Observations of the fine structure of superdislocations in Ni₃Al by field-ion microscopy

Abstract

The fine structures of two common types of superdislocation which exist in the intermetallic Ni₃Al are experimentally characterized by field-ion microscopy. It is shown how the a<110> type superdislocations and the a/3<211> type superlattice partial dislocations may be distinguished in the field-ion image and the geometrical contrast theory is extended to include these observations. The a<110> type super-dislocations are found to be dissociated, having a fault width of close to 3 nms, and the antiphase boundary energy is hence calculated to lie between 0·25 and 0·35 Jm-². The a/3<211> type dislocations do not appear to be dissociated in the classical sense. The possibility of distinguishing between an intrinsic and an extrinsic stacking fault in this alloy is demonstrated and an example is given of the contrast from a dislocation pile-up. Finally, the observations of dislocations, and ordering defects in field-ion images from ordered alloys are discussed.