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

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Abstract
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 relatively small Burgers vectors and small spacings of many of the grids expected in these regions. On the basis of expected energy versus Θ behaviour of such boundaries, it was concluded that misfit dislocation structures may actually exist over a substantial portion of the total Θ range.