S H O R T C O M M U N I C A T I O N S 409

linckx, 1964). As the stacking fault energy is reduced, the probability of slip by partical dislocations is increased. Consequently, the probability of cross slip, which is neces- sary for cell formation, is also reduced. This correlation seems verified by the electron microscopy results presented by Swann (1963) on AI, a metal of high stacking fault energy, and Au and Cu metals of low stacking fault ener- gies. Briefly, his results show that good cell wails are formed in AI, with no dislocations within the cells, and ragged walls in Cu and Au with dislocations within the cells. In addition, metals which form a good cell structure during deformation should contain more dislocations with screw orientation than edge orientation since the cell walls are composed mostly of screw dislocations networks (Hirsch, H o r n e & Whelan, 1956). The formation of cell structure may also be hindered in pure metals by lowering the de- formation temperature (Swann, 1963).

The present results add credit to the above concepts of cell formation and emphasize the usefulness of the X-ray technique in studying cold-worked metals. The results on A1 show that a well defined cell structure was not produced at low temperatures; the ratio of r.m.s, strains in the (100) and (111 ) directions was unity when deformation was made at - 196 °C. This ratio indicates a random dislocation array in this relatively isotropic metal. It also seems to indicate an excess of edge dislocations, since the calculated ratio was 1.03 for edge and 1.23 for screw dislocations. When the deformation temperature was increased to room tem- perature, both the particle size and ratio increased. This result indicates the development of a better cell structure containing more screw dislocations. This trend continued after one week at room temperature (RT). Although the results for Ni, another metal of high stacking fault energy, are less striking, they do seem to indicate more screw than edge dislocations such as would accompany the develop- ment of a cell structure. Au, a metal of low stacking fault energy, shows a r.m.s, strain ratio which suggests a less well developed cell structure at low than at high tempera- tures. The difference in the calculated values for screw and

edge dislocations for Cu, Ni and Au are not as significant as those for AI and, consequently, only show trends. The results of the Cu measurements, however, show a change similar to that shown by A1 on annealing. The large an- isotropy in the r.m.s, strains obtained on annealing are indicative of the development of a non-random array of dislocations, i.e. the formation of a well defined cell struc- ture.

The authors would like to acknowledge the support of Project Themis (F.W.L.) and the U.S. Atomic Energy Commission (EAS) during the course of this work.

References

AMELINCKX, S. (1964). The Direct Observations of Disloca- tions, Solid State Physics. Edited by F. SErrs & D. TURN- BULL, Suppl. 6, New York: Academic Press.

GOSWAMI, K. N., SEN GLrP'rA, S. P. & QtJADER, M. A. (1966). Acta Met. 14, 1559.

HmSCH, P. B. (1958). Internal Stresses and Fatigue in Metals, p. 139. New York: Elsevier.

HIRSCH, P. B., HORNE, R. W. & WrtELAN, M. J. (1956). Phil. Mag. 1,677.

HIRTH, J. P. & LOTHE, J. (1968). Theory of Dislocations, p. 762. New York: McGraw-Hill.

RACHINGER, W. A. (1948). J. Sci. Instrum. 25, 254. RYABOSHAPKA, K. P. & TIKHONOV, L. V. (1961). Fiz. Metal.

Metalloved. 11,489; 12, 1. SXOKES, A. R. (1948). Proc. Phys. Soc. Lond. 361, 382. SWANN, P. R. (1963). Electron Microscopy and Strength of

Crystals. Edited by G. TaOMAS & J. WASHBURN. New York : Interscience.

WAGNER, C. N. J. (1957). Acta Met. 5, 477. WAGNER, C. N. J. (1958). Rev. M~tall. 55, 1171. WAGNER, C. N. J. (1960). Z. Metallk. 51, 259. WARREN, B. E. (1959). Progress in Metal Physics, 8. New

York: Pergamon Press.

J. Appl. Cryst. (1970). 3, 409

High-speed X-ray diffraction photography. By C.T. PREwrrr, Department of Earth and Space Sciences, State Univer- sity of New York, Stony Brook, New York 11790, U.S.A. and E. P. MOORE and L. F. LARDEAR, Central Research Depart- ment,* E. I. du Pont de Nemours and Co., Wilmington, Delaware 19898, U.S.A.

(Receit, ed 27 May 1970)

The speed of the conventional process of X-ray diffraction photography can be greatly improved by the use of a new industrial X-ray film and a fluorescent screen.t This process produces a transparent film with about a tenfold decrease in exposure time and at a cost less than that of standard nonscreen film.

Use of a fluorescent screen to decrease exposure time is a commonly used technique in X-ray diffraction photography. The most notable application of this technique has been the Polaroid high-speed exposure and development process

* Contribution No. 1698. t The combination used in these experiments was a Du Pont

experimental industrial NDT Non-Destructive Testing X-ray film and 'Cronex' NDT screens.

which gives a negative image on photographic paper. For various reasons, however, there has been less interest in using fluorescent screens with conventional transparent film and instead, nonscreen films have been widely used which require considerably longer exposures and longer develop- ment times than does the Polaroid process. Since it is often desirable to have transparent films for certain applications, we have devised a relatively high-speed method in which the conventional processes are optimized.

