Grain Boundary Kinetics Research Articles

Inclination dependence of grain boundary energy and its impact on the faceting and kinetics of tilt grain boundaries in aluminum

The shape evolution and migration of <1 0 0> and <1 1 1> tilt grain boundaries with rotation angles θ in the range between 6° and 24° were investigated in situ in a scanning electron microscope at elevated temperatures. The results revealed that boundaries with misorientation θ < 15° did not attain a continuously curved shape in the entire temperature range up to the melting point and, thus, did not move under a capillary driving force. Instead, they remained straight or formed several facets which were inclined to the initial boundary orientation. Molecular statics simulations suggest that the observed behavior of low-angle boundaries is due to the anisotropy of grain boundary energy with respect to boundary inclination. This anisotropy diminishes with increasing misorientation angle, and high-angle boundaries assume a continuously curved shape and move steadily under the curvature driving force.

Simulation of Fracture of Nanocrystalline Metals

A computation sample of nanocrystallion metal with a crack is proposed in this work. The structural evolution scheme is employed to model the mass flow, the grain boundary cavitation, and the crack growth of the specimen under remote loading. The scheme leads to a system of ordinary differential equations that can be solved by Euler integration. The simulation validates the proposal that the ductile versus brittle transition of nano-grained metals is dictated by the competition of creep deformation promoted by grain-boundary kinetics, and the decohesion of grain boundaries.

Grain Boundary Engineering by Application of Mechanical Stresses

It is shown that an externally applied mechanical stress field can change the kinetics of individual grain boundaries. Moreover, such mechanical stresses also have influence on grain growth and recrystallization kinetics and can strongly affect the microstructure evolution, so that the application of mechanical stresses during annealing can be used as a new approach in the field of grain boundary engineering.

High-Pressure Influence on the Kinetics of Grain Boundary Segregation in the Cu–Bi System

The effect of pressure on the kinetics of grain boundary (GB) segregation in the Cu–50 at. ppm Bi alloy has been studied. It was found by means of Auger electron spectroscopy that at a temperature of 1173 K the segregation level decreases from 2 to 1.5 monolayer as the pressure increases from 0.01 to 1.2 GPa. This behavior was explained by considering the physical parameters controlling kinetics and thermodynamics of GB segregation. A simplified model based on dislocation pipe diffusion, proposed previously and discussed in more details in this work, was used to calculate the non-equilibrium GB segregation during cooling under high pressure. The pressure effect on bulk diffusion is responsible for the suppression of GB segregation, while the pressure effect on the phase stability in Cu–Bi alloys provides a negligible contribution on GB segregation in the pressure range studied.

A serial sectioning technique for evaluating grain and twin boundary precipitate growth kinetics in bulk specimens

A new measurement technique for the growth kinetics of both grain boundary and twin boundary bainite precipitates in bulk specimens is demonstrated. The key advance is the use of coarse-scale serial sectioning to determine the angle made by the grain or twin boundary to the plane of polish, so an explicit stereological correction to the precipitate thickness can be made. This technique is applied to the thickening of bainite at the bay temperature of Fe–0.24C–4Mo, where grain boundary bainite growth kinetics had been measured previously in much thinner specimens using the “bamboo-specimen” method developed by Bradley et al. Several key assumptions implicit in the “bamboo-specimen” method are critically examined using the data obtained from this new technique. Twin boundary bainite growth thickening kinetics, which cannot be obtained using the bamboo method, are also obtained for the first time, and are measured in the same steel.

Two-Dimensional Faceting Kinetics of Grain Boundaries

Faceting is characteristic of both special and incommensurate grain boundaries. The orientation of a facet is related to the misorientation angle between the grains. The faceting effect is analyzed here in a kinetic model for the faceting phase transition and the formation of pyramids of facets adjacent to one another.

On grain boundary wetting during deformation

We use numerical experiments to analyze the kinetics of grain boundary wetting in polycrystalline aggregates under the influence of an external applied shear stress. Grain boundary wetting is driven by the stress concentration at the tip of the fluid front. The extent of wetting of favorably oriented grain boundaries depends on the fluid mobility number β, which is defined as the ratio of the rate of stress relaxation by grain boundary wetting to the rate of stress relaxation by grain boundary diffusion. The applied stress can be relaxed by grain boundary diffusion, by grain boundary wetting or by a combination of both. In the wetting regime, an anisotropic grain boundary fluid network will develop due to the external shear stress.

