Stable configurations of dislocations in homogenized nickel aluminide single crystals

Abstract

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° 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 μ mean radius, and >210< facetting was observed rather than arcs at the corners 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° to the predicted stable directions of ±(27–42°) and ±(82–98°) from the average screw orientation, up to 15% of the zigzagging extended to orientations beyond the nearest stable ones.