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Submitted on 1 Jan 1971
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INTERACTION BETWEEN DISLOCATIONS AND POINT DEFECTS AS SEEN BY ANELASTIC
METHODS
K. Lucke
To cite this version:
K. Lucke. INTERACTION BETWEEN DISLOCATIONS AND POINT DEFECTS AS SEEN BY ANELASTIC METHODS. Journal de Physique Colloques, 1971, 32 (C2), pp.C2-117-C2-118.
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I N TERA C TION DISL OCA TIONS. DEFA UTS PONTUELS
INTERACTION BETWEEN D1 SLOCATIONS
AND POINT DEFECTS AS SEEN BY ANELASTIC METHODS
by K. LUCKE
Institut fiir Allgemeine Metallkunde und Metallphysik der Technischen Hochschule Aachen
The interaction between point defects and disloca- tions is, for the purpose of internal friction, mostly simplified in such a way, that the short range interac- tion is assumed to be very strong and the long range interaction to be very weak. This leads to the concept of pinning points which, because of its simplicity, also here will be used. It must be kept in minds, howe- ver, that also the long range interaction with point defects might influence dislocation motion.
If pinning points are distributed along a disloca- tion line the length of the free dislocation segments will be given by an exponential (Koehler-type) distri- bution. This result is obtained if both the contribu- tions of the vibrational entropy of the dislocation segments (Alefeld) and also the enegry increase due to formation of a zick-zack form of the dislocation is considered.
Concerning the internal friction phenomena connec- ted with the interaction between dislocations and point defects, four areas will be considered :
1. The movement of point defects to the dislocations.
- Here it is assumed that the arriving point defects (or a certain fraction of them) form pinning points so that the arrival of point defects can be measured with the help of the dislocation resonance damping (Gra- nato/Lucke-theory). For all investigated metals it has been found that the pinning occurs in certain temperature ranges which can be compared with the annealing stages of point defects as observed by resis- tivity measurements. For the case of stage 111-pinning of electron irradiated Cu which is rather thoroughly investigated it was possible to show that after suffi- ciently small dosis the time dependence of pinning is controlled by a first order kinetics (as approximately to be expected for diffusion of point defects to dislo- cations) so that also the activation energy could be determined by direct methods. Mostly, however, the kinetics of migration to the dislocation is obscured by additional process occuring directly at the dislocations (to be described in (2)).
tion the number of pinning points measured by inter- nal friction experiments is mostly more than an order of magnitude smaller than the number of point defects which are calculated to arrive at the dislocation. There- fore, one must assume that most of the point defects migrate to jogs, clusters, nodal points etc. These pro- cesses lead to saturation effects for the pinning point numbers as function irradiation dose and to depinning phenomena. It can be shown that the clusters immo- bile at lower temperatures can be broken up at higher temperatures so that in the case of irradiation induced pinning points at high temperatures a complete depinning is obtained.
3. The movement of dislocation with the pinning points kept fixed. - Because of the interaction between pinning point and dislocation this movement has the character of breakaway of the dislocation from the pinning points. The resulting amplitude dependent internal friction has originally been calculated by Granato and Lucke for O O K . The most important recent development is the inclusion of the influence of temperature into the theory which, by now, is largly completed. This consideration of the thermal vibra- tions leads to a certain understanding of the deviations from the Granato-Lucke plots and requires in cases where good Granato-Lucke plots are obtained a re- evaluation of the measurements. Another recent development in this area leads to a more direct determination of the interaction energy between point defects and dislocation. By application of slowly changing external stress and simultaneous testing of the degree of breakaway by loop length measure- ments with the help of the dislocation resonance dam- ping, the stress at which breakaway occurs can be determined directly.
4. Movement of point defects together with a moving dislocation. - Here 3 cases must be considered :
a) A motion of the point defects with the disloca- tion perpendicular to the originally straight disloca- 2. The movement of point defects along the disloca- tion line.
tion. - Such a motion is concluded from indirect b) A motion parallel to the dislocation line.
arguments. For example, one finds that during irradia- c) A rotation of point defects with tetragonal sym-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971225
C2-118 K. LUCKE metry due to the change of the local stress field of a moving dislocation (induced Snoek-effect ; such a rotation can occur also near to a non-moving disloca- tion if the orientation of the defects is not in equifi- brium and the temperature is increased.) While the theoretical treatment of these effects is largely comple- ted, it is still rather unclear to which of the observed effects these models apply. It is assumed that they are
of importance for the understanding of the Hasiguti peaks. This is in contrast to some theoretical explana- tions of the Hasiguti peaks in which breakaway pro- cesses are assumed. It is felt here that breakaway processes are not responsible to these peaks since they would lead to a strong strain amplitude depen- dence of the peak temperature what is in contrast to the observations.
