Dmitri A. Molodov

On the effect of purity and orientation on grain boundary motion

The influence of impurities on grain boundary mobility in a Σ7 (38.2° ) and in an off-coincidence boundary (40.5° ) was investigated. The grain boundary mobility was found to strongly depend on grain boundary crystallography and material purity. The measured concentration dependence of activation enthalpy and preexponential mobility factor did not comply with predictions of traditional impurity drag theory. An extended impurity drag theory is presented that takes into account interaction of the adsorbed atoms in the boundary. This theory predicts a concentration dependence of the activation enthalpy. For the Σ7 boundary it can explain qualitatively the frequently observed high values of preexponential mobility factor and activation enthalpy. The compensation temperature was found to depend on composition.

True absolute grain boundary mobility: motion of specific planar boundaries inBi-bicrystals under magnetic driving forces

The migration of planar 90°〈112〉 tilt grain boundaries with the same misorientation but different boundary inclination in Bi-bicrystals was investigated. The driving force for grain boundary motion was created by the action of a magnetic field on bicrystals of bismuth, which is magnetically anisotropic with different susceptibilities parallel and perpendicular to the trigonal axis. The driving force dependency of boundary velocity was measured and the absolute value of grain boundary mobility was determined. The grain boundary mobility was found to strongly depend on grain boundary inclination. The mobility parameters (activation enthalpy and preexponential factor) for a symmetrical 90°〈112〉 tilt grain boundary were much smaller than for an asymmetrical (ψ=45°) boundary. The mobility of the asymmetrical grain boundary depended on the direction of motion. For the given crystallography the boundary moved faster, if the trigonal axis in the growing grain was parallel rather than perpendicular to the direction of motion.

Magnetically induced texture development in zinc alloy sheet

Highly textured Zn–1.1%Al sheet was annealed in a direct-current magnetic field of 25.5 MA/m. Depending on the orientation to the field the texture components strengthened, retained their original intensity or disappeared. The results obtained are interpreted in terms of magnetically induced selective grain growth. 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc.

TRIPLE JUNCTION MOTION IN ALUMINUM TRICRYSTALS

The results of an investigation of the steady-state motion of grain boundary systems with triple junctions in high-purity aluminum are presented. In particular, the migration of systems with 111> and 110> tilt boundaries was studied. The experimental results demonstrate that the motion of grain boundary systems with triple junctions in aluminum can be controlled by slowly moving triple junctions. The influence of triple junctions depends on temperature, and it is particularly strong at low temperatures. In the high- temperature regime the motion of a connected grain boundary system is less affected by the triple junction, and, therefore, effectively controlled by the grain boundary mobility. The experiments reveal a drastic differ- ence between activation enthalpy of grain boundary and triple junction motion. Therefore, there is a tempera- ture below which triple junctions govern the motion of the connected boundary system. This temperature was found to depend on the particular grain boundary and triple junction geometry.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Mobility of 〈111〉 tilt grain boundaries in the vicinity of the special misorientation ∑=7 in bicrystals of pure aluminium

The temperature dependence of the mobility of tilt boundaries in aluminum with angle of misorientation between 35[degree] and 43[degree] was investigated in aluminum bicrystals in the temperature regime between 370 and 500 C. The mobility depends on the angle of misorientation and this dependence changes with temperature. These results explain some contradictory observations in bicrystal (and recrystallization) experiments and growth selection experiments.

Grain Boundary Migration in Metals: Recent Developments

Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.

Evidence of grain boundary dislocation step motion associated to shear-coupled grain boundary migration

Abstract The present work reports dynamical observations of the grain boundary (GB)-mediated plasticity during in situ transmission electron microscopy straining experiments at moderate temperature (400 C) both in a 76.4 bicrystalline and a polycrystalline Al sample. We show that the GB migration occurs by the lateral motion of elementary GB dislocation steps. The accumulation of GB dislocation steps eventually form macro-steps. This observation agrees with the idea that GB dislocation steps generally operate in high angle GBs similarly as in twinning or martensitic transformations. The coupling factor, i.e. the strain produced by the motion of the steps was measured using fiducial markers and image correlation. The migration process involves different types of GB dislocation steps, producing different amounts of strain both parallel (coupling factor) and perpendicular to the GB plane.

Boundary migration in Zn bicrystal induced by a high magnetic field

A bicrystal of Zn with an originally flat 89° 〈1010〉 symmetric tilt boundary was annealed in a magnetic field of 25 T. The boundary migrated under the action of a magnetic driving force in the direction of the grain with higher diamagnetic susceptibility. In addition, the boundary changed its crystallographic orientation, decreasing length and becoming almost perpendicular to the free surfaces. The results were interpreted in terms of magnetically forced grain boundary motion due to the anisotropy of the magnetic susceptibility in Zn. The absolute boundary mobility was measured to be about 5.1×10−9 m4/J s.

Wetting transition on grain boundaries in Al contacting with a Sn-rich melt

The contact angle θ at the intersection of a grain boundary in Al bicrystals with the solid Al/liquid Al−Sn interphase boundary has been measured for two symmetric tilt <011> {001} grain boundaries with tilt angles ϕ of 32° and 38.5°. The temperature dependencies θ(T) present the evidence of the grain boundary wetting phase transition at Tw. The observed hysteresis is consistent with the assumption that the wetting transition is of first order. The determined discontinuity in the temperature derivative of the grain boundary energy is−5.6 μJ/m2K (Tw1=617°C) for the boundary with a low energy (ϕ=38.5°) and −17 μJ/m2K (Tw2=604°C) for the grain boundary with a high energy (ϕ=32°).

Mechanically driven migration of tilt grain boundaries in Al-bicrystals

The stress induced migration of planar grain boundaries in aluminium bicrystals was measured. Symmetrical <100> tilt grain boundaries with misorientation angles in the range between 5.7° and 17.8° were examined. Boundary migration under a shear stress was observed to be ideally coupled to the lateral translation of grains. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with the respective boundary geometry. The temperature dependence of grain boundary mobility was measured, and the corresponding activation parameters were determined. The activation enthalpy of boundary migration was found to be independent of misorientation angle in the investigated misorientation range and amounts to H=1.44 eV.