STABLE CONFIGURATIONS OF DISLOCATIONS IN HOMOGENIZED NICKEL ALUMINIDE SINGLE-CRYSTALS

Dislocations produced in quasi-stoichiometric NiAl (about 50.6 at.% Ni) by homogenizing and slow cooling have been analysed. The dislocations were primarily on {100} planes with [100] Burgers vectors in one of three configurations: vacancy-fed edge concentric loops, spiral dislocations and zigzag dislocations with screw resultants. Calculated anisotropic elastic line energies around the sessile loops, interpreted by inverse Wulff plots, predicted a +/- 11-degrees range of unstable orientations around [110]. In contrast with the predicted curvature of the loops, straight [100] dislocations connected by continuous arcs were observed for loops smaller than 1.5 mum mean radius, and [210] facetting was observed rather than arcs at the comers for larger loops. This indicates that the 10% variation in line energy predicted by the calculation was not enough to overcome diffusion rate and lattice friction effects. The breakdown of screw dislocations lying on a {100} plane with b = a [100] into zigzags is explained by the wide ranges of unstable configurations of {100} [100] screw dislocations. Although the general orientations of the dislocation segments conform within 5-degrees to the predicted stable directions of +/-(27-42-degrees) and +/-(82-98-degrees) from the average screw orientation, up to 15% of the zigzagging extended to orientations beyond the nearest stable ones.