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

Abstract

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d001 ¢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 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 2 nm. The temperature window of stability for these a⟨101⟩ dislocations depends upon the strain rate. At a strain rate of 1.4 210−4 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.