Anomalous Weak Beam Contrast from Extrinsic Stacking Faults

Abstract

This paper concerns observations of anomalous fault contrast arising from weak beam images of extrinsic faulted dislocation loops. Weak beam images are of considerable use when examining lattice defects due to the reduced image width associated with dislocations and the fine detail revealed in the fringe contrast from stacking faults. The observations described here arose from a study of interstitial (extrinsic) dislocation loops produced by radiation damage in a commercial nickel base superalloy, Nimonic PE16(1).Figure 1 shows a bright field image revealing faulted loops on all four {111} planes of the f.c.c. structure. Complementary images taken in a variety of different two beam diffracting conditions showed the faulted loops to be <111> interstitial faults surrounded by <111> pure edge (Frank) partial dislocations. When images were taken under weak beam conditions, contrast from the stacking faults was found to be critically dependent on the sign of g.R (where g is the diffracting vector and R the displacement vector of the fault).