Papers by Author: Eugen Rabkin

Abstract: We consider the kinetics of chemical interdiffusion along the grain boundaries in stressed thin metal film attached to inert substrate. We show that the kinetics of stress relaxation in the film can be either accelerated or slowed down if compared with the same kinetics in a single-component film, depending on the difference of intrinsic GB diffusion coefficients of the two components. In the case of faster matrix atoms the tensile stress in the film significantly increases beyond its initial value at the beginning of interdiffusion process, while in the case of faster diffuser atoms the compressive stresses develop in the film at the intermediate stages of stress evolution.

Abstract: A grain boundary interdiffusion in a semi-infinite bicrystal under the conditions of negligible bulk diffusion is considered. We show that the inequality of intrinsic grain boundary diffusion coefficients of the two components leads to plating out of additional material at the grain boundary in the form of extra material wedge, which generates an elastic stress field in the vicinity of the grain boundary. We solved a coupled diffusion/elasticity problem and determined the time-dependent stress field and concentration distribution in the vicinity of the grain boundary.

Abstract: A model that considers diffusion in nanocrystalline materials undergoing recrystallization was developed. Application of this model enabled us deriving 63Ni radiotracer diffusion coefficients along the grain boundaries in ultrafine grain copper produced by equal channel angular pressing from the experimentally measured radiotracer penetration profiles.

Abstract: We considered a polycrystalline cylindrical nanowire with bamboo microstructure strained uniaxially by an external load. Our molecular dynamic computer simulations demonstrated that grain boundary grooving plays an important role in determining the morphological stability of nanowires. Also, an exceptionally high yield stress of nanowires emphasizes the importance of diffusion in their plastic deformation under applied load. We formulated a phenomenological diffusion-based model describing morphological stability and diffusion-controlled deformation behaviour of polycrystalline nanowires. The kinetics of the shape changes was calculated numerically.

Abstract: The effect of pressure on the kinetics of grain boundary (GB) segregation in the Cu–50 at. ppm Bi alloy has been studied. It was found by means of Auger electron spectroscopy that at a temperature of 1173 K the segregation level decreases from 2 to 1.5 monolayer as the pressure increases from 0.01 to 1.2 GPa. This behavior was explained by considering the physical parameters controlling kinetics and thermodynamics of GB segregation. A simplified model based on dislocation pipe diffusion, proposed previously and discussed in more details in this work, was used to calculate the non-equilibrium GB segregation during cooling under high pressure. The pressure effect on bulk diffusion is responsible for the suppression of GB segregation, while the pressure effect on the phase stability in Cu–Bi alloys provides a negligible contribution on GB segregation in the pressure range studied.

Abstract: ECAP (equal channel angular pressing) is a well-known severe plastic deformation method used to produce ultra-fine grained materials. The dimensions of ECAP specimens are usually in the centimeter range. For producing high strength wires or fibres with diameter in the micrometer/millimeter range, downscaling of the ECAP process may be a viable option. To achieve this, several experiments were carried out. For downscaling to the micrometer range, porous steel discs can be used as processing tools. In this case, a solid state infiltration method as a variant of the forcefill process can be used. Extremely large strain is introduced due to the material flow through the tortuous channels inside a porous pre-form leading to grain refinement depending on processing conditions. To obtain specimens with a typical dimension in the millimeter range, the forcefill approach was altered by using die channels produced by conventional drilling. The tool geometry used is equivalent to conventional ECAP, but with a multi-channel die. Microstructure investigations demonstrating significant grain refinement confirm the viability of this approach.

Abstract: We considered a polycrystalline cylindrical wire of the initial radius R0 composed of identical cylindrical grains of the length L0, strained uniaxially by an external stress P. At the temperatures at which some surface and grain boundary diffusion are allowed the thinning of the nanowire in the vicinity of grain boundaries occurs due to the phenomenon of grain boundary grooving. We calculated the equilibrium shapes of the nanowire achieved after long annealing times. Our calculations demonstrated that for any given L0/R0 ratio some critical value of the applied stress exists above which the nanowire is unstable and breaks down into the string of isolated spherical particles, in full analogy with the Rayleigh instability of long cylinders. The kinetics of the shape change was calculated numerically. It was shown that the rate of thinning of unstable wires diverges as the moment of breakdown is approached. We also demonstrated that the breakdown may occur even for nominally stable wires “on the way” to achieving their equilibrium shape. Therefore, the stability of nanowire is determined by a combination of geometric (L0/R0), thermodynamic (grain boundary energy), and kinetic (ratio of grain boundary and surface diffusivities) parameters. An application of external tensile stress accelerates the breakdown of nanowires.

Abstract: Microstructure and hardness of ternary Al–Zn–Mg alloys were studied both in as cast state and after high pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) grains and of reinforcing second phase precipitates decreases drastically after HPT reaching nanometer range. During HPT, the Zn- and Mg-rich supersaturated (Al) solid solution decomposes and reaches the equilibrium state corresponding to the room temperature. In the as cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to the solid-solution hardening. However, after HPT the work hardening and Hall-Petch hardening due to the decreasing grain size competes with softening due to the decomposition of a supersaturated solid solution. In the net effect, the severe plastic deformation results in softening of ternary Al–Zn– Mg alloys.

Abstract: It was shown that external magnetic field of 5 kOe slows down the kinetics of grain boundary grooving in iron at 750 °C by about one order of magnitude. This observation is discussed in terms of magnetic effect in surface diffusion.

Abstract: We considered the flattening of perturbed surface of a thin stress-free polycrystalline film with columnar microstructure deposited on rigid substrate. We show that the mass transport along the film/substrate interface and along the grain boundaries significantly contributes to the overall rate of surface flattening of the film. The diffusion along the film/substrate interface and along the grain boundaries is driven by the capillary stresses in the film. Using the approximation of small surface slopes, we calculated the distribution of capillary stresses in the film, and derived an explicit expression for the temporal behavior of the film topography. The initial distribution of the capillary stresses rapidly relaxes to the steady-state one that does not allow the accumulation of bending strain in the film. For the films with passivated or contaminated surfaces exhibiting reduced surface diffusivity the interfacial and grain boundary diffusion play a leading role in kinetics of surface flattening. The flattening process can be accelerated in this case by several orders of magnitude. The results of our work can be helpful in design of thin films and coatings with enhanced selfhealing capabilities.