Dislocation Motion in Metals Investigated by Means of Pulsed Nuclear Magnetic Resonance
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DISLOCATION MOTION IN METALS INVESTIGATED BY MEANS OF PULSED NUCLEAR MAGNETIC RESONANCE H. TAMLER, H. J. HACKELOER, and 0. KANERT Institute of Physics, University of Dortmund,
46 Dortmund-50,
W. Germany
W. H. M. ALSEM and J. Th. M. DE HOSSON Dept. of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands
ABSTRACT We report the first use of nuclear magnetic resonance to investigate dislocation motion in metals. The spin-lattice re27 Al in polylaxation rate in the rotating frameT l p -1, of crystalline, ultrapure Aluminium foils has been measured as a function of plastic-deformation rate ý for two different temperatures (77K and 300K). For ý = 0, the relaxation rate is determined by conduction electrons. For a finite deformation rate E, an additional contribution to the relaxation rate arising from fluctuations in the nuclear quadrupole interaction due to dislocation motion is observed. From the motion-induced part of the relaxation rate the mean jump distance of a mobile dislocation is calculated which is determined by the density of lattice defects acting as obstacles for moving dislocations.
INTRODUCTION Dislocations play a key role in the explanation of the phenomena of slip in crystals, the mechanism of the process of plastic deformation which is so important in materials science. Strengthening of crystals is brought about by obstructing the movement of dislocations. A realistic description of the dislocation motion is essential for an understanding of plastic deformation. A few years ago, we have shown that nuclear magnetic resonance (NMR) is a useful tool for studying dislocation motion in non-metallic materials like alkali halide crystals (1, 2, 3). In the work, the nuclear spin relaxation rate in the rotating frame, T 1 p-1, has been instantaneously measured while a sample is plastically deforming with a constant deformation rate L. In particular, results have been presented as a function of the deformation rate ý, the direction of deformation with respect to the crystal axes, temperature, and concentration of impurity atoms. Parallel to the experimental work, the theoretical basis for evaluation of the nuclear spin relaxation data has been established (4). It turned out that from such NMR experiments two microscopic informations of the dislocation motion can be obtained in principle: (i) The mean jump distance and (ii) the mean time of stay between two consecutive jumps of a mobile dislocation. Now, for the first time dislocations at various velocities in metals, especially in ultrapure polycrystalline aluminium are studied by means of pulsed NMR technique. In this paper, first results of the investigation are presented.
422 THEORETICAL BACKGROUND As discussed for example by Argon (5), the mechanical behaviour of crystalline material under the influence of a plastic-deformation rate E is governed by the Orowan equation (6) =
Lb.p .v = L.b.p.L/T
.()
The physical model underlying Eq.(1) assumes a thermally activated jerky motion of
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