Control of Grain Boundary Microstructures in Liquid-Phase Sintered Alumina

1999 ◽  
Vol 586 ◽  
Author(s):  
N. Ravishankar ◽  
C. Barry Carter
Keyword(s):  
Free Surface ◽  
Liquid Phase ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Driving Force ◽  
Free Surfaces ◽  
Surface Energies ◽  
The Difference ◽  
Sintered Materials

ABSTRACTThe elimination of the grain boundary liquid in liquid-phase sintered materials is examined for the case of anorthite liquid in alumina grain boundaries. It is shown that under suitable conditions the liquid can exude from the grain boundary to the free surface. The proposed driving force is provided by the difference in energies and wetting behavior of the grain boundary and the free surface at high temperatures. The results emphasize the importance of the crystallography of the boundary and the nature of free surfaces (i.e., the surface energies) on the exudation behavior.

Glass-Crystal Boundaries in Liquid-Phase Sintered Ceramics

2000 ◽  
Vol 620 ◽  
Author(s):  
N. Ravishankar ◽  
C. Barry Carter
Keyword(s):  
Free Surface ◽  
Liquid Phase ◽  
Surface Energy ◽  
Grain Boundary ◽  
Surface Structure ◽  
Glass Crystal ◽  
The Difference ◽  
Crystal Boundaries

ABSTRACTThe interface between dewet glass droplets and the free surface of a crystal and the interface between the intergranular glass and adjoining crystalline grains have been examined with particular emphasis on the influence of the crystallography of the free surface and the grain boundary. The wetting of liquid on the free surface has been shown to depend on the surface structure. The migration of boundaries containing a liquid phase has been studied. The migration is initiated by the difference in surface energy of the bounding planes. Faceting of the grain boundary planes has been examined. It is proposed that the boundary migrates by the motion of the facets.

Simulation of Microstructural Evolution in Polycrystalline Films

1990 ◽  
Vol 202 ◽  
Author(s):  
H. J. Frost
Keyword(s):  
Free Surface ◽  
Surface Energy ◽  
Film Thickness ◽  
Grain Boundaries ◽  
Grain Growth ◽  
Driving Force ◽  
Three Dimensional ◽  
Grain Structure ◽  
Two Dimensional ◽  
Free Surfaces

ABSTRACTThis paper will review the topic of computer simulation of the evolution of grain structure in polycrystalline thin films, with particular attention to the modelling of the grain growth process. If the grain size is small compared to the film thickness, then the grain structure is three-dimensional. As the grains grow to become larger than the film thickness, so that most grains traverse the entire thickness of the film, the microstructure may approach the conditions for a two-dimensional grain structure. Both two- and three-dimensional grain growth have been simulated by various authors.When the grains become large enough for the microstructure to be two-dimensional, the surface energy associated with the two free surfaces of the film becomes comparable to the surface energy of the grain boundaries. In this condition, the free surface may profoundly effect the grain growth. One effect is that grooves may develop along the lines where the grain boundaries meet the free surfaces. This grooving may pin the boundaries against further migration and lead to grain-growth stagnation. Another possible effect is that differences in the free surface energy for grains with different crystallographic orientation may provide a driving force for the migration of the boundaries which is additional to that provided by grain boundary capillarity. Grains with favorable orientations will grow at the expense of grains with unfavorable orientations. The coupling of grain-growth stagnation with an additional driving force can produce abnormal or secondary grain growth in which a few grains grow very large by consuming the normal grains.

