Papers by Author: Dmitri A. Molodov

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Authors: Christoph Günster, Dmitri A. Molodov, Günter Gottstein
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.
Authors: Tatiana Gorkaya, Thomas Burlet, Dmitri A. Molodov, Günter Gottstein
Abstract: A novel set-up developed to continuously observe and measure stress driven grain boundary migration is presented. A commercially available tensile/compression SEM unit was utilized for in-situ observations of mechanically loaded samples at elevated temperatures up to 850°C by recording orientation contrast images of bicrystal surfaces. Two sample holders for application of a shear stress to the boundary in bicrystals of different geometry were designed and fabricated. The results of first measurements are presented.
Authors: Dirk M. Kirch, A. Ziemons, I. Lischewski, Dmitri A. Molodov, Günter Gottstein
Abstract: A novel high temperature heating method in combination with automated EBSD-data acquisition is presented. A commercially available infrared laser is utilized to heat samples up to a temperature of about 1000°C in high vacuum in a SEM while acquiring EBSD-data of the microstructure. First results on the γ-α-γ phase transformation between 840°C and 865°C in a microalloyed ferritic low carbon steel is presented.
Authors: Dmitri A. Molodov, U. Czubayko, Günter Gottstein, Lasar S. Shvindlerman
Authors: Zhanna Yanushkevich, Andrey Belyakov, Rustam Kaibyshev, Christian Haase, Dmitri A. Molodov
Abstract: The regularities of static recrystallization in an Fe-0.3C-17Mn-1.5Al TWIP steel subjected to cold rolling and annealing were studied. The cold rolling led to noticeable increase in the dislocation density, extensive mechanical twinning and shear banding. The subsequent annealing resulted in the development of recovered or recrystallized microstructure depending on the rolling reduction and the annealing temperature. An increase in the rolling reduction promoted the recrystallization development, which led to ultrafine-grained microstructure with a grain size below 10 μm. The developed ultrafine-grained steel samples are characterized by beneficial mechanical properties.
Authors: D. Mattissen, D. Kirch, Dmitri A. Molodov, Lasar S. Shvindlerman, Günter Gottstein
Abstract: The motion and geometry of connected grain boundary systems with triple junctions in aluminium -10 ppm magnesium was investigated in-situ with a special designed SEM heating stage. The results show that triple junctions can have a marked influence on grain boundary motion. The grain area change with annealing time was from a hot stage in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (Von Neumann-Mullins relation). This is attributed to the effect of triple junction drag on grain growth.
Authors: V.A. Ivanov, Dmitri A. Molodov, Lasar S. Shvindlerman, Günter Gottstein, D. Kolesnikov, Witold Łojkowski
Abstract: A new method is introduced to determine the absolute value of the boundary excess free volume. Along with the boundary energy the excess free volume belongs to the major thermodynamic properties of grain boundaries. The method utilizes the dependence of the contact angle at triple junctions of grain boundaries in Al-tricrystals on hydrostatic pressure. We investigated <111> tilt boundaries in the pressure range up to 14 kbar. In particular, for a 40° <111> tilt boundary with 2° twist component the boundary free volume was found to be equal to 5.03×10-11 m3/m2.
Authors: Dirk M. Kirch, Bing Bing Zhao, Dmitri A. Molodov, Günter Gottstein
Abstract: The kinetic and structural behavior of symmetrical <100> tilt grain boundaries with rotation angles 8.4°, 12.0°, 14.3° and 16.0° were investigated in-situ in a hot stage SEM in the temperature range between 380°C and 640°C. The results revealed that depending on the rotation angle the boundary either remained straight, became faceted or curved under the driving force provided by the boundary surface tension during high temperature annealing. The transition “facetedcurved boundary” was also found to depend on temperature. The observed behavior is attributed to the anisotropy of grain boundary energy with respect to boundary inclination.
Authors: Dmitri A. Molodov
Abstract: The latest research on dynamics of grain boundaries in non-magnetic materials in high magnetic fields is reviewed. A control of grain boundary migration means control of microstructure evolution, which is a key for the design of materials with desire properties. Grain boundary motion can be affected by a magnetic field, if the anisotropy of the magnetic susceptibility generates a gradient of the magnetic free energy. In contrast to curvature driven boundary motion, a magnetic driving force also acts on planar boundaries so that the motion of crystallographically well-defined boundaries can be investigated, and the true grain boundary mobility can be determined. The results of migration measurements obtained on bismuth and zinc bicrystals are addressed. Selective growth of new grains in locally deformed zinc single crystals driven by a magnetic force is reported as well. Implications for materials processing, in particular the effect of magnetic fields on texture development in hcp metals are finally discussed.
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