A field ion microscope study of atomic configuration at grain boundaries

Abstract

The structure of grain boundaries can be studied in atomic detail with the field ion microscope. A number of grain boundaries in tungsten and tungsten-rhenium alloys have been examined and the information from these studies has been collected together for presentation in this paper. The distribution of strain energy in the region of a low angle boundary leads to preferential field evaporation of material from this region. The field ion microscope has proved more useful for elucidating the atomic structure at high angle grain boundaries. The width of high angle boundaries is observed to be very narrow (approximately 2 atom diameters) and such a boundary shows regions of good and bad fit. These observations are correlated with a model for high angle boundaries, presented in this paper and developed from coincidence site lattice theory. For two grains related by particular misorientations about specific axes, high density coincidence site lattices exist and these special relationships are tabulated. It is also shown that deviations from these relationships can be accommodated by the introduction of a dislocation sub-boundary. The effect of the orientation of the boundary plane is also given and it is shown that good fit regions are developed where the boundary follows the most densely packed planes in the coincidence lattice. A stepped structure is generated where the boundary runs at an angle to these planes. Segregation effects at grain boundaries are illustrated and it is shown that the processes giving rise to an image from a finite concentration solid solution make it difficult to determine the atomic structure of a grain boundary in such a material.