S.L. Sass

The detection of the periodic structure of high-angle twist boundaries

Abstract An electron diffraction technique was developed to study the structure of high-angle grain boundaries. Diffraction patterns were obtained from the grain boundary region, and it was demonstrated for the first time that high-anglo 〈001〉 twist boundaries have a periodic nature over the entire range of possible misorientations. Relatively strong grain boundary reflections corresponding to reflections from the 0-lattice wore obtained from all boundaries studied. These results indicate that atomic relaxations occur around the 0-lattice points and those relaxations appear to be qualitatively similar to those which are well known to occur in low-angle twist boundaries.

Control of the mechanical properties of metal-ceramic interfaces through interfacial reactions

Abstract The use of interfacial reactions to control the structure and shear strength of metal-ceramic interfaces was studied in the NiO-Pt system. Interfaces were formed by hot1pressing together thin NiO single crystals and thick Pt polycrystalline films. Suitable choice of the annealing temperature, time and oxygen partial pressure allowed the introduction at the interface of a layer of either an intermetallic compound NiPt with thickness between 1 and 65 nm or a Ni-Pt solid solution.The shear strength of the NiO-Pt interface with and without the different interlayers present was measured by the periodic cracking method. Compared to its originally hot pressed state the shear strength of the NiO-Pt interface was increased by a factor of at least 4 by the presence of the NiPt and by ∼10 by the solid solution. The use of interfacial reactions to control interfacial strength may also be applicable in other metal-ceramic systems where the metal and the cation form intermetallic compounds, and where the oxidation potentials of the metal and the cation are significantly different.

Grain boundary dislocation networks as electron diffraction gratings

Abstract Electron diffraction patterns from thin-film gold bicrystal specimens having grain boundaries containing periodic networks of grain boundary dislocations (GBDs) have been studied. It has been demonstrated that planar GBD networks (which were examined in the electron microscope at normal incidence) give rise to arrays of extra diffraction sports centred around the main f.c.c. gold reflections. The periodic structure of the grain boundary region there fore acts very much as a difraction grating. Examples of this effect are presented for orthogonal grids of screw GBDs in twist boundaries and arrays of parallel edge GBDs in tilt boundaries. The importance of this phenomenon in the study of GBD structure is discussed. It is pointed out that, since the spacing of the GBD spots is reciprocally related to the actual GBD spacing, this effect would be particularly useful in examining fine GBD networks. Also, the possibility of forming a direct image of the GBD network using the GBD spots is emphasized.

Structure, chemistry and bonding at grain boundaries in Ni3Al—I. The role of boron in ductilizing grain boundaries

Abstract Boron segregation is known to change the fracture mode from intergranular to transgranular in Ni-rich Ni3Al. This paper addresses the question of why boron segregation at grain boundaries improves their resistance to fracture. Grain boundaries in B-free and B-doped Ni-rich Ni3Al (76 at. % Ni) were examined using spatially resolved electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS) and annular dark field (ADF) imaging in an UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni-enrichment was seen in a 0.5 – 1.0 nm wide region at large angle boundaries, both in the absence and presence of B. Using EELS, B segregation to the boundary was observed to vary along the interface. EELS of the Ni L2,3 edge showed that the B-rich regions have a bonding similar to that in bulk Ni3Al, while the B-free regions have a bonding similar to the more Ni-like character of undoped boundaries. These results demonstrate that boron segregation increases the cohesive strength of grain boundaries in Ni3Al by making the bonding at the boundary similar to that in the bulk and, in this manner, increases their fracture resistance.

Deformation and fracture of intermetallics

Abstract Recent advances in our understanding of the fundamental mechanisms of strength and ductility in ordered intermetallic alloys are summarized. State of the art experimental techniques and innovative computational advances have together stimulated the current activity in this field and opened new opportunities for research. The current status and scientific issues in the area of bulk and defect properties, deformation and strengthening mechanisms, and fracture and toughening mechanisms are assessed. Future research areas of major importance for understanding of deformation and fracture behavior in intermetallic compounds are identified.

The Initial Stages of the Omega Phase Transformation in Ti-V Alloys.

Abstract : The as-quenched omega phase and its transition to the aged form of the omega phase was examined using selected area diffraction and dark field electron microscopy. (Author)

Electron diffraction and microscopy studies of the structure of grain boundaries in silicon

Abstract The diffraction effects expected from the periodic structure of twist boundaries in Si are determined by an examination of the reciprocal lattice of these boundaries. Methods of analysis are developed to distinguish between the real diffraction spots due to the periodic boundary structure and those due to double diffraction effects. The electron microscope images from the boundaries studied in Si bicrystals were frequently found to be complex and contained moire fringes which provided no information on the actual boundary structure. By analysing the electron diffraction patterns from these boundaries for the presence of new diffraction spots it is possible to show that all the Σ1 [001], Σ1 [111] and Σ3 [111] twist boundaries examined have a periodic structure.

Diffraction from periodic arrays of dislocations

Abstract The structure factor of a square grid of screw dislocations was computed using the displacement field of van der Merwe for a low-angle twist boundary. The calculations predict, for dislocations spaced d D apart, that the electron diffraction pattern from the dislocation array viewed at normal incidence consists of square arrays of reflections spaced I/d D apart, centred around f.c.c. reflections, in the same orientation as the square dislocation grid. No extra reflections were predicted around the reciprocal space origin. The calculated pattern showed excellent agreement with the observed electron diffraction pattern from a low-angle twist boundary in gold. The use of these reflections to study periodic arrays of closely spaced dislocations was discussed and it was estimated that d Ds as small as 4|b|, corresponding to a misorientation of 14°, would be detectable.

Observation of the effect of solute segregation on grain boundary structure

It is frequently observed that solute atoms segregate to grain boundaries in alloy systems where there is limited solubility in the solid solution. Auger spectroscopy has shown the presence of typically up to a monolayer of solute at internal interfaces in such systems. The practical consequence of such segregation is frequently the embrittlement of the grain boundary region, making it susceptible to fracture at relatively low stresses. It is of considerable interest to learn how the presence of the solute changes the mechanical properties of the boundary. Is the solute changing the chemical bonding across the interface as suggested by Messmer and Briant or is it changing the atomic structure, or perhaps both. In this paper an experiment is described which addresses the question of whether the presence of solute at the boundary influences the structure of the boundary. It is shown that in a small angle (001) twist boundary in the Fe-Au system, a small amount of Au causes a major change i the dislocation structure of the interface. These observations are evidence for the occurrence of a two dimensional phase transformation in the grain boundary.

Fabrication of nanoperiodic surface structures by controlled etching of dislocations in bicrystals

A method for the fabrication of periodic arrays of surface features with controlled spacings of 2–100 nm has been developed. This process relies on the selective etching of dislocations formed at a twist–bonded interface in a bicrystal. The production of nanoscale periodic silicon surface features with a mean spacing of 38 nm is reported. The etch rate of edge and screw dislocations is compared, and the rate of dislocation etching is found to be poorly correlated to strain. This observation calls long-held theories of dislocation etching into question.