Nuclear Spin Relaxation Investigations on the Influence of Impurities and Temperature on the Mean Free Path of Mobile Di
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NUCLEAR SPIN RELAXATION INVESTIGATIONS ON THE INFLUENCE OF IMPURITIES AND TEMPERATURE ON THE MEAN FREE PATH OF MOBILE DISLOCATIONS IN NaCI
W. H. M. ALSEM, J. Th. M. De HOSSON Dept. of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands H. TAMLER, H. J. HACKELOER, 0. KANERT Institute of Physics, University of Dortmund, Postfach 50 05 00, 46 Dortmund 50, W. Germany
ABSTRACT Dislocation motion in alkali halide single crystals is strongly impeded by the presence of impurities, apart from obstacles built by the forest dislocations. The mean free path L of stepwise moving dislocations is measured by determination of the spin-lattice relaxation rate 1/T1 p as a function of the strain rate E, varying the content of impurities and the temperature. The latter influences the distribution of the point defects and the activation rate of dislocations before obstacles, while the former merely shorten L, thereby raising 1/T 1 p,
INTRODUCTION To obtain insight in the macroscopic plasticity of crystals it is crucial to study moving dislocations on a microscopic scale. This has been done now by the technique of nuclear spin relaxation measurements. The motion of dislocations under the influence of external stress is characterized as "jerky glide" (i). Dislocations are assumed to move in zero elapsed time between obstacles. At obstacles they may be released either by thermal fluctuations or by a change in applied stress or in the obstacle structure. In fact one could distinguish between dislocations which are actually running and those waiting to be released at obstacles. Generally, the running time is small compared to the waiting time and almost all mobile dislocations are waiting. Introducing pm as the mobile dislocation length per unit volume and L as the average distance swept out by a released dislocation segment, one gets for the shear a produced by the dislocation motion: a = bp L, (1) m where b is the Burgersvector of the dislocations; then given by: a
Lbp m t w
the shear strain rate A is
(2)
where tw is the mean waiting time before obstacles. Parameters like the mean free path L determine to a great extent the plastic deformation behaviour of crystals. For instance, the stress needed to continue further deformation, the flow stress, increases with strain. This increase is well known to be correla-
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ted to a decrease in L. The main obstacles at which dislocations can be held up are points where other immobile dislocations (forest dislocations) cut the glide plane or point defects like impurities. The intersection of mobile dislocations with forest dislocations can be hard to accomplish because of formation of small nodal segments in the dislocations, which lower the local energy significantly. Therefore these are referred to as strong obstacles. In alkali halide crystals of monovalent ions divalent impurities are effective obstacles. To preserve charge neutrality in the lattice each impurity ion, e.g. a Ca++ ion in NaCl, is associated with a c
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