Experiments on the production of large copper crystals

Five years ago one of the authors with Miss Elam devised a method of preparing “ single-crystal ” test-pieces of aluminium from bars of the polycrystalline metal by the method of critical straining and subsequent heat treatment. As a result of this work it has been possible to prepare a large number of single-crystal test-pieces of this metal whose properties have been investigated by Taylor, Farren, Gough, Hanson, Wright and the authors themselves. How aluminium distorts under various forms of stress is now well established. This method has also proved applicable to the production of single-crystal test-pieces of alloys of aluminium and zinc composed of one phase which possesses the same symmetry as that of the aluminium crystal itself (face-centred cubic lattice). Furthermore, Edwards and Pfeil§ have used it in producing single-crystal test-pieces of iron. It is of some interest to notice that the method has proved successful in the case of iron, because this metal possesses a different crystal symmetry from that of aluminium (body-centred cubic lattice) and distorts in an entirely different way. The method of distortion of these crystals under stress, both tensile and compressive, has recently been determined by Taylor and Elam. Investigations carried out by the present authors during the last two years have, however, shown that there are cases in which this method of producing large metal crystals is attended with only a moderate degree of success. The present paper described these experiments which have been carried out with copper. An outstanding difference on straining and heating between this metal and those previously mentioned is that the former produces a large number of twins (while the latter do not produce any) which have a profound influence when the strained metals are subsequently heated, and limit, to a considerable extent, the size of the crystals which can be produced. This