading a high temperature gas flow in a channel which walls are made of reactable material there appears a problem dealing with the possibility of their ignition by the flow.

Abstract: The experimental results of plastic behavior investigation of Al-6%Zn-3%Mg alloy are compared with the mathematical model of plastic deformation of dispersion hardened FCC materials with undeformed particles. Were determined the factors, when model properly describes the regularities of slip development in this alloy.