Dislocation Dynamics In B.C.C. Metals: A Nuclear Magnetic Resonance and Transmission Electron Microscopic Study
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DISLOCATION DYNAMICS IN B.C.C. METALS: A Nuclear Magnetic Resonance and Transmission Electron Microscopic Study J. Th. M. De Hosson° and 0. Kanert ** ° Department of Applied Physics, University of Groningen, Materials Science Centre, Nijenborgh 18, 9747 AG Groningen, The Netherlands Department of Physics, University of Dortmund, 46 Dortmund 50, FRG ABSTRACT Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in b.c.c. metals. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been measured in Vanadium as a function of temperature. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. The NMR experiments show that the mean jump distance is nearly constant below 230 K whereas it decreases substantially above 230 K to 300 K indicating a transition that marks two different mechanisms. NMR observations in combination with TEM support the physical picture that above that transition temperature dislocation segments are stopped between localized obstacles whereas below Tc the lattice friction controls the plastic behaviour. INTRODUCTION Plastic deformation of b.c.c. metals in general exhibits characteristic features at low temperatures [1], e.g. a strong temperature dependence of the yield stress and anomalous slip. The latter has been studied most extensively in a-Fe and in Nb, however, this slip mode has been observed in other b.c.c. metals as well, such as Ta [2-5], V [6-8] and Mo [9-10]. The occurrence of the slip is anomalous as the slip plane observed belongs to one of the {110) planes with a small Schmid factor [11-16]. In the past, in-situ TEM experiments have been performed [17-18] at low and intermediate temperatures on Nb, Mo and (x-Fe. In all these b.c.c. materials a transition has been observed in the dislocation dynamics between a low temperature behaviour, characterized by an almost continuous movement of long screw dislocations and an intermediate temperature behaviour, where mixed dislocations predominate. The present paper reports on the dislocation dynamics measured using a complementary new technique for b.c.c. metals, i.e. pulsed nuclear magnetic resonance as a function of temperature. The great strength of nuclear magnetic resonance is that the resonance signal is characteristic of the particular nucleus studied. Moreover, the surrounding of a nucleus may affect NMR properties like relaxation time. Accordingly, NMR can be used to study the environment of the nuclei providing microscopic information of atomic motion. In contrast to in-situ transmission electron microscopic observations of dislocation motion, NMR is detecting more of the bulk of the material and does not allow only investigation of dislocations near free surfaces, like in TEM where the existence of image forces may cause their behaviour very different from that in the interior of the material. EXPERIMENTS In this investigat
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