L.S. Shvindlerman

Stress induced grain boundary motion

We investigated the motion of planar symmetrical and asymmetrical tilt boundaries in high-purity aluminium with - and -tilt axes under the influence of an external mechanical stress field. It was found that the motion of low-angle grain boundaries as well as high-angle grain boundaries can be induced by the imposed external stress. The observed activation enthalpies allow conclusions on the migration mechanism of the grain boundary motion. The motion of planar low- and high-angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary. A sharp transition between low-angle grain boundaries and high-angle grain boundaries was observed at 13.6°, which was apparent from a step of the activation enthalpy for the grain boundary motion. For the investigated boundaries the transition angle was independent of tilt axis, impurity content and tilt boundary plane.

Influence of triple junctions on grain boundary motion

Abstract The paper is dedicated to the steady state motion of the grain boundary systems with the triple junctions. The main features of one of the systems where the steady state motion is possible are considered. In the experimental part the special technique of in-situ observations and recording of the triple junction motion and the results of the experiments on the tricrystals of Zn are described comprehensively. It was shown, in particular, that the described method makes it possible to study the motion of a grain boundary system with a triple junction and, what is of importance to measure its mobility. The shape of the moving half-loop in the tricrystal fits the theoretically calculated. The transition from the motion controlled by the triple junction kinetics to the boundary kinetics is observed. By this is meant that the triple junction along with the other structural defects can drag the boundary motion, or, conversely, and their role and properties should be taken into consideration in theories of grain growth.

MISORIENTATION DEPENDENCE OF INTRINSIC GRAIN BOUNDARY MOBILITY: SIMULATION AND EXPERIMENT

Abstract Both experimental and atomistic simulation measurements of grain boundary mobility were made as a function of temperature and boundary misorientation using the same geometry that ensures steady-state, curvature-driven boundary migration. Molecular dynamics simulations are performed using Lennard–Jones potentials on a triangular lattice. These simulations represent the first systematic study of the dependence of intrinsic grain boundary mobility on misorientation. The experiments focus on high purity Al, with 〈111〉 tilt boundaries, which are isomorphic to those examined in the simulations. Excellent agreement between simulations and experiments was obtained in almost all aspects of these studies. The boundary velocity is found to be a linear function of the curvature and the mobility is observed to be an Arrhenius function of temperature, as expected. The activation energies for boundary migration varies with misorientation by more than 40% in the simulations and 50% in the experiments. In both the simulations and experiments, the activation energies and the logarithm of the pre-exponential factor in the mobility exhibited very similar variations with misorientation, including the presence of distinct cusps at low Σ misorientations. The activation energy for boundary migration is a logarithmic function of the pre-exponential factor in the mobility, within a small misorientation range around low Σ misorientations.

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.

The effect of triple-junction drag on grain growth

Abstract Current theories of grain growth presume that grain boundary migration is the rate-limiting step, and either explicitly or implicitly assume that triple junctions can always move with sufficient speed to accommodate the changing positions of the grain boundaries. Following from some recent observations of triple-junction drag effects in tricrystals of zinc and in molecular dynamics models, an analytical theory is developed to explore the effects of triple-junction drag upon grain growth, for a two-dimensional solid. The theory is developed in the framework of the Von Neumann–Mullins formulation, and demonstrates that drag effects operating exclusively at the triple junctions result in a retardation of grain growth. The stability of six-sided grains in the isotropic, drag-free case of the Von Neumann–Mullins analysis is successively extended to grains of 6± N sides, where N increases with the strength of the triple-junction drag.

Triple junction drag and grain growth in 2D polycrystals

Abstract The process of grain growth in 2D systems is analyzed with respect to the controlling kinetics: from solely boundary kinetics, when grain growth in a polycrystal is determined by the Von Neumann–Mullins relation, to exclusively triple junction kinetics, when grain growth is governed by the mobility of triple junctions. It is shown that in the “intermediate” case, when the driving force for grain boundary motion and the characteristic mobility are grain boundary curvature and grain boundary mobility, respectively, a limited mobility of triple junctions essentially influences grain boundary motion. The Von Neumann–Mullins relation does not hold anymore, and this is the more pronounced the smaller the triple junction mobility. In the case where grain growth is determined by the mobility of grain boundary triple junctions (triple junction kinetics) all grains are transformed into polygons in the course of grain growth. Grain growth would cease if all grains assumed the shape of regular polygons, not only hexagons like in the Von Neumann–Mullins case. The only exceptions are triangles: they collapse without transforming into a polygon. The respective relation for the rate of a change of grain area under triple junction kinetics is obtained and discussed with regard to microstructure evolution.

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.

Molecular dynamics simulation of triple junction migration

We present a molecular dynamics simulation study of the migration of grain boundaries with triple junctions. We have monitored the grain boundary profile, triple junction angles and rate of grain boundary migration with and without triple junctions as a function of grain size, grain misorientation, direction of migration and temperature in a series of configurations designed to ensure steady-state migration. The present results demonstrate that triple junction mobility is finite and can be sufficiently small to limit the rate of grain boundary migration. The drag on grain boundaries due to limited triple junction mobility is important at small grain sizes, low temperature and near high symmetry grain misorientations. This drag limits the rate of grain boundary migration and leads to triple junction angles that differ substantially from their equilibrium value. Simulation data suggest that triple junction drag is much more a factor at low temperature than at high temperature. The triple junction mobility is shown to depend upon the direction of triple junction migration. The present results are in excellent qualitative agreement with experimental observations.

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.

Penetration of Tin and zinc along tilt grain boundaries 43° [100] in Fe-5 at.% Si alloy : premelting phase transition ?

Abstract Tin and zinc penetration along the tilt grain boundary 43° [100] in b.c.c. Fe-5 at.% Si alloy is studied in the temperature range from 652 to 975°C. Wetting transition of grain boundary by the tin-rich melt at Tw = 810 ± 5°C is observed. About Tw there is a thin wetting film at grain boundary. With zinc penetration along the grain boundary a wetting film has been observed at all temperatures studied. Behind that film there is a region with an unusually high diffusivity of zinc, and below that region there is a region of “ordinary” grain boundary diffusivity. Such a phenomenon may be explained in terms of the phase transition “grain boundary-thin wetting film on the boundary”, which is commonly known as a premelting phase transition. A model is proposed which explains the form of the temperature dependence of the concentration cBt, at which such transition occurs, and, in particular, the influence of the “paramagnet-ferromagnet” transition in the bulk on the premelting transition. The influence of the temperature dependence of the volume solubility limit, c0, on the cBt(T) dependence is also discussed. In critical region below Curie point Tc critical exponents d of magnetic part of activation free energy of bulk and grain boundary diffusion are calculated. Critical index d for grain boundary diffusion by premelting layer, as well as activation energy in paramagnetic region, lies in the interval between bulk values of d and estimation of d for truly two-dimensional grain boundary diffusion.