Deformation mechanism in NiAl single crystals at low temperatures M.Z. Butt a, * , Dilawar Ali b a Rafi M. Chaudhri Chair, Centre for Advanced Studies in Physics, GC University, Lahore, 54000, Pakistan b Department of Physics, GC University, Lahore, 54000, Pakistan article info Article history: Received 20 July 2014 Received in revised form 2 September 2014 Accepted 14 October 2014 Available online Keywords: A. Aluminides B. Yield stress B. Plastic deformation mechanisms D. Point defect abstract Available data on the temperature dependence of yield stress and of associated activation volume of high-purity NiAl single crystals between 76 and 205 K have been analyzed within the framework of a solid-solution hardening model, which is based on the concept of depinning of an edge-dislocation segment from several randomly dispersed, isolated, point defects simultaneously. The vacancies in NiAl single crystals act as point-defect obstacles to the stress-assisted thermally-activated glide of edge dislocations, and their concentration is estimated as about 10 at.%. The product of yield stress and associated activation volume (z0.146 eV) is found to be independent of temperature and yield stress, as envisaged in the model. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Theories of plastic flow in binary solid-solution crystals can be divided into two groups. In the models of first group, the rate process of yielding is assumed to be breakaway of a dislocation segment from individual, point-like, obstacles [e.g. Refs. [1‒3]]. The models in second group are based on the concept of depinning of an edge-dislocation segment from several, randomly dispersed, iso- lated, point defects simultaneously [e.g. Refs. [4‒18]]. Computer simulation predicts that the critical concentration of point defects below and above which depinning process will be of “individual”- and “collective”- type, respectively, lies in the range 10 5 e 10 4 [19]. In 2001, Brunner and Gumbsch [20] studied the temperature dependence of yield stress of high-purity NiAl single crystals with stoichiometric composition over a wide range of temperature be- tween 76 and 325 K. The residual resistivity ratio of the crystals was 12 and the dislocation density 4  10 11 m 2 . Tensile tests performed at a constant plastic shear-strain rate 1  10 4 s 1 , were combined with stress-relaxation experiments. The tensile axis was close to <111> direction (so-called “soft” orientation) which facilitated plastic deformation primarily by the motion of <100> dislocations on {011} slip plane. They observed that the yield stress (t) of NiAl single crystals decreases rapidly with increasing temperature from 76 to 230 K (region I), and then it decreases slowly up to 320 K (region II). Similarly, the strain-rate sensitivity (vt=v _ g) T increases rapidly with decreasing temperature below 230 K, reaches a maximum at about 100 K and decreases again below this temper- ature. Brunner and Gumbsch [20] assumed that the high concen- tration of above equilibrium vacancies in NiAl single crystals at temperature below 1000 K as well as the low mobility of vacancies makes these point defects to act as localized obstacles to dislocation glide. The stress necessary to overcome the short-range interaction of these immobile vacancies with dislocations in the glide plane determines the flow stress of NiAl single crystals. Brunner and Gumbsch [20] analyzed the experimental data of region I in terms of Fleischer's model [1] of solid-solution hard- ening, which is based on depinning of a dislocation segment from an individual, point-like, obstacle as the rate process of plastic deformation. However, the mean obstacle spacing of only 3b determined from Fleischer's model appears to be far too small to be considered realistic, and the idea of depinning of a dislocation segment from an individual, point-like, obstacle (vacancy in this case) as the rate process of plastic deformation was therefore ruled out by them in view of the too small mean obstacle spacing and the limited flexibility of dislocations. Based on the atomic simulation, Schroll and Gumbsch [21] have also concluded that the tremendous decrease of yield stress with increasing temperature cannot be explained on the basis of dislocation interactions with isolated structural point defects alone. In order to explore an acceptable picture of deformation mechanism for the observed temperature dependence of yield stress and of strain-rate sensitivity in NiAl single crystals referred to above, the data have been re-analyzed within the framework of * Corresponding author. Tel.: þ92 42 99214601. E-mail addresses: mzbutt49@yahoo.com, mzakriabutt@gmail.com (M.Z. Butt). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet http://dx.doi.org/10.1016/j.intermet.2014.10.006 0966-9795/© 2014 Elsevier Ltd. All rights reserved. Intermetallics 57 (2015) 93e97