Vera G. Sursaeva

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.

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.

Review: grain boundary faceting–roughening phenomena

Similar to free surfaces, the grain boundaries (GBs) in metals, semiconductors and insulators can contain flat (faceted) and curved (rough) portions. In the majority of cases, facets are parallel to the most densely packed planes of coincidence sites lattice formed by two lattices of abutting grains. Facets disappear with the increasing temperature (faceting–roughening transition) and the increasing angular distance from coincidence misorientation. The temperature of GB faceting–roughening transition TR decreases with the increasing inverse density of coincidence sites Σ. In case of fixed Σ, TR decreases with the decreasing density of coincidence sites in the GB plane. The intersection line (ridge) between facets or between facets and curved (rough) portions of surfaces can be of first order (two different tangents in the contact point) or of second order (common tangent, continuous transitions). The rough (curved) portions of GB can also form the first-order rough-to-rough ridges (with two tangents). GB facets control the transition from normal to abnormal grain growth and strongly influence the GB migration, diffusion, wetting, fracture and electrical conductivity.

The Effect of Triple Junctions on Grain Boundary Motion and Grain Microstructure Evolution

The theory of steady state motion of grain boundary sytems with triple junctions and the main features of such systems are considered. A special technique of in-situ observations and recording of triple junction motion is introduced, and the results of experimental measurements on Zn tricrystals are discussed. It is shown, in particular, that the described method makes it possible to measure the triple junction mobility. It was found that the measured shape of a moving half-loop with a triple junction agrees with theoretical predictions. A transition from triple junction kinetics to grain boundary kinetics was observed. This means that triple junctions can drag boundary motion. It is demonstrated that the microstructural (granular) evolution is slowed down by triple junction drag for any n-sided grain. The second consequence pertains to six-sided grains. For a boundary system with dragging triple junctions there is no unique dividing line between vanishing and growing grains with respect to their topological class anymore, like n = 6 in the Von Neumann-Mullins relation.

Influence of faceting-roughening on triple-junction migration in zinc

½� tilt grain boundaries (GBs) with misorientation angles h of 43° ,3 7° and 6°. The stationary shape of the migrating triple junction was studied, and the migration rate was measured in-situ between 670 and 688 K using polarized light. In some experimental runs, a facet was formed on the h ¼ 37° 10 1 ½� tilt GB. This facet was parallel to the close-packed plane in the constrained coincidence site lattice (CCSL). The length of this facet decreases with increasing temperature and becomes zero at 688 K. The temperature dependence of the facet length is better described by the meanfield Andreev approximation than by the solid-on-solid model. The step energy estimated in the framework of the Bonzel approximation is about 0.1 eV/atom. In other experimental runs, the h ¼ 37° 10 1 ½� tilt GB did not facet and remained rough in the same temperature interval. This fact allowed us to compare the stationary migration of the same TJ with faceted and rough GBs. A TJ formed by faceted GBs migrates one to two orders of magnitude more slowly in comparison with a rough TJ. An unrealistically high value of the apparent migration activation enthalpy of faceted TJs can appear due to the changing geometry of faceted GBs, similar to the case of migration of faceted twin tips.

Faceting and migration of twin grain boundaries in zinc

Abstract Faceting and migration of twin tips in slightly deformed Zn [112¯0] flat single crystals have been investigated. The stationary shape of the slowly migrating tip of the twin plate has been studied, and the migration rate has been measured between 592 and 692 K in situ using polarized light. Below 632 and above 682 K, the twin tip contains only one facet which is parallel to the (1¯102¯)2 and (11¯00)1 plane of the matrix single crystal, respectively. The non-physically high values of the apparent migration activation enthalpy between 632 and 682 K are discussed using the model of simultaneous migration of co-existing grain boundary facets based on the concept of weighted mean curvature. The experimentally determined temperature dependence of the twin tip mobility can be rationalized under the assumption that the migration of individual facets is characterized by identical activation enthalpies of 46.6 kJ/mol, but different pre-exponential factors (1.1 × 10−11 and 7 × 10−9m2/s, respectively).

Grain boundary misorientation changes during grain growth in pure aluminium

A new method is described for data-logging large amounts of grain boundary misorientation information from channelling patterns in the scanning electron microscope (SEM). The method relies on producing specimens where the grain size is larger than the specimen thickness and where the grain boundary planes are perpendicular to the specimen plane (the so-called columnar structure). Results for grain growth in pure aluminium at 460 and 500°C are presented. There is an increase in the proportion of low angle boundaries at the expense of high angle boundaries during growth times of up to a few hours. The reasons are thought to be partly connected with lower low angle boundary mobility compared with high angle boundaries. However, the growth kinetics appear to be normal over the entire growth time range.

Faceting and Roughening of the Asymmetric Twin Grain Boundaries in Zinc

Faceting of grain boundaries (GBs) or surfaces can be considered as a phase transition when the original surface or GB dissociates onto flat segments whose energy is less than that of original surface or GB. Zn [1120] flat single crystals were grown using the modified Bridgman technique from Zn of 99.999 wt% purity. Individual elongated twin plates having very uniform thickness were produced with the aid of slight deformation of single crystals. Parallel elongated sides of the twin plate are formed by the coherent symmetric twin (1102)1‖(1102)2 grain boundary (STGB) facets. Due to its optical anisotropy, zinc allows one to study the shape of the GB with the aid of polarised light. The stationary shape of the slowly migrating tip of the twin plate has been studied in situ. The hot stage of optical microscope was used. The temperature interval from 592 to 692 K was investigated. Below 632 K the twin tip contains only one plane facet 1 which is nearly parallel to the (1102)2 plane and has the angle of 84° with the coherent STGB. Above 632 K the second facet 2 appears at the tip of the twin plate. This facet is nearly parallel to the (1100)1 plane and has the angle of 46° with the coherent STGB. Between 632 and 682 K both 84° and 46° facets coexist, and 84° facet gradually disappear with increasing temperature. Above 682 K only 46° facet is present in the twin tip. The indications of the GB roughening phase transition were also observed, namely the edges of the facets become smoother with increasing temperature. The GB phase diagram for the twin GBs in zinc containing the lines of two GB faceting phase transitions has been constructed. Schematic Wulff-Herring diagrams explaining these transitions are presented.

Reversible transformation of a grain-boundary facet into a rough-to-rough ridge in zinc

The transition of two first-order facet-to-rough ridges into a single first-order rough-to-rough ridge theoretically predicted for free surfaces has been observed in a slowly migrating grain boundary (GB). The reversible faceting-roughening transition of the tilt GB in a Zn bicrystal has been studied in situ. Above T R+ = 673 K, the slowly migrating GB semi-circle was discontinuously curved, e.g. two rounded (rough) GB portions intersected with a slope discontinuity. Above T R+, a single first-order facet-to-rough GB ridge has been observed. Below T R− = 668 K, a facet appeared and intersected with rounded (rough) GB portions with a slope discontinuity. Below T R−, two first-order facet-to-rough GB ridges existed. The GB facet and tangents to the rounded (rough) GB portions were parallel to the closely packed planes of the constrained coincidence sites lattice. The steady state length of the facet increased with decreasing temperature. A hysteresis of about 5 K between roughening temperatures measured on ...

Migration of faceted high-angle grain boundaries in Zn

Abstract The migration of a faceted high angle tilt grain boundary 85° in Zn was investigated. The migration behavior complies with theoretical predictions of a recently proposed model on the joint motion of curved and faceted boundary segments. The results are compared to data published for a 30° boundary. It is shown that the observed dynamic facet length corresponds to a maximum dissipation rate of free energy.