NiAl single crystals that are well annealed and oriented along the 0 0 1 orientation to suppress 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 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 slip is blocked by dislocation locking along both of the two 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 slip by dislocation locking long two line directions is in contrast to dislocations in NiAl that are locked only along the screw orientation, and to dislocations that are not locked at all. This property explains why slip is more difficult than and slips, the general absence of 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 slip in 0 0 1 NiAl are discussed and compared.
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