The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.

The influence of grain boundaries on mechanical properties

Overview no. 61 On geometric criteria for low interfacial energy

The fine structure of (001) twist boundaries in gold was systematically studied by transmission electron microscopy. About 200 such boundaries were prepared under controlled conditions by welding single crystal films together face-to-face at various twist angles Θ. An almost continuous study of the structure was then made over the entire range 0 ⋜ Θ⋜45°. Orthogonal grids of grain boundary misfit screw dislocations were found in the vicinities of critical angles Θc (Θc=0, 22–6, 28.1 and 36.9°). producing various high density coincidence site boundaries. Measured geometrical properties of the grids near each Θc (i.e. grid spacings, orientation and dislocation diffraction contrast) were completely consistent with those predicted for a boundary structure consisting of a suitable misfit dislocation grid embedded in the high density coincidence site interface corresponding to Θc. Fine structure was not detected in appreciable angular ranges lying between the Θc. This may have been due in part to the re...

Quantitative observation of misfit dislocation arrays in low and high angle twist grain boundaries

https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/24025/1/ea198303.pdf

Interface controlled diffusional creep

Electrochemical studies were conducted in 2 N H2SO4 at 303 K in order to assess the applicability of the CSL/DSC model of interface structure to intergranular corrosion susceptibility at grain boundaries in high purity (99.999%) polycrystalline nickel. Susceptibility to the initiation of localized corrosion at grain boundaries was manifested through characteristic overpotentials for passive film breakdown. These characteristic overpotentials were found to (1) decrease with increasing bulk sulphur concentration (0.3–50 ppm), and (2) be strongly dependent on interface structure (CSL/DSC). Boundaries close (Δθ) to low ΣCSL relationships were observed to be most resistant to the initiation of localized corrosion. A limiting structural field was determined, not extending beyond Σ25, and restricted to an angular deviation limit defined by a relation of the type: Δθ = 15° Σ−5/6. Results were determined to be consistent with a mechanism whereby susceptibility to intergranular corrosion is dictated by the (1) geometry, and (2) chemistry (i.e. solute concentration) of intrinsic grain boundary dislocations.

Structure-dependence of intergranular corrosion in high purity nickel

A technique for producing any type of planar dislocation sub-boundary in thin-film bierystal specimens of gold under controlled conditions is described. Single crystal thin films of the desired orientations are first grown epitaxially on appropriate substrates from the vapour phase. Two films, while still on their substrates, are then welded together face-to-face at any desired mis-orientation to produce a thin-film bierystal containing a planar dislocation array at the welded interface. The substrates are then dissolved away to allow examination of the sub-boundary at normal incidence by transmission electron microscopy. The technique is applied to the study of pure twist sub-boundaries on (001) and (111) planes in gold. Features of the structures of these boundaries are described. Further possible applications are discussed.

Dislocation sub-boundary arrays in oriented thin-film bicrystals of gold

A study of extraneous grain boundary dislocation (GBD) structures in low and high angle (001) twist boundaries in gold was carried out by transmission electron microscopy using bicrystal specimens prepared by welding evaporated single crystal thin films together face-to-face GBD structures were observed which could be explained by the introduction of dislocations with lattice Burgers vectors ±a/2[101], ±a/2[101], ±a/2[011], or ±a/2[011] into the boundaries and the subsequent interactions of these dislocations with the networks of misfit screw GBD's which are associated with the intrinsic equilibrium structure of the twist boundaries (Schober and Balluffi 1970 b) In many cases the interactions in the boundary essentially eliminated the components of the Burgers vectors of the extraneous GBD's in the plane of the boundary and produced effective Burgers vectors almost (or exactly) perpendicular to the boundary In all cases the components normal to the boundary of the effective Burgers vectors wer

Extraneous grain boundary dislocations in low and high angle (001) twist boundaries in gold

The fine structure of high angle (110) twist boundaries in gold was studied using electron microscopy. A considerable number of boundaries was prepared by welding epitaxial thin films of gold together at various twist angles. Orthogonal arrays of grain boundary misfit dislocations were detected in the vicinity of the critical angles at which the two highest density coincidence site interfaces are formed in this system. These orthogonal arrays were shown to consist of grids of screw dislocations embedded in the interfaces corresponding to the exact high density coincidence misorientations. The misfit dislocations therefore served to ‘correct’ for effects due to rotational deviations from the exact coincidence site orientations.

Observation of misfit dislocation arrays in high angle (110) twist grain boundaries in gold

Bardeen–Herring type grain boundary dislocation source (or sink) spirals were found in high-angle (001) twist boundaries in gold by transmission electron microscopy. The possible roles that these configurations could play in supporting grain boundary point-defect source (or sink) action and boundary migration were pointed out.

T. Schober

Papers

Quantitative observation of misfit dislocation arrays in low and high angle twist grain boundaries

Dislocation sub-boundary arrays in oriented thin-film bicrystals of gold

Extraneous grain boundary dislocations in low and high angle (001) twist boundaries in gold

Observation of misfit dislocation arrays in high angle (110) twist grain boundaries in gold

Grain boundary dislocation spirals in high-angle (001) twist boundaries in gold