410 S H O R T C O M M U N I C A T I O N S

Photographs were taken with a Buerger precession camera using a specially modified 5 x 7 inch cassette* containing the fluorescent screen. The film is held flat against the phosphor side of the screen with a ~- inch bakelite sheet and placed so that the X-rays pass through the film before striking the screen. The bakelite sheet absorbs about 3% of the Mo Kct radiation and, in general, the exposures are about a factor of ten shorter than for conventional nonscreen film. The resolution is not as good, however, since the darker spots are larger in diameter than on the nonscreen film. For routine orientation, cell, and space-group work, utilizing precession, back reflection, cone axis and other fiat plate type cassettes, we find the this industrial film to be per- fectly satisfactory.

Manual development of the films is at 85 :;F in 'Cronex' X I M D developer for thirty seconds, followed by rinsing in clear water and fixing for sixty seconds in X-ray fixer. Because of its low gelatin content, this film can be sa- tisfactorily washed and dried to a usable condition in five minutes or less at 85°F. All solutions are maintained at 85 °F in a stainless steel, dental-film tank equipped with a thermostated tap water mixing valve.

After it appeared that the general technique was feasible, we obtained an automatic developing machine which can produce a developed and dry film in 90 seconds. Our ex- perience shows that this machine is of value when a large number of films is being processed each day, but the steady maintenance required for good quality photographs makes the machine unsuitable for small processing volumes.

* Supplied by the Charles Supper Co., Tech. Circle, Natick, Massachusetts, U.S.A.

Since the cost of the this film is less than that of non- screen film and since a complicated cassette is not re- quired, the techniques described above represent a very economical way of improving the efficiency of diffrac- tion photography. The 5 x 7 inch cassette is a great advan- tage because standard 5 x 7 inch films can be used without having to cut them in the darkroom. For those who wish, a smaller piece of film can be cut and used in the same cas- sette.

For Weissenberg photography we used a thin fluorescent screen optimized for Cu Ke radiation (Herglotz, 1968). Here the screen and film are placed inside a black paper envelope so that the X-rays will strike the screen first. This is necessary because the film would absorb too much of the Cu Ke radiation if the X-rays passed through it first. Using this same technique in the Debye-Scherrer powder camera, the exposure time relative to nonscreen films was reduced by a factor of 2. We believe this factor of 2 can be used to good advantage in routine identification work and in high- temperature powder diffraction studies. Further develop- ments of other diffraction techniques using the screen-film combination are in progress.

The authors wish to thank Mr A. F. Biddle for his evaluation of the powder experiments and Dr K. F. Mucker for his evaluation of the back-reflection experiments.

Reference

HERGLOTZ, H. K. (1968). Rev. Sci. Instrum. 39, 1658.

J. Appl. Cryst. (1970). 3, 410

On the occurrence of extrinsic stacking faults in some copper-base and silver-base alloys. By S. P. SEN GUPTA and M. DE, Department of General Physics and X-rays, Indian Association for the Cultivation of Science,

Calcutta-32, India

(Received 12 August 1969 and in revised form 21 May 1970)

The earlier X-ray data of stacking-fault probabilities in some cold-worked Cu-base and Ag-base alloys have been reanalysed to detect the presence of extrinsic faults. Results indicate that these are present in the alloys having higher solute content. The probability of their occurrence increases with the increase of the valency of the solutes and with the increase of intrinsic faults and decrease of twin faults. However, in certain cases, the measured twin-fault probabilities not only decrease, but become unreasonably large and negative. The observations are in accordance with recent findings on the occurrence of extrinsic faults in cold-worked face-centred cubic alloys.

X-ray measurements of stacking-fault densities in cold- worked (CW) metals and alloys have been undertaken by various workers in recent years and in most of the investi- gations the interpretation is based on the fact that only in- trinsic or single deformation faults and twin faults are pre- sent in the cold-worked filings. The presence and the dif- fraction effects of extrinsic or double-deformation faults along with intrinsic and twin faults in deformed materials have, however, been investigated theoretically by Johnson (1963) and Warren (1963). But X-ray measurements of stacking faults in ~-Cu-Zn and ~-Cu-Sn alloys by Wagner & Helion (1965) could not produce any evidence of the oc- currence of extrinsic faults in the cold-worked filings of

these substances. Recently, Lele (1967) has reinterpreted ear- lier X-ray data in Ag-Sb alloys (Sastry, Rao & Ananthara- man, 1966) and has been able to detect a small concentra- tion of extrinsic faults. Extrinsic faulting due to deforma- tion has also been observed recently in Au-4.8 Sn (Loretto, 1964), Ag-6.8 Sn (Ives & Ruff, 1966), Ag-7.5,11-8 In (Gal- lagher, 1966; Gallagher & Washburn, 1967), pure Ag and Ag-0.5 In (Gallagher & Washburn, 1966), Cu-22, 30 Zn and Cu-7.5 AI (Gallagher, 1968) and Cu-16.6 AI (Swann, 1966) alloys by transmission electron microscopy. This informa- tion points to the need for the re-evaluation of our earlier X-ray data of fault probabilities in some copper-base and silver-base alloys (Goswami, Sen Gupta & Quader, 1966;