Scripta METALLURGICA Vol. 28, pp. 1561-1564, 1993 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved

DEPINNING EFFECT OF HIGH HYDROSTATIC PRESSURE ON DISLOCATIONS IN "/-IRRADIATED MONOCRYSTALLINE COPPER

Zbigniew WITCZAK and Jerzy KROLIKOWSKI*) High Pressure Research Center, Polish Academy of Sciences, Sokotowska 29, 01-142 Warszawa, Poland

*)Institute of Fundamental Technological Research, Polish Academy of Sciences, Swi~tokrzyska 21, 00-049 Warszawa, Poland

(Received December i, 1992) (Revised April i, 1993)

Introduction

Up to now the irreversible changes in monocrystals after applying a hydrostatic pressure to them have been observed with transmission electron microscopy. The observed changes were caused by the interaction elastic inhomogeneities existing in the material with the matrix. New mobile dislocations were generated at interfaces (see [1-5]). In the case of a totally homogeneous and pure monocrystal no visible changes in the dislocation structure were observed with the use of the electron microscope [6]. In the present work, homogeneous copper monocrystals have been studied after subjecting them to hydrostatic pressurization. The irreversible changes in the properties of dislocations of the monocrystals have been observed by means of an ultrasonic method. The possible sources of those changes are discussed.

Experimental

Cubic samples of the dimension of 10 mm were cut off with a wire saw from a copper monocrystal (99.999% purity) oriented in <111> direction. The samples were finely polished, etched, annealed (vacuum, 600 °C, 3 h), and "/-irradiated up to 40 Mrad by a Co 6° source. The samples were again annealed at 90 °C for 1 h to allow the pinning of the dislocations by point defects. The samples were subjected to hydrostatic pressurization at various pressures and after each pressurization the attenuation of longitudinal ultrasonic waves in <111> direction was measured. The waves were generated by a quartz transducer (10 MHz resonant frequency, 6 mm in diameter)_ Nonaq stopcock grease was used as a bonding material between the samples and the transducers. The measurements were made in the frequency range of 10-90 MHz at ambient temperature with MATEC measuring equipment. An echo method was used. High hydrostatic pressure was generated by a hydrostatic amplifier. The test vessel was equipped with a manganin gauge and a thermocouple to monitor the pressure and the temperature, respectively. Pentane was used as a pressure medium to insure a hydrostatic state in the whole pressure range (up to 2 GPa). The scheme of the pressure cycles was as follows: 5-10 minutes of increasing pressure, 5 minutes constant pressure at a given level, 5 minutes of decreasing pressure. All investigations were carried out at ambient temperature. The specimens were pressurized with or without the attached transducer in order to determine the effect of bonding material on the results of the ultrasonic measurements_

1561 0956-716X/93 $6.00 + .00

Copyright (c) 1993 Pergamon Press Ltd.

1562 DISLOCATIONS IN Cu Vol . 28, No. 12

Results and Discussion

The attenuation of the longitudinal ultrasonic waves for all frequencies used was increasing asymptoti- cally to the constant value with an increase in the hydrostatic pressure previously applied to the specimen. On the basis of the obtained values of the attenuation and the material constants of copper [9], the density of mobile dislocations and the mean length of dislocation loops were computed according to the Granato-Lficke theory [10]. The results are plotted in Fig. 1.

3

,.2 2

1

0

- • • • •

0 O O0 O0 0 0 0 0 0 0 0

El

a

O N

I I I I I I I I I I

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 P (GPa)

3O -'-'

(9 O

20 <

10

0

Fig. 1. Mean length of dislocation loops (L) and density of mobile dislocations (A) in pure copper monocrys- tal vs. applied hydrostatic pressure.

