Glide Mechanisms of <001> Dislocations in NiAl
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ABSTRACT The glide properties of dislocations have been studied by in situ straining experiments at and below room temperature, with the aim of studying slip, cross-slip, Peierls friction forces, and pinning at small obstacles. Most results are in a good agreement with atomistic calculations. It is concluded that unpinning from small extrinsic obstacles is probably the rate controlling mechanism in this temperature range and in the soft orientation. INTRODUCTION Plastic deformation of NiAl single crystals takes place by the glide of dislocations for all straining axes different from . Since the corresponding activation volumes are rather small (40-80 b 3 at room temperature, according to[ 1-4]), and since atomistic calculations yield non planar cores along several directions [5-9], Peierls frictional forces have frequently been proposed as a rate controlling mechanism. Previous in situ experiments at room temperature did not however confirm this hypothesis [10] but showed abundant cross-slip and pinning of screw dislocations at small obstacles. This paper reports additional results at and below room temperature, with the aim of determining the respective roles of Peierls forces, cross-slip, and local pinning, in the glide process of dislocations. EXPERIMENTAL PROCEDURE In situ straining experiments were conducted on a JEOL 200 CX electron microscope, at 300 K and 143 K. Microsamples were cut in a single crystal of stoichiometric NiAl grown by General Electric, in a plane within 5' from (120), with tensile axes parallel and perpendicular to [421]. The Burgers vectors were identified by the classical extinction method, and the slip planes were deduced from the Burgers vectors and slip traces directions. The accuracy of slip plane determination is better than 5'.
RESULTS Fig. 1 shows the two different behaviors which were observed at 300 K in different areas of the same microsamples. In (a), dislocations with [010] Burgers vectors have trailed straight slip traces corresponding to glide in (101) planes. In fig. l(b), dislocations with [100] Burgers vectors have glided on wavy slip surfaces with average planes comprised between (021) and (03 1), and many loops or dipoles have been formed in the wake of gliding dislocations. This behavior is typical of a very intensive cross-slip, as discussed in [10, 11]. In spite of these two different situations, dislocations exhibit several common features: same zig-zag shape along the screw direction, same high density of pinning points (average distance 100200 nm), same mobility corresponding to fast movements between anchored positions. Fig. 2 shows a dislocation noted d interacting with a preexisting loop (B) at 300 K, and forming by cross-slip a large dipole over the preexisting loop. KK9.9.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
Figure 1. Dislocation structures at 300 K. a) Stable slip in (101), b = [010], g = il0, tensile axis I[421]. b) Intensive cross-slip, average slip trace direction noted tr, b -[100], g -101, tensile axis/I [421].
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