Experimental Investigations of Grain Boundary Ensembles in Polycrystals

The plastic deformation behaviors of crystalline materials are usually determined by lattice dislocations. However, below a certain particle or grain size, focus is placed on the grain-boundary-mediated mechanisms (e.g., grain rotation, grain boundary sliding, and diffusion), which has been observed during recrystallization, grain growth, and plastic deformation. However, the underlying mechanisms of grain rotation remain to be studied. In this article, we review the theoretical models, molecular dynamics simulations, and experimental investigations on grain rotation. Especially, the development of in situ transmission electron microscopy (TEM) and X-ray characterization methods for probing grain boundary processes during plastic deformation provides a better understanding of the mechanisms of grain rotation. Moreover, the ability to acquire high-quality X-ray diffraction patterns from individual nanograins is expected to find broad applications in various fields such as physics, chemistry, materials science, physics, and nanoscience.

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Dependence of Intergranular Fracture on Boundary Topography and Cohesion

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100071193 ◽

1973 ◽

Vol 31 ◽

pp. 150-151

Author(s):

J. E. Doherty ◽

A. F. Giamei ◽

B. H. Kear ◽

C. W. Steinke

Keyword(s):

Grain Boundary ◽

Room Temperature ◽

Nickel Base Superalloy ◽

Boundary Shear ◽

Room Temperature Ductility ◽

Solid Solution Matrix ◽

Nickel Base ◽

Solution Matrix ◽

Different Levels ◽

Base Superalloy

Recently we have been investigating a class of nickel-base superalloys which possess substantial room temperature ductility. This improvement in ductility is directly related to improvements in grain boundary strength due to increased boundary cohesion through control of detrimental impurities and improved boundary shear strength by controlled grain boundary micros true tures.For these investigations an experimental nickel-base superalloy was doped with different levels of sulphur impurity. The micros tructure after a heat treatment of 1360°C for 2 hr, 1200°C for 16 hr consists of coherent precipitates of γ’ Ni3(Al,X) in a nickel solid solution matrix.

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Two Investigations into Grain Boundary Structure

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100080833 ◽

1977 ◽

Vol 35 ◽

pp. 688-691

Author(s):

P. Humble

Keyword(s):

Grain Boundary ◽

Dark Field ◽

Boundary Structure ◽

Boundary Dislocation ◽

Bright Field ◽

Intrinsic Structure ◽

Weak Beam ◽

Grain Boundary Structure ◽

Independent Information ◽

Diffraction Patterns

There has been sustained interest over the last few years into both the intrinsic (primary and secondary) structure of grain boundaries and the extrinsic structure e.g. the interaction of matrix dislocations with the boundary. Most of the investigations carried out by electron microscopy have involved only the use of information contained in the transmitted image (bright field, dark field, weak beam etc.). Whilst these imaging modes are appropriate to the cases of relatively coarse intrinsic or extrinsic grain boundary dislocation structures, it is apparent that in principle (and indeed in practice, e.g. (1)-(3)) the diffraction patterns from the boundary can give extra independent information about the fine scale periodic intrinsic structure of the boundary.In this paper I shall describe one investigation into each type of structure using the appropriate method of obtaining the necessary information which has been carried out recently at Tribophysics.

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Metallurgical Effects of Grain Boundary Ledges

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100078225 ◽

1977 ◽

Vol 35 ◽

pp. 126-129

Author(s):

L.E. Murr

Keyword(s):

Grain Boundary ◽

Quantitative Data ◽

Mechanical Treatment ◽

Unit Length ◽

Physical And Mechanical Properties ◽

Direct Observations ◽

Transmission Electron ◽

Properties Of Materials ◽

Thermo Mechanical Treatment ◽

Ledge Density

Ledges in grain boundaries can be identified by their characteristic contrast features (straight, black-white lines) distinct from those of lattice dislocations, for example1,2 [see Fig. 1(a) and (b)]. Simple contrast rules as pointed out by Murr and Venkatesh2, can be established so that ledges may be recognized with come confidence, and the number of ledges per unit length of grain boundary (referred to as the ledge density, m) measured by direct observations in the transmission electron microscope. Such measurements can then give rise to quantitative data which can be used to provide evidence for the influence of ledges on the physical and mechanical properties of materials.It has been shown that ledge density can be systematically altered in some metals by thermo-mechanical treatment3,4.

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Measurement of the Displacement Across a Coincidence Grain Boundary

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100078213 ◽

1977 ◽

Vol 35 ◽

pp. 124-125

Author(s):

J. W. Matthews ◽

W. M. Stobbs

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

Stacking Fault ◽

The Other ◽

Measuring Technique ◽

High Angle ◽

Twin Boundaries ◽

Rigid Displacement ◽

Cubic Crystals ◽

Lattice Displacement

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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Phase Transformation In Ti-6al-4v Alloys

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100096175 ◽

1972 ◽

Vol 30 ◽

pp. 584-585

Author(s):

Shiro Fujishiro

Keyword(s):

Phase Transformation ◽

Grain Boundary ◽

Cryogenic Temperature ◽

Β Phase ◽

Heat Treated ◽

Mixed Microstructure ◽

Ductile Behavior

The Ti-6 wt.% Al-4 wt.% V commercial alloys have exhibited an improved formability at cryogenic temperature when the alloys were heat-treated prior to the tests. The author was interested in further investigating this unusual ductile behavior which may be associated with the strain-induced transformation or twinning of the a phase, enhanced at lower temperatures. The starting materials, supplied by RMI Co., Niles, Ohio were rolled mill products in the form of 40 mil sheets. The microstructure of the as-received materials contained mainly ellipsoidal α grains measuring between 1 and 5μ. The β phase formed an undefined grain boundary around the a grains. The specimens were homogenized at 1050°C for one hour, followed by aging at 500°C for two hours, and then quenched in water to produce the α/β mixed microstructure.

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On the Shape of Field-Ion Emitters

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100092566 ◽

1976 ◽

Vol 34 ◽

pp. 520-521

Author(s):

H.C. Eaton ◽

B.N. Ranganathan ◽

T.W. Burwinkle ◽

R. J. Bayuzick ◽

J.J. Hren

Keyword(s):

Grain Boundary ◽

Spherical Shape ◽

Theoretical Strength ◽

Evaporation Process ◽

Field Evaporation ◽

Geometric Information ◽

Field Ion Microscopy ◽

Considerable Error ◽

True Shape ◽

Ion Microscopy

The shape of the emitter is of cardinal importance to field-ion microscopy. First, the field evaporation process itself is closely related to the initial tip shape. Secondly, the imaging stress, which is near the theoretical strength of the material and intrinsic to the imaging process, cannot be characterized without knowledge of the emitter shape. Finally, the problem of obtaining quantitative geometric information from the micrograph cannot be solved without knowing the shape. Previously published grain-boundary topographies were obtained employing an assumption of a spherical shape (1). The present investigation shows that the true shape deviates as much as 100 Å from sphericity and boundary reconstructions contain considerable error as a result.Our present procedures for obtaining tip shape may be summarized as follows. An empirical projection, D=f(θ), is obtained by digitizing the positions of poles on a field-ion micrograph.

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