Papers by Author: Dmitri A. Molodov

Abstract: The motion of grain boundaries in zinc bicrystals (99.995%) driven by the “magnetic” driving force was investigated. Planar symmetrical and asymmetrical tilt grain boundaries with rotation angles in the range between 60° and 90° were examined. At a given temperature the boundary migration rate was found to increase linearly with an applied driving force. The absolute grain boundary mobility was determined. The boundary mobility and its temperature dependence were found to depend on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90%) Zn-1.1%Al sheet specimens resulted in an asymmetry of the two major texture components. This is interpreted in terms of magnetically affected grain growth kinetics.

Abstract: The response of various structurally different planar grain boundaries in aluminum bicrystals to an applied stress was experimentally investigated. Stress induced boundary migration was observed to be coupled to a tangential translation of the grains for symmetrical and asymmetrical and tilt boundaries with both low and high misorientation angles. The activation enthalpy of high angle boundary migration was found to vary non-monotonously with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The Σ7 CSL boundaries in bicrystals of different geometry were observed to move under an applied stress, but their migration did not produce shear. These crystallographically equivalent boundaries, however, were found to behave different with respect to migration rate and its temperature dependence. The stress driven migration of the mixed tilt-twist boundary was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal. This behavior was interpreted in terms of the structure of the investigated tilt-twist boundary.

Abstract: The migration of planar grain boundaries induced by a magnetic field was measured in specially grown zinc bicrystals (99.995%). Particularly, symmetrical and asymmetrical <> tilt grain boundaries with rotation angles in the range between 60° and 90° were investigated. Boundary migration was measured in-situ in the temperature range between 330°C and 415°C and the absolute values of grain boundary mobility were obtained. The results revealed that grain boundary mobility essentially depends on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90%) Zn-1.1%Al sheet specimens substantially affected the texture and microstructure evolution. This effect is attributed to the additional magnetic driving force for grain growth arising due to the magnetic anisotropy of zinc.

Abstract: The recrystallization behavior of 50% cold rolled Fe-22%Mn-0.376%C alloy during annealing at 560°C, 630°C and 700°C was investigated. Microhardness tests were applied for characterization of the recrystallization kinetics, X-ray diffraction and EBSD measurements were utilized to characterize the crystallographic texture and the grain microstructure. The obtained experimental data were evaluated in terms of the JMAK model. The obtained values of the Avrami exponent varied in the range between 0.70 and 1.37. The inhomogeneous grain microstructure after recrystallization is interpreted in terms of non-randomly distributed nuclei. Shear bands, lamellar lines intersecting with mechanical twins and grain boundaries with localized high misorientation gradients were identified to be preferential nucleation sites. No pronounced texture was observed after annealing at 630°C.

Abstract: The results of investigations of magnetically driven grain boundary migration in high purity (99.995%) zinc bicrystals are presented. In-situ measurements were conducted by means of a specially designed and fabricated polarization microscopy probe. The migration of planar tilt grain boundaries with various misorientation angles in the range between 60° and 90° was studied. The absolute grain boundary mobility and its temperature dependence was measured in the regime between 330°C and 415°C and the corresponding migration activation parameters were determined. The results revealed that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The migration activation enthalpy was found to vary between 1.18 eV and 2.15 eV. The obtained activation parameters comply with the compensation law, i.e. the migration activation enthalpy changes linearly with the logarithm of the pre-exponential factor.

Abstract: Recent results of experimental research into stress induced grain boundary migration in aluminum bicrystals are briefly reviewed. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for any <100> tilt boundary in the entire misorientation range (0-90°). Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

Abstract: Effect of carbide precipitation on pinning force and migration mechanism of boundaries of martensite laths was considered in a 3%Co modified P911. The dimensions of second phase precipitations, martensite laths and dislocation densities were measured by means of transmission electron microscopy. The pinning forces retarding the motion of the lath boundaries, that arise from M(C,N) nanoscale precipitations and M₂₃C₆ particles were evaluated by using different models. The pinning pressure evaluated by taking into account a non-uniform distribution of M₂₃C₆ particles was high enough to stabilize the lath martensite structure during tempering and long term ageing. On the other hand, significant coarsening of martensite laths occurred in neck portions of samples subjected to long-term creep tests. Additional effects from dislocation density and applied stress on the motion of lath boundaries are considered in some details.

Abstract: The effect of a magnetic field on texture and microstructure development in cold rolled (80%) commercially pure zirconium (Zr701) was investigated. X-ray diffraction and EBSD measurements were utilized for the texture and microstructure characterization. The results revealed that a magnetic field promotes grain growth in the investigated material. During annealings at 550°C this is particularly apparent from the faster development of specific (0/180, 35, 30) texture components and the bigger mean grain size after magnetic annealing. The magnetic annealing at 700°C resulted in an asymmetry of the two major texture components. This is due to a magnetic driving force for grain growth arising from the anisotropic magnetic susceptibility of zirconium. During annealing at 700°C the abnormal grain growth occurred. This behavior is attributed to the higher mobility of grain boundaries between grains misoriented by 30° around [000. The magnetic field essentially enhanced the observed abnormal grain growth.

Abstract: The impact of a magnetic field on texture and microstructure evolution in Zn-1.1%Al was investigated. Specifically oriented cold rolled (85%) sheet specimens were annealed in a magnetic field of 17 Tesla at 340°C for 15, 30 and 90 minutes. X-ray diffraction and EBSD-measurements were utilized to characterize crystallographic texture and grain microstructure. The results confirmed that grain growth in magnetically anisotropic zinc can be substantially affected by a magnetic field. This manifested itself by significant changes in the development of the grain growth texture during magnetic annealing compared to annealing at zero field. The magnetically induced texture changes are caused by the generation of an additional magnetic driving force which arises from a difference in magnetic free energy density between differently oriented grains. The grain microstructure evolution was also essentially affected by a magnetic field such that grains with energetically favoured orientations grow faster and their fraction becomes larger, than that of grains with disfavoured orientations. The results will be also discussed with respect to the magnetically altered grain boundary character distribution.

Abstract: A cellular automaton and a vertex model were used, respectively, for the simulation of recrystallization and grain growth in a Fe-0.374%C-21.64%Mn alloy. The results of the recrystallization simulations revealed that the preferential nucleation during the annealing of the rolled sheet occurs at shear bands, which is corroborated by experimental observations. Subsequently, grain growth simulations were carried out with a 2D vertex model. The model used experimental data as input for its validation in this specific steel. The simulations showed a good agreement with the experimental results.