Energetics of Ideal Grain Boundary Fracture in Iron and the Thermodynamic Criterion of Impurity Embrittlement

1996 ◽  
Vol 458 ◽  
Author(s):  
Genrich L. Krasko
Keyword(s):  
Free Surface ◽  
Grain Boundary ◽  
Interatomic Interaction ◽  
Empirical Method ◽  
Empirical Modeling ◽  
Impurity Atoms ◽  
Surface Energies ◽  
Thermodynamic Criterion ◽  
The Difference ◽  
Semi Empirical

ABSTRACTModeling of grain boundary (GB) relaxation during ideal fracture, and the fracture energetics of a Σ3 (111) GB in Fe was performed using the modified Finnis-Sinclair semi-empirical method, and utilizing the so-called “environment-sensitive embedding energies” of impurity atoms, introduced earlier by the author. The calculations were done for both the clean GB, and GB with the following impurity atoms: H, B, C, N, O, P and S. The ideal fracture was modeled by separating the two halves of crystal normal to the GB, step-wise, minimizing the total crystal energy at every step. The interplanar distances were varied, while the Fe interatomic spacing within the hexagonal planes was held fixed. When the distance between the two crystal halves: one with the impurity and another without, exceeded the interatomic interaction cut-off radius (3.6 Å), two different free surfaces - with and without the impurity - emerged. The GB and surface energies were found both for the pure Fe, and that with impurity atoms at the GB or free surface. Both the (111) GB energy and the (111) surface energy of pure Fe agree well with experimental data and results of previous semi-empirical modeling. In general, the correlation between the embrittling/ cohesion enhancing effect of impurities in GB and the difference between the GB and free surface energies agrees with the thermodynamic criterion of embrittlement.

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.

Grain Boundary Diffusion and Segregation in the Solid State Phase Transformations

1998 ◽  
Vol 527 ◽  
Author(s):  
E. Rabkin ◽  
W. Gust
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Diffusion ◽  
Grain Boundary Diffusion ◽  
Solute Diffusion ◽  
Apparent Difference ◽  
Impurity Drag ◽  
The Difference ◽  
Boundary Width ◽  
Dynamic Segregation

ABSTRACTWe consider the problem of solute diffusion and segregation in the grain boundaries moving during a phase transformation in the framework of Cahn's impurity drag model. The concept of a dynamic segregation factor for the diffusion along moving grain boundaries is introduced. The difference between static and dynamic segregation factors may cause the apparent difference of the triple product of the segregation factor, grain boundary width and grain boundary diffusion coefficient for stationary and moving grain boundaries. The difference between static and dynamic segregation is experimentally verified for the Cu(In)-Bi system, for which the parameters of static segregation are well-known. It is shown that the complications associated with the dynamic segregation may be avoided during the study of the discontinuous ordering reaction. From the kinetics of this reaction, the activation energy of the grain boundary self-diffusion can be determined.

Effects of Alloying Elements on Iron Grain Boundary Cohesion

1997 ◽  
Vol 492 ◽  
Author(s):  
X. Chen ◽  
D. E. Ellis ◽  
G. B. Olson
Keyword(s):  
Molecular Dynamics ◽  
Free Surface ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
First Principles ◽  
Electronic Structure Calculations ◽  
Alloying Elements ◽  
Alloying Element ◽  
Long Time ◽  
Structure Calculations

For a long time, understanding the mechanisms of impurity-promoted embrittlement in iron and the consequent cohesion(decohesion) effects has been a challenge for materials scientists. The role alloying elements play in impurity-promoted embrittlement is important due to either their direct intergranular cohesion(decohesion) effects or effects upon embrittling potency of other impurities. Some alloying elements like Pd and Mo are known to be helpful for intergranular cohesion in iron and some other alloying elements like Mn are known to segregate to and weaken iron grain boundaries dramatically[1]. There have been intensive investigations on these mechanisms for a long time and especially, with the progress in computing techniques in recent years, calculations on more realistic models have become possible[2–4]. In this paper we briefly present our studies on some selected alloying-element/iron grain boundaries(GB) and free surface(FS) systems. The effects of Pd, Mo, Mn and Cr on the Fe Σ5 (031) grain boundary and its corresponding (031) free surface are examined, using a combination of molecular dynamics(MD) and first-principles electronic structure calculations. Section 2 gives a brief introduction to the methods used and Section 3 gives the main results.

Competitive Motions of Grain-Boundary and Free Surface in Selecting Thin Film Morphology

1996 ◽  
Vol 441 ◽  
Author(s):  
B. Sun ◽  
Z. Suo ◽  
W. Yang
Keyword(s):  
Thin Film ◽  
Free Energy ◽  
Free Surface ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Analytic Solutions ◽  
Film Morphology ◽  
Polycrystalline Thin Film ◽  
Continuous Film

AbstractDuring annealing of a polycrystalline thin film, grain-boundaries and film surfaces move. If the grain-boundaries move faster, the grains having the lowest free energy grow at the expense of others, resulting in a continuous film with large grains. If the film surfaces move faster, they groove along their junctions with the grain-boundaries, breaking the film to islands. This paper describes analytic solutions for steady surface motions, and discusses the morphology selection.

Crystallite Rotation Experiments Revisited: the Contribution of Free-Surface Interactions

1993 ◽  
Vol 319 ◽  
Author(s):  
L. Balasubramanian ◽  
A.H. King
Keyword(s):  
Free Surface ◽  
Grain Boundary ◽  
Boundary Energy ◽  
Surface Interactions ◽  
Grain Boundary Energy ◽  
Free Surfaces ◽  
Rotation Rates ◽  
Crystal Rotation

AbstractWhile crystallite-rotation experiments of the type pioneered by Gleiter and his co-workers have provided a great deal of information about the variation of grain boundary energy with crystal misorientation, certain aspects of the experiments still remain puzzling. Notably, crystal rotation rates do not follow the form predicted by consideration of the Read-Shockley equation; and Chan and Balluffi have shown that in some cases certain crystallites are able to escape from the energy cusps that trap others [5]. In this paper we re-examine the crystallite rotation mechanism and rotation rates from the perspective of the interactions of grain boundary dislocations with the free surfaces that terminate the grain boundary.

Experimental Evidence of Iron Segregation in Copper Grain Boundaries as Deduced from Type B Diffusion Measurements

2006 ◽  
Vol 249 ◽  
pp. 161-166 ◽  
Author(s):  
Jean Bernardini ◽  
Christophe Girardeaux ◽  
Andree Rolland
Keyword(s):  
Grain Boundary ◽  
Driving Force ◽  
Positive Curvature ◽  
Pure Copper ◽  
Boundary Segregation ◽  
Grain Boundary Segregation ◽  
Type B ◽  
Self Diffusion ◽  
Surface Energies ◽  
Respective Surface

Grain-boundary heterodiffusion of iron in pure copper and self diffusion of iron in copper–0.091at% iron were measured by the serial sectioning technique in the Harrison B-regime. The penetration profiles corresponding to iron heterodiffusion in pure copper show a strong positive curvature far beyond the (Dvt)1/2 depth . This peculiar shape, which does not exist for self diffusion in the solid solution, proves the presence of a strong non linear grain-boundary segregation of iron in copper in spite of the respective surface energies of these metals. This segregation is linked to the size effect which is, as predicted by numerical simulation, the main driving force for grainboundary segregation.

Study of Bonding of Grain Boundaries in Steels Using EELS

2005 ◽  
Vol 475-479 ◽  
pp. 4063-4066
Author(s):  
X. Zhang ◽  
Lina Zhang ◽  
Jun Jie Qi ◽  
Yue Ma
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Intergranular Fracture ◽  
Fracture Mode ◽  
Fracture Property ◽  
Transgranular Fracture ◽  
The Difference ◽  
Relationship Of ◽  
The Relationship

A novel EELS technique was developed to study bonding of grain boundary in many kinds of steels. We measured the normalized intensities of Fe white lines and calculated the occupancies of 3d states of iron, and then analyzed the relationship of the occupancies of 3d states of iron and the fracture property of the steels. We found that if the grain boundary has a different occupancy of 3d state of iron from that of the bulk, the steel tends to have an intergranular fracture, whereas if the grain boundary has almost the same occupancy of 3d state as the bulk, the steel tends to have a transgranular fracture. Our result shows that the difference in the occupancy of 3d state between bulk and grain boundary can be used to study the fracture mode at grain boundary in steel.

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