Grain Boundary Kinetics in Oxide Ceramics with the Cubic Fluorite Crystal Structure and its Derivatives

Kinetics of grain boundaries in oxides with the cubic fluorite structure and its derivatives has been investigated using fine grain ceramics that are fully dense. Grain growth measurements in these materials have provided information on grain boundary diffusivity over a diffusion distance of the order of the initial grain size. With the addition of solute cations, grain boundary mobility can be varied over many orders of magnitude, often with very different activation energies. This is caused by the variation of defect population and the defect-solute association. Definitive evidence for solute drag has also been observed in some cases, but solute drag can not be confirmed as a general mechanism in solid solutions. Lastly, while grain boundary at low temperature may continue to serve as a fast diffusion path, it may not be able to migrate because of additional pinning mechanisms such as those exerted by grain boundary nodal points or lines. This means that sintering without grain growth is possible, opening up an avenue for obtaining ultrafine ceramics by pressureless sintering.

Effect of boron on the ductility of ordered Ni-Ni4Mo alloys

Boron was identified to have a beneficial effect on the room temperature ductility of ordered Ni-Ni4Mo alloys. An addition of 0.01 to 0.03 wt % boron to hypostoichiometric alloys was found to increase the tensile ductility from about 5% to 30% in the ordered state produced by exposure at 600 to 800 °C, of the annealed material. The boron effect was, however, found to diminish with exposure time at a given temperature and not to be maintained at elevated temperatures. The beneficial effect of boron on room temperature ductility was also found to be considerably less pronounced in stoichiometric Ni4Mo alloy. Both electron energy loss spectroscopy and Auger electron spectroscopy techniques revealed no preferential segregation of boron to grain boundaries. Experimental results suggested that boron decelerates the kinetics of heterogeneous grain boundary ordering which leads to an improvement in ductility.

Overview no. 57 Morphology, crystallography and kinetics of sympathetic nucleation

The phenomenon of sympathetic nucleation was investigated in detail, employing the Widmanstatten α precipitation in Ti-6.6 at% Cr and Ti-8.6 at% Mn alloys as a typical example. Two types of morphological arrangement, resulting from edge-to-edge and edge-to-face sympathetic nucleation are clearly identified. The question of whether or not the latter arrangement is formed by branching in the style of solidification dendrities was considered. By expressing the orientation relationship of precipitates in the form of a matrix, a search was made for the relationships which permit branching with equivalent pairs of conjugate habit planes. From the results of such analyses, carried out on commonly observed orientation relationships among f.c.c., b.c.c. and h.c.p. phases, it is concluded that crystallographic constraints essentially prohibit branching in all of these systems. A comparative theoretical study of the relative nucleation kinetics of homogeneous, grain boundary and sympathetic nucleation demonstrated that the activation barrier associated with these processes become comparable when sympathetic nucleation occurs on the terraces of plate shaped precipitate and the matrix:precipitate interface is replaced by a relatively low energy precipitate: precipitate boundary. TEM examination of regions in which sympathetic nucleation occurred indicated that the interphase boundaries are invariably replaced by small-angle precipitate:precipitate boundaries essentially in accord with theory.

Nucleation, growth, and overall transformation kinetics of grain boundary allotriomorphs of proeutectoid alpha in Ti-3.2 At. Pct Co and Ti-6.6 At. Pct Cr alloys

The nucleation, growth, and overall transformation kinetics of grain boundary α allotriomorphs were measured in Ti-3.2 at. pct Co and Ti-6.6 at. pct Cr alloys with optical microscopy. Nucleation kinetics were interpreted with classical heterogeneous nucleation theory, using the pillbox model of the critical nucleus. Growth kinetics of allotriomorphs were significantly accelerated by the rejector plate mechanism, but much less so than allotriomorphs formed in fcc substitutional matrices at comparable homologous temperatures. Overall transformation kinetics were accounted for with a modified version of Cahn’s theory of grain boundary nucleated reactions.

Migration Kinetics of (001) Twist Grain Boundaries in Silver Bicrystalline Films

Migration Kinetics of (001) Twist Grain Boundaries in Silver Bicrystalline Films

The kinetics of grain boundary grooving in copper

In an earlier paper the diffusion equation associated with grain boundary grooving was solved and it was concluded that surface diffusion was the dominant transport mechanism for copper. For this mechanism the theory predicts that the shape of the groove profile has a time independent form and that the groove width and depth increase linearly with t 1 4 . These two predictions have been tested by studying the grooving of tilt boundaries in copper annealed in dry hydrogen at 930°C and 1035°C. Both predictions agree with the experimental results. The validity of the theory allows one to calculate the surface diffusion coefficient, D s . The advantages of this method of determining D s are discussed.