Field Evaporation of a Screw Dislocation in Tungsten

A complex crystal defect in tungsten has been examined in detail by means of the field ion microscope. Field evaporation has been employed to remove the metal surface, making visible the three‐dimensional structure of the defect. The defect has been identified as a grain boundary appearing in$\text{(1̄16)}$ of one grain and$\text{(11̄6)}$ of the other, so that the grains are misoriented by an angle of 26.5° rotation about the direction [110], which corresponds to the axis of the wire sample. A screw dislocation intersects (110), which is shared by both grains. The intersection of the screw dislocation with the surface produces a spiral on (110) which can be made to rotate under field evaporation. The direction of the Burgers vector of the dislocation has been found to be [110], and the Burgers vector magnitude is an integral multiple of the (110) plane spacing. The Burgers vector magnitude of the dislocation increases as the sample is removed, producing multiple spirals on (110).