Determination of the sign of screw dislocations viewed end-on by weak-beam diffraction contrast

Abstract

This paper describes how the sign of a screw dislocation or of the screw component of a mixed dislocation in a thin elastically isotropic foil, viewed end-on, can be determined from the dark-field weak-beam diffraction contrast arising from surface relaxation displacements. The contrast consists of black-white lobes, with the line of no-contrast parallel to g , similar to that found previously by Tunstall et al . [Phil. Mag. 9 99 (1964)] for bright-field imaging of screw dislocations in thick foils. Unlike weak-beam images of inclined dislocations, the image profiles are very broad (∼10 nm for the strongest) owing to the long-range nature of surface relaxation strain-field. For dislocations spaced at ∼10 nm or less, the overlap of the strain-field from nearby dislocations has to be taken into account. The paper also discusses the nature of the contrast from mixed dislocations slightly tilted from the incident beam direction, when contrast from the edge component is expected, and the possibility of determining the sign of the screw component in this case.