The onset and blocking of slip in NiAl

Abstract

NiAl single crystals that are well annealed and oriented along the [0 0 1] orientation to suppress <1 0 0> dislocations have been used to study the onset of slip. Under a constant stress, slip is initiated abruptly with increasing temperature. The slip vector is identified to be <0 1 1> by slip line and transmission electron microscopy (TEM) observations. There is a rate-independent slip initiation temperature that decreases approximately linearly with stress. TEM observations show that the <0 1 1> slip is blocked by dislocation locking along both of the two <1 1 1> directions in the {0 1 1} glide plane. The locking is explained by the non-planar splitting of the screw component along the three-fold line directions. The blocking of <0 1 1> slip by dislocation locking long two line directions is in contrast to <1 1 1> dislocations in NiAl that are locked only along the screw orientation, and to <1 0 0> dislocations that are not locked at all. This property explains why <0 1 1> slip is more difficult than <1 1 1> and <0 0 1> slips, the general absence of <0 1 1> dislocations in annealed crystals, and the need for these dislocations to be nucleated in [0 0 1]-oriented NiAl. Possible mechanisms for the onset of <0 1 1> slip in [0 0 1] NiAl are discussed and compared.