The initial length of the dislocation loops after irradiation and annealing was equal to 7.7 x 10 -6 cm. This value was in a good agreement with previous investigations by Inagaki et al. [11] on the thermal treatment of -/-irradiated monocrystalline copper. The hydrostatic pressurization increased the mean loop length linearly to the constant value of 3 × 10 -5 cm at the pressure of about 1 GPa. Further pressurization of the specimens did not have any effect on that value. That constant value is not equal but slightly lower than obtained for the material before the irradiation (~ 3.7 x 10 -s cm). The density of mobile dislocations remained constant regardless of the pressure at the level of the value obtained for the material before the irradiation (~ 6 x 10 s cm-~).

Vol. 28, No. 12 DISLOCATIONS IN Cu 1563

Such a response of the pure copper monocrystals to the hydrostatic pressurization cannot be explained on the basis of a supposition that the elastic inhomogeneities (e.g. voids) in the monocrystals existed. In such a case, new dislocations, mainly of a prismatic type, would be generated in the material [2-5,12]. This should have increased the density of dislocations while the mean length of dislocation loops should have been rather constant_ It would be the opposite phenomenon to the observed one. For the same reason the hypothesis of non-hydrostatic state of stresses in the material during pressurization can be rejected. It should be mentioned, additionally, that the usage of pentane as a pressure medium secures the hydrostatic pressurization, and that the results for the pressurization effect were the same independently of pressurization procedure, with or without the bonded transducer to the sample. Therefore the supposition about the effect of a bonding layer as a source of significant stresses in the tested monocrystals cannot be true.

On the basis of previous investigations [11] one can expect that pinning points of dislocations in the initial monocrystMs are small imperfections densely arranged at dislocation lines. The most probable features of those imperfections are narrow pairs of jogs [13,14] which are not very stable in the lattice. Hydrostatic pressurization could be thought of as a means to remove the jogs from dislocations which gives rise to a depinning effect. That could not be the diffusion controlled process as the time was too short and the temperature too low [11]. Also, the experimental results [6] and theoretical predictions [7,8] do not support the idea of climbing forces being acted on dislocations under pressure. Additionally, in the present study, the depinning effect linearly increases with pressure (increase in the mean dislocation loop length, Fig. 1) which is in opposition to the decrease of the diffusion coefficient for metals with pressure according to the exponential function [15]_

The above reasoning indicates that the hydrostatic pressure has an indirect effect on the dislocations lines which are very irregular after -7-irradiation and low temperature annealing. Most probably hydrostatic pressurization makes the dislocation lines smoother by annihilation of newly created dislocation jogs. This process must be stimulated by pressure which reorders the dislocation cores, which can be caused by the increase of the separation between the Shockley partials of extended dislocations. This increase was estimated by Dulin et al. [16,17] for Olnr/Op ~ 0.08 GPa -1, where r is the separation between partials, p - hydrostatic pressure. The stacking fault energy in copper is low. Its most probable value equals 45 mJ/m 2 [18], which gives 3.16 nm for the value of r at ambient conditions. While the hydrostatic pressure is increasing to the value of 1 GPa the separation between the partials increases to the value of 3.42 nm. That means that the hydrostatic pressure of 1 GPa causes the increase in the separation between partials of 0.26 nm. This value is nearly the same as the value of the Burgers vector in copper (0.255 nm), so the fact of removing the effects of "r-irradiation when the pressure achieves 1 GPa does not seem to be accidental.

Conclusions

1. Hydrostatic pressurization irreversibly removes the pinning points on dislocation lines introduced by "~-irradiation and low temperature annealing in pure copper.

2. Ultrasonic investigations of properties of dislocations (e.g. damping constant of dislocations) under high hydrostatic pressure should be carried out after hydrostatic pressurization at a pressure high enough to stabilize the state of dislocations. This requires a pressure of 1 GPa for pure copper.

1564 DISLOCATIONS IN Cu Vo l . 28, No. 12

Acknowledgements

We wish to thank Dr. Ewa Drescher-Krasicka for pure copper monocrystaJ.

References

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