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  • 237

    An Investigation of Some Banded Structures Metal Crystals.By C. F. E lam, D.Sc., with an appendix by Prof. G. I. Taylor, F.R.S.

    (Communicated by G. 1. Taylor, F.R.S.Received August 28, 1928.

    [Plate 3.]

    Crystals of the native metals gold, silver and copper have been found which exhibit twinning on an octahedral plane of the spinel type. Metallurgists have described as twins the banded structures which are of very frequent occurrence in these metals when prepared commercially, as they resemble lamellar twinning in calcite and felspar very closely. A few quantitative measurements have been made to determine the relationship between such bands in a metallic crystal; but the value of the results depends on the correct determination of the orientation of the respective parts, and this can only be done by means of X-rays or by indirect methods such as the measurement of slip-bands, etching-pits, etc., which are not always trustworthy.

    McKeehan* measured some structures resembling twins in nearly pure iron by obtaining reflections from the deeply etched surface of a wire mounted in a goniometer. He concluded that the crystals were twins in a crystallographic sense, the twin plane being of the form {211} and the twin axis of the form [111].

    A. J. Phillipst made some measurements of the angles between more than one set of bands in the same crystal in specimens of copper and brass, and found that they were inclined at approximately 70 to each other. (The angle between octahedral planes of a cubic crystal is 70 31'.) Similarly, he measured the inclinations of traces of secondary bands in the principal bands, and found that these also could be considered as being traces of octahedral planes in the bands, provided that the two were related to each other as the two parts of the spinel twin. From this he concluded that the bands were true twins. No direct measurements of the orientation of either the original crystal or of the bands were made, and in many cases the bands could not be traced over the edge between the two faces of the specimen, so that there was a certain amount of conjecture as to whether two traces really belonged to the same plane.

    In a letter to Nature J the present writer pointed out that banded* Amer. Inst. Mining Met. Eng., September, 1927. t Amer. Inst. Mining Met. Eng., February, 1927. t Nature, vol. 120, p. 259 (1927).

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  • 238 C. F. Elam.

    structures in aluminium and copper liad been obtained which resembled twins in appearance, but which did not have the correct relationship for norma! twins. Also, in a paper describing some experiments with crystals of a copper- aluminium alloy,* two examples were given of twin-like structures. One could be described as parallel growth, part of the crystal being rotated 7 with regard to the other about a common axis. The other was the result of distortion. The crystal was so oriented that the shear stress was nearly equal on two octahedral planes and one part slipped on one plane and one on the other.

    Some measurements have now been made in order to obtain further information about these structures, and to determine their true nature with greater accuracy than before.

    The most complete measurements have been made on some samples of aluminium that contained exceptionally large bands. Although aluminium has the same crystal structure as copper, silver and gold (face-centred cubic), the crystals are rarely founded with a banded structure after treatment which would normally produce it. When, however, a large aluminium crystal is strained about 10 per cent, and heated until it recrystallises, the new crystals

    have very straight boundaries, and some of these take on the typical banded structure of twinned copper.

    Two examples with very clear banding were chosen and the orientation of each part relative to the surface determined by means of X-rays. The inclinations of the bands on two surfaces were also measured so that the plane of intersection between the bands and the original crystal could be compared with the X-ray measurements. Fig. 1 (Plate 3) is a photograph showing two sides at right angles, and fig. 2 a diagram of one crystal cut from a sheet

    ig. 2. Diagram of Crystal, i thick. The principal crystal went throughthe sheet, but the twin did not, as the photo

    graph shows. Further, although the junction between the two parts on one face was straight, on the other it was uneven and like an ordinary crystal boundary. It was not possible to obtain any trustworthy measurements of the inclination of the line of intersection on this face. Particulars of the

    * Roy. Soc. Proc., A, vol. 116, p. 694 (1927).

    FACE

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  • Banded Structures in Metal Crystals. 239

    X-ray data are given in Table I. and are plotted on a stereographic diagram in fig. 3. The plane of the paper represents the face of the specimen and the

    Table I.Reference Plane = Face of Specimen.6 as Angle between normal to plane and line of intersection of face and edge.tp = Angle between plane containing the normal of the reflecting plane and face of

    specimen, measured anti-clockwise.

    Reflecting Planes. Twin Plane.

    1k ;=115 9 = 136 9 = 157 30'! 7 II 2 o A = 359 30'1 86 e = 123 9 = 156r w II II1 8 II Ci t/s= 0

    r a / * = 83 10' e = 57 30' 9 = 153 308 30' tp= 81 xjj = 14

    .2. w II II 99 30' 9 = 42 10' 9 = 154 30'

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