GALT 473-31

Mobility of Dislocations in Aluminum at 74°K and 83°K*

J. A. Gorman, D. S. Wood, and T. Vreeland, Jr.

W. M. Keck Laboratories, California Institute of Technology, Pasadena, California

In a previous article1

the authors reported determinations of the velocity

of dislocations in high purity single crystals of aluminum as a function of applied

stress in the temperature range 123°K to 343°K. These results were obtained using

a stress pulse technique in which dislocation positions were observed before and

after the stress pulse using the Berg-Barrett x-ray technique. This note reports

an extension of the dislocation velocity measurements in aluminum to lower

temperatures, 74°K and 83°K, and discusses their significance.

Dislocation velocities at 74°K and 83°K were obtained by the method

1 described previously except that the aluminum single crystal test specimens

were bonded to the torsion stress pulse machine using mixtures of isopentane and

methylcyclohexane which form glasses at temperatures slightly below the test

temperatures. The dislocation velocity data obtained at 74°K and 83°K are

* This work was supported by the U. S. Atomic Energy Commission

- 1 -

- 2 -

shown as functions of the applied resolved shear stress in Figure 1. The

dashed lines in Figure 1 are straight lines from the origin to a point obtained

by averaging all the stress-velocity data for the given temperature.

The dislocation drag coefficient, which is the ratio of the drag force

acting on a unit length of dislocation to the dislocation velocity, was obtained

from the stress-velocity data as described previously1

The values obtained

-4 2 0 -4 2 were 1.33 x 10 dyne-sec/em at 74 K and 1.31 x 10 dyne-sec/em at 83°K.

These values, together with the values reported previously for higher temperatures,

are plotted as circles in Figure 2. Figure 2 also shows dislocation drag

2 coefficients for aluminum obtained by Mason and Rosenberg from ultrasonic

attenuation measurements (indicated by+) and from the phonon viscosity3 ' 4 and

electron viscosity5

'6

theories (indicated by dashed lines). The experimental

drag coefficients from Mason and Rosenberg and the theoretical values from the

phonon and electron viscosity theories are plotted to a different scale than

the present results to facilitate comparison of their temperature dependencies.

For temperatures greater than 150°K, Figure 2 shows that the magnitude of

the dislocation drag coefficient obtained by the present authors is considerably

less than that obtained by Mason and Rosenberg by ultrasonic measurements and

by the phonon viscosity theory, but that the temperature dependencies agree

rather well. In the authors' previous paper1

possible reasons for the dif-

ferences in the magnitude of the dislocation drag coefficient were discussed and

will not be repeated here. It was concluded that the agreement in the temperature

dependence tended to support the phonon viscosity theory.

The temperature dependence of the dislocation drag coefficient obtained

by the present authors in the range 74 °K to 150 °K does not exhibit the increase

with decreasing temperature which is shown by Mason and Rosenberg's experimental

and theoretically predicted results.

The present results thus indicate in contrast to Mason and Rosenberg that

in aluminu.'Tl the dislocation drag due to electron viscosity is ne g ligible compared

- 3 -

to the phonon drag for temperatures greater than 74°K. Further direct velocity

measurements at temperatures below 74°K are needed to establish the importance

of electron viscosity in aluminum.

REFERENCES

1. J. A. Gorman, D. S. Wood, and T. Vreeland, Jr., submitted for publication

to J. Appl. Phys., 1968.

2. w. P. Mason and A. Rosenberg, Phys. Rev. 151, 434 ( 1966).

3. w. P. Mason, J . Acoust. Soc. Am. 32, 458 ( 1960).

4. w. P. Mason, J. Appl. Phys. 35, 2779 ( 1964).

5. w. P. Mason, Ph~sical Acoustics and the ProEerties of Solids (D. Van

Nostrand Co., Inc . , Princeton, 1958) p. 323.

6. W. P. Mason, Appl. Phys. Letters~. 111 (1965).

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Fig. 1 Dislocation velocity vs resolved shear stress.

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TEMPERATURE, °K 0

Fig . 2 Disloca ti on dr a g coe ffi c i e nt vs t emperature .