The orientation dependence of deformation mode and structure in stoichiometric NiAl single crystals deformed by high temperature steady-state creep

Abstract

Single crystals of stoichiometric NiAl having predetermined orientations have been deformed by steady-state creep in the temperature range 750° to 1055°C, the stresses being varied from 1.76 to 7.02 kg mm_-2. In crystals oriented 7.5 deg from [101], only (100) slip was observed; the primary slip plane was 100 although secondary slip occurred on 110. As predicted by elastic anisotropy considerations, dislocations on 100 with b = α(100) have a zig-zag shape with the segments aligned along (110). On 110 they tend to lie along (111) and (112). Cross-slip of short segments having screw orientations can occur between l00 and 110, giving rise to dipoles and prismatic loops. Crystals with tensile axes 14 deg from [1ll] slip in all three (100) directions on both cube and dodecahedral planes. Characteristic structures are dislocation entanglements, dipoles, loops, and networks containing nodes. Often two (100) type dislocations react and form a segment of (110) dislocation. Crystals approximately 19 deg from the cube orientation slip in both the (100) and (110) directions, and the contribution of (110) slip to the total glide strain increases at higher temperatures. The (110) dislocations are mostly pure screw as predicted by elastic anisotropy. Two sets of α(100) dislocations with mutually perpendicular Burgers vectors can form a network of twist or mixed character. If the contact plane is not one of the cube planes, the network is lozenge shaped, and small (110) segments form at the node points. Slip in the (111) direction was not observed with certainty; (100) slip occurred in all specimens, and (110) slip only in crystals reasonably close to the cube orientation.