Comment on “The role of special high angle grain boundaries in precipitate nucleation” by P.H. Pumphrey

Many high-angle grain boundaries in cubic crystals are thought to be either coincidence boundaries (1) or coincidence boundaries to which grain boundary dislocations have been added (1,2). Calculations of the arrangement of atoms inside coincidence boundaries suggest that the coincidence lattice will usually not be continuous across a coincidence boundary (3). There will usually be a rigid displacement of the lattice on one side of the boundary relative to that on the other. This displacement gives rise to a stacking fault in the coincidence lattice.Recently, Pond (4) and Smith (5) have measured the lattice displacement at coincidence boundaries in aluminum. We have developed (6) an alternative to the measuring technique used by them, and have used it to find two of the three components of the displacement at {112} lateral twin boundaries in gold. This paper describes our method and presents a brief account of the results we have obtained.

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Elucidation of block boundaries as the dissolution channels in hydrated cement

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100109562 ◽

1978 ◽

Vol 36 (1) ◽

pp. 484-485

Author(s):

N.V. Belov ◽

U.I. Papiashwili ◽

B.E. Yudovich

Keyword(s):

Liquid Phase ◽

Grain Boundaries ◽

Benzoyl Peroxide ◽

Phase Contrast ◽

Active Role ◽

Point Of View ◽

Hardened Cement ◽

Hydrated Cement ◽

Hardened Cement Paste

It has been almost universally adopted that dissolution of solids proceeds with development of uniform, continuous frontiers of reaction.However this point of view is doubtful / 1 /. E.g. we have proved the active role of the block (grain) boundaries in the main phases of cement, these boundaries being the areas of hydrate phases' nucleation / 2 /. It has brought to the supposition that the dissolution frontier of cement particles in water is discrete. It seems also probable that the dissolution proceeds through the channels, which serve both for the liquid phase movement and for the drainage of the incongruant solution products. These channels can be appeared along the block boundaries.In order to demonsrate it, we have offered the method of phase-contrast impregnation of the hardened cement paste with the solution of methyl metacrylahe and benzoyl peroxide. The viscosity of this solution is equal to that of water.

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Observations of dislocation interactions with recrystallized grain boundaries

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100105205 ◽

1988 ◽

Vol 46 ◽

pp. 628-629

Author(s):

C. W. Price

Keyword(s):

Dislocation Density ◽

Grain Boundary ◽

Strain Rate ◽

Grain Boundaries ◽

Dislocation Structure ◽

Hot Deformation ◽

Static Recrystallization ◽

High Dislocation Density ◽

High Angle ◽

Dislocation Interactions

Little evidence exists on the interaction of individual dislocations with recrystallized grain boundaries, primarily because of the severely overlapping contrast of the high dislocation density usually present during recrystallization. Interesting evidence of such interaction, Fig. 1, was discovered during examination of some old work on the hot deformation of Al-4.64 Cu. The specimen was deformed in a programmable thermomechanical instrument at 527 C and a strain rate of 25 cm/cm/s to a strain of 0.7. Static recrystallization occurred during a post anneal of 23 s also at 527 C. The figure shows evidence of dissociation of a subboundary at an intersection with a recrystallized high-angle grain boundary. At least one set of dislocations appears to be out of contrast in Fig. 1, and a grainboundary precipitate also is visible. Unfortunately, only subgrain sizes were of interest at the time the micrograph was recorded, and no attempt was made to analyze the dislocation structure.

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Determination of the lattice translation across {110} APBs in GaAs

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100105060 ◽

1988 ◽

Vol 46 ◽

pp. 600-601

Author(s):

D.R. Rasmussen ◽

N.-H. Cho ◽

C.B. Carter

Keyword(s):

Grain Boundaries ◽

Lower Energy ◽

Antiphase Boundary ◽

The Other ◽

Boundary Structure ◽

Interface Plane ◽

Inversion Symmetry ◽

High Angle ◽

Equilibrium Distance

Domains in GaAs can exist which are related to one another by the inversion symmetry, i.e., the sites of gallium and arsenic in one domain are interchanged in the other domain. The boundary between these two different domains is known as an antiphase boundary [1], In the terminology used to describe grain boundaries, the grains on either side of this boundary can be regarded as being Σ=1-related. For the {110} interface plane, in particular, there are equal numbers of GaGa and As-As anti-site bonds across the interface. The equilibrium distance between two atoms of the same kind crossing the boundary is expected to be different from the length of normal GaAs bonds in the bulk. Therefore, the relative position of each grain on either side of an APB may be translated such that the boundary can have a lower energy situation. This translation does not affect the perfect Σ=1 coincidence site relationship. Such a lattice translation is expected for all high-angle grain boundaries as a way of relaxation of the boundary structure.

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Carbide precipitation and chromium depletion on coherent twin boundaries in deformed 304 stainless steels

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100130717 ◽

1992 ◽

Vol 50 (2) ◽

pp. 1216-1217

Author(s):

A.H. Advani ◽

L.E. Murr ◽

D.J. Matlock ◽

W.W. Fisher ◽

P.M. Tarin ◽

...

Keyword(s):

Grain Boundaries ◽

Stainless Steels ◽

Interfacial Free Energy ◽

Carbide Precipitation ◽

Twin Boundaries ◽

Invariant Structure ◽

Heat Treated ◽

Constant Structure ◽

Cr Depletion

Coherent annealing-twin boundaries are constant structure and energy interfaces with an average interfacial free energy of ∼19mJ/m2 versus ∼210 and ∼835mJ/m2 for incoherent twins and “regular” grain boundaries respectively in 304 stainless steels (SS). Due to their low energy, coherent twins form carbides about a factor of 100 slower than grain boundaries, and limited work has also shown differences in Cr-depletion (sensitization) between twin versus grain boundaries. Plastic deformation, may, however, alter the kinetics and thermodynamics of twin-sensitization which is not well understood. The objective of this work was to understand the mechanisms of carbide precipitation and Cr-depletion on coherent twin boundaries in deformed SS. The research is directed toward using this invariant structure and energy interface to understand and model the role of interfacial characteristics on deformation-induced sensitization in SS. Carbides and Cr-depletion were examined on a 20%-strain, 0.051%C-304SS, heat treated to 625°C-4.5h, as described elsewhere.

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Precipitation on Low Angle Boundaries in Al-Zn-Mg

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s1431927600001574 ◽

1980 ◽

Vol 38 ◽

pp. 374-375

Author(s):

Diane M. Vanderwalker

Keyword(s):

Grain Boundaries ◽

High Strength ◽

Boundary Structure ◽

Aircraft Industry ◽

Precipitate Free Zone ◽

High Angle ◽

Adjacent Region ◽

Free Zone ◽

Widespread Interest

There is a widespread interest in understanding the properties of Al-base alloys so that progress can be made toward extending their present applications in the aircraft industry. Al-Zn-Mg is precipitation hardened to gain its high strength; however, during aging the formation of heterogeneous precipitates on the grain boundaries creates a precipitate-free zone in the adjacent region. Since high angle grain boundaries are not easily characterized, it is difficult to establish a relationship between the precipitate and the boundary structure. Therefore, this study involves precipitation on low angle grain boundaries where the boundary and the precipitate can be fully analyzed.

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