Abstract

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d 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 d 111 ¢ slip to non- a d 111 ¢ slip at intermediate temperatures. In Ni-50 at.% Al single crystals, only a d 010 ¢ dislocations are observed above the slip transition temperature. In contrast, a d 101 ¢ {101} glide has been observed to control deformation beyond the slip transition temperature in Ni-44 at.% Al. a d 101 ¢ dislocations are observed primarily along both d 111 ¢ directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a d 101 ¢ dislocations along these directions is decomposed into two a d 010 ¢ dislocations, separated by a distance of approximately 2 nm. The temperature window of stability for these a d 101 ¢ dislocations depends upon the strain rate. At a strain rate of 1.4 2 10 m 4 s m 1 , a d 101 ¢ dislocations are observed between 800 and 1000 K. Complete decomposition of a d 101 ¢ dislocations into a d 010 ¢ dislocations occurs beyond 1000 K, leading to a d 010 ¢ climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a d 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 d 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 d 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