Observations of glide and decomposition of a(101) dislocations at high temperatures in Ni-Al single crystals deformed along the hard orientation

Ni-44at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard <001> orientation. The dislocation processes and deformation behaviour were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from a<111> slip to non-a<111> slip at intermediate temperatures. In Ni-50 at. % Al single crystals, only a<010> dislocations are observed above the slip transition temperature. In contrast, a<101>{101} glide has been observed to control deformation beyond the slip transition temperature in, Ni-44 at.% Al. a<101> dislocations are observed primarily along both <111> directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a<101> dislocations along these directions is decomposed into two a<010> dislocations, separated by, a distance of approximately 2nm. The temperature window of stability for these a<101> dislocations depends upon. the strain rate. At a strain rate of 1.4 x 10(-4) s(-1), a<101> dislocations are observed between 800 and 1000 K. Complete decomposition of a<101> dislocations into a<010> dislocations occurs beyond 1000 K, leading to a<010> climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a<101> dislocations has been observed to occur along the edge orientation at temperatures below 1000 K. Embedded-atom method calculations and experimental results indicate that a<101> dislocations have a large Peierls stress at low temperatures. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for a<101>{101} glide in Ni-44 at.% Al, and for the observed yield strength versus temperature behaviour of Ni-Al alloys at intermediate and high temperatures.