Wetting Transitions of Grain Boundaries

1999 ◽  
Vol 586 ◽  
Author(s):  
John Blendell
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Abrupt Change ◽  
Equilibrium Shape ◽  
Second Phase ◽  
Curved Surfaces ◽  
Wetting Layer ◽  
Wetting Transitions ◽  
Microscopic Morphology ◽  
Morphology Changes

ABSTRACTIt has been shown that the concepts used to determine the equilibrium shape of crystals can be extended to determine the conditions under which grain boundaries will be fully wetted, partially wetted, or not wetted by a second phase. Recent experimental observations on the equilibrated morphologies of solid or fluid wetting phases along grain boundaries, reveal features that are predicted, and in some cases required, by this construction. Theory distinguishes between cases where surfaces are smoothly curved or where there are facets, edges and corners. In the latter case the conventional comparison of the energy of the original grain boundary with the sum of the surface energy of the two surfaces of the wetting layer leads to erroneous predictions. The correct predictions are obtained by comparing the Wulff shape of the grain boundary (the interfacial energy minimizing shape for a fixed volume of material) with a carefully defined sum of Wulff shapes of the surfaces of the wetting layer. Where orientations that are wetted join with those that are not, there is almost always an abrupt change of orientation. Faceting on two hierarchical levels can occur. Microscopic morphology changes along macroscopically curved surfaces follow well defined rules predicted by theory. The analogy between the thermodynamics of interface faceting and phase transformations allows the well known concepts of phase equilibria to be used to understand the predicted structures. The predictions of the model will be used to identify the nature of the faceting observed in alumina in the presence of a second phase.

Morphology of Σ3{/70.53°} grain boundaries in a C15 intermetallic compound

1994 ◽  
Vol 52 ◽  
pp. 684-685
Author(s):  
Fuming Chu ◽  
D. P. Pope ◽  
D. S. Zhou ◽  
T. E. Mitchell
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Crystallographic Analysis ◽  
Laves Phase ◽  
Second Phase ◽  
Bright Field Image ◽  
Boundary Edge ◽  
Arc Melting ◽  
Fcc Lattice ◽  
Diffraction Patterns

A C15 Laves phase, HfV2+Nb, shows promising mechanical properties and here we describe the structure of its grain boundaries. The C15 Laves phase has a fcc lattice with a=7.4Å. An alloy of composition Hf14V64Nb22 (including a C15 matrix and a second phase of V-rich bcc solution) was made by arc-melting. The alloy was homogenized at 1200°C for 120h. Preliminary study concentrated on Σ3{<110>/70.53°} grain boundaries in the C15 phase using Philips 400T and CM 30 microscopes.The most-commonly observed morphology of Σ3{<110>/70.53°} grain boundaries in the C15 phase is a faceted boundary. A bright field image (BFI) of the faceted boundary and the corresponding diffraction patterns with the grain boundary edge-on are shown in Fig. 1(a). From the diffraction patterns using a <110> zone axis for both grains, it is obvious that this is a Σ3{<110>/70.53°} grain boundary. Crystallographic analysis shows that the Σ3{<110>/70.53°} grain boundaries selectively facet with the following relationships between the two grains: {111}1//{111}2, {112}1//{112}2, {111}1//{115}2, and {001}1//{221}2.

Research on Micro-Mechanism of Nanocomposite Ceramic in Two-Dimensional Ultrasound Grinding

2007 ◽  
Vol 359-360 ◽  
pp. 344-348 ◽  
Author(s):  
Bo Zhao ◽  
Yan Wu ◽  
Guo Fu Gao ◽  
Feng Jiao
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Deformation Mechanism ◽  
Ground Surface ◽  
Grain Boundary Sliding ◽  
Second Phase ◽  
Two Dimensional ◽  
Composite Ceramics ◽  
Nano Composite ◽  
Boundary Sliding

Surface microstructure of nano-composite ceramics prepared by mixed coherence system and machined by two-dimensional ultrasonic precision grinding was researched using TEM, SEM, XRD detector and other equipments. Structure, formation mechanism and characteristic of metamorphic layer of ground surface of nano-composite ceramics were researched. The experiment shows micro deformation mechanism of ceramic material in two-dimensional ultrasound grinding is twin grain boundary and grain-boundary sliding for Al2O3, and it is crystal dislocation of enhanced phase, matrix grain boundary sliding, coordination deformation of intergranular second phase as well as its deformation mechanism for nano-composite ceramics. The fracture surfaces of nano-composite materials with different microscopic structure were observed using TEM and SEM. Research shows that ZrO2 plays an important influence on the generation and expansion of crack, and enhances the strength of grain boundaries. When grain boundaries is rich in the ZrO2 particles, the crack produced in grinding process will be prevented, and the surface with plastic deformation will be smooth. The results shows nanoparticles dispersed in grain boundary prevents crack propagation and makes materials fracture transgranularly which makes the processed surface fine.

The “Solidification” of Grain Boundaries with Increasing Temperature

1994 ◽  
Vol 357 ◽  
Author(s):  
Witold Lojkowski ◽  
Bogdan Palosz
Keyword(s):  
Solid State ◽  
Melting Point ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Low Temperatures ◽  
Melting Behavior ◽  
Wetting Transitions ◽  
Increasing Temperature ◽  
Segregation Of Impurities

AbstractThe aim of the paper is to explain the recently observed de-wetting grain boundary transition with increasing temperature. On the example of a bicrystal from the Fe-6at.%Si alloy, it was found recently that as temperature is increased, the following GB transitions take place: “solid” (or regular) GB-→“premelted” GB →“solid” GB. At the same time the wetting/de-wetting transitions have taken place. Another example of such GB behavior was discovered during sintering of alumina. The inverse melting behavior is explained as follows: low melting point impurities cause GB premelting at low temperatures, However de-segregation of impurities at high temperatures causes return of the GB structure to its regular “solid” state.

Texture Formation and Improvement of Grain Boundary Weak‐Links in Tape Shaped Wire Prepared by the Unidirectional Solidification Technique.

1989 ◽  
Vol 169 ◽  
Author(s):  
Michiya Okada ◽  
Toyotaka Yuasa ◽  
Tosimi Matsumoto ◽  
Katuzo Aihara ◽  
Masahiro Seido ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Fine Particles ◽  
Unidirectional Solidification ◽  
Weak Links ◽  
Second Phase ◽  
Texture Formation

AbstractAu‐Sheathed Y‐Ba‐Cu‐O(YBCO) and Tl‐Ba/Sr‐Ca‐Cu‐O(TBSCCO) tapes were fabricated by the drawing‐rolling and subsequent unidirectional solidification. A typical microstructure of melt‐textured polycrystallite including fine particles of second phase(e.q. Y‐211) was observed. The Tl‐2223 tapes prepared at an optimized condition yielded Jc=15,300 A/cm2 at 77K in the absence of magnetic field, and l,100A/cm2 in a magnetic field of IT. The enhancement of Jc in a magnetic field is suggested to be due to the improvement of weak‐links in grain boundaries.

Equilibrium Shapes of Pb Inclusions at 90° Tilt Grain Boundaries in Al

1996 ◽  
Vol 54 ◽  
pp. 364-365
Author(s):  
E. Johnson ◽  
U. Dahmen ◽  
S.-Q. Xiao ◽  
A. Johansen
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Nearest Neighbor ◽  
Lattice Mismatch ◽  
Equilibrium Shape ◽  
Minimum Energy ◽  
Aluminum Matrix ◽  
Spherical Cap ◽  
Polycrystalline Aluminum ◽  
Aluminum Films

Ion implantation of lead in aluminum leads to spontaneous phase separation and formation of dense distributions of nanosized lead inclusions[1]. The inclusions have fee structure, and despite the large lattice mismatch (aA1 = 0.4048 nm and aPb = 0.495 nm) they grow in parallel-cube topotaxy with the matrix. Their shape is cuboctahedral with larger {111} facets and smaller {100} facets which is the minimum- energy shape for an fee crystal in equilibrium with its vapor, as calculated by considering only nearest neighbor bonds. Implantation of polycrystalline aluminum films is accompanied by preferential nucle- ation and enhanced growth of inclusions in the grain boundaries. In adapting their equilibrium shape, grain boundary inclusions will be subject to a larger number of constraints than inclusions in the bulk matrix. This may result in a variety of morphologies characteristic for different types of grain boundaries.In the present study we have used a well-defined bicrystal geometry to study the morphology and structure of lead grain boundary inclusions in mazed bicrystal aluminum films containing mainly 90°<110> tilt boundaries with fixed misorientation but variable inclination[2]. It was found that the shape, size and orientation of the inclusions in the grain boundaries depend on the inclination, i.e. the orientation of the grain boundary plane. Inclusions were all single crystalline and invariably faceted toward one aluminum grain and more rounded toward the other grain (fig.l). Independent of grain boundary inclination, the faceted side was a section of the cuboctahedral equilibrium shape of inclusions in parallel topotaxy with the bulk aluminum matrix. The rounded side, where the inclusions were rotated by 90° with respect to the aluminum lattice, approximated a spherical cap consisting partly of somewhat flatter segments with complex faceting, illustrating the lack of distinctly flat low-energy facets.

Annealing in a Natural Laboratory: an EBSD and Cl Study of Calcite and Quartz Growth from Volumes of Rock Heated by a Nearby Melt Intrusion

2007 ◽  
Vol 550 ◽  
pp. 333-338 ◽  
Author(s):  
Sandra Piazolo ◽  
David J. Prior ◽  
M.D. Holness ◽  
Andreas O. Harstad
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Crystallographic Orientation ◽  
Lattice Distortion ◽  
Temperature History ◽  
Second Phase ◽  
Gradual Change ◽  
Twin Boundaries ◽  
Second Phase Particles ◽  
Crystallographic Orientations

Annealing is an important mechanism of microstructural modification both in rocks and metals. In order to relate directly changes in crystallographic orientation to migrating boundaries the researcher has the option to investigate either samples where the grain boundary motion can be directly tracked or a series of samples exhibiting successively higher degrees of annealing. Here we present results from rock samples collected from two well characterised contact aureoles (a volume of rock heated by the intrusion of a melt in its vicinity): One quartz sample in which patterns revealed by Cathodoluminescence (CL) indicate the movement of grain boundaries and a series of calcite samples of known temperature history. Electron backscatter diffraction (EBSD) analysis is used to link the movement of grain, twin boundaries and substructures with the crystallographic orientation / misorientation of a respective boundary. Results from the quartz bearing rock show: (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area, and (c) a gradual change in crystallographic orientation in the direction of growth. Observations are compatible with a growth mechanism where single atoms are attached and detached both at random and at preferential sites i.e. crystallographically controlled sites or kinks in boundary ledges. Strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. Calcite samples show with increasing duration and temperature of annealing: (a) systematic decrease of the relative frequency of low angle grain boundaries (gbs), (b) decrease in lattice distortion within grains, (c) development of distinct subgrains with little internal lattice distortion, (d) change in lobateness of gbs and frequency of facet parallel gbs and (e) change in position of second phase particles. These observations point to an increasing influence of grain boundary anisotropy with increasing annealing temperature, while at the same time the influence of second phase particles and subtle driving-force variations decrease. This study illustrates the usefulness of using samples from natural laboratories and combining different analysis techniques in microprocess analysis.

Measurement of compositional changes accompanying solid-state transformations at grain boundaries in metals

1987 ◽  
Vol 45 ◽  
pp. 296-299
Author(s):  
Ernest L. Hall ◽  
Clyde L. Briant
Keyword(s):  
Solid State ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Precipitation Process ◽  
Second Phase ◽  
Slow Cooling ◽  
Collector Plate ◽  
Compositional Changes ◽  
Property Changes ◽  
Boundary Chemistry

In many multicomponent metallic systems, solid-state precipitation processes can occur upon slow cooling or isothermal aging of solutionized material. Frequently, the precipitates form at grain boundaries, which are preferred sites for the nucleation and growth of the second phase. The precipitates generally grow through a combination of matrix and grain boundary diffusion, in which the grain boundary acts as a collector plate for the delivery of the solute to the growing precipitate. The precipitation process is thus accompanied by significant changes in the chemistry of the grain boundary and matrix regions near the grain boundary. These grain boundary chemistry changes can have a profound effect on the macroscopic properties of the material, including corrosion resistance, strength, and ductility. In order to understand the mechanism associated with these property changes, it is necessary to obtain a complete and precise description of the magnitude and extent of the compositional changes which have occurred at the grain boundaries.

Pseudopartial Grain Boundary Wetting: Key to the Thin Intergranular Layers

2013 ◽  
Vol 333 ◽  
pp. 175-192 ◽  
Author(s):  
Boris B. Straumal ◽  
Alexey Rodin ◽  
A.E. Shotanov ◽  
Alexander B. Straumal ◽  
Olga A. Kogtenkova ◽  
...  
Keyword(s):  
Contact Angle ◽  
Phase Transitions ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Thin Layers ◽  
Second Phase ◽  
Triple Junctions ◽  
Complete Wetting ◽  
Grain Boundary Wetting

The thin layers of a second phase (also called complexions) in grain boundaries (GB) and triple junctions (TJs) are more and more frequently observed in polycrystals. The prewetting (or premelting) phase transitions were the first phenomena proposed to explain their existence. The deficit of the wetting phase in case of complete wetting can also lead to the formation of thin GB and TJ phases. However, only the phenomenon of pseudopartial (or pseudoincomplete, or constrained complete) wetting permitted to explain, how the thin GB film can exist in the equilibrium with GB lenses of a second phase with non-zero contact angle.

Grain Boundary Structure as a Function of Aluminum Level in Ni3Al

1990 ◽  
Vol 213 ◽  
Author(s):  
J.A. Horton ◽  
C.T. Liu ◽  
S.J. Pennycook
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Structure ◽  
Boundary Phase ◽  
Second Phase ◽  
Contrast Imaging ◽  
Minimum Width ◽  
Undoped Material ◽  
Boron Doped ◽  
Aluminum Level

ABSTRACTUnderstanding the boron effect in Ni3Al currently centers on determining thestructure of the region near grain boundaries, especially the presence of a disordered γ phase. In this study, a series of alloys was examined by transmission electron microscopy (TEM) as a function of aluminum level, boron level, and cooling rate for thepresence of any grain boundary phases. The base alloy series all contained 0.5 at. % Hf. At 21.5 at. % Al,γ formed in the matrix as expected. However, in slowly cooled specimens with 22 and 22.5% Al, a second phase with thicknesses from 25 to 50 nm formed on some of the grain boundaries. At higher aluminum levels no evidence of any second phase was observed. The minimum width of grain boundary images formed by normal diffraction contrast imaging was generally 0.5 to 1 nm for grain boundaries that presumably had no second phase and were parallel to the electron beam. Therefore, a second phase with a thickness of 1 nm or less would not be discernable by TEM. High resolution Z-contrast imaging by scanning TEM of a very low angle boundary in directionally solidified (DS) material also showed no evidence of disordering, but did show a surprising amount of non-planarity of the boundary.Whereas slower cooling rates might be expected to aid the formation of any grain boundary phase and thereby increase the ductility, the ductility was slightly lower for the more slowly cooled alloys than for the furnace cooled alloys. From 21.5 to 24% Al, the tensile elongations remain fairly constant with values ranging from 45 to 52%. Above 24%, the ductility drops off rapidly but is still much higher for the boron-doped material than for the undoped material. At 25.2% Al, the ductility is still 4% as compared to essentially zero for undoped material. Therefore the ductility improvements in boron-doped Ni3Al do not require the presence of any grain boundary disordered phases discernable by TEM.

Morphological Changes in Rod-Shaped Precipitates With Internal Boundaries: Finite Difference Analysis

1991 ◽  
Vol 237 ◽  
Author(s):  
Jun-Ho Choy ◽  
S. A. Hackney ◽  
J. K. Lee
Keyword(s):  
Aspect Ratio ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Morphological Changes ◽  
Equilibrium Shape ◽  
Interfacial Free Energy ◽  
Dynamic State ◽  
Shape Evolution ◽  
Boundary Groove ◽  
The Relationship

ABSTRACTShape evolution of rod-shaped precipitates due to surface diffusion has been studied under the conditions of constant volume and isotropie interfacial free energy. The shape evolution depends strongly on both the initial aspect ratio and the grain boundary groove angle. For a finite rod with one grain boundary, the morphology evolves into an equilibrium shape made of spherical portions if its aspect ratio and the groove angle are small. Increase in the aspect ratio causes a boundary splitting. For an infinite rod with periodic boundaries, three types of morphological evolutions are observed. When the relationship between the aspect ratio and the groove angle satisfies a certain critical condition, the shape evolves into an equilibrium. If the relationship deviates significantly from this condition, an ovulation process takes place at each location of the internal grain boundaries. When the deviation is intermediate, the morphology undergoes an oscillation in a quasi-dynamic state between the process toward an equilibrium shape and the ovulation process. The ovulation process due to internal grain boundaries is found to precede the Rayleigh spheroidization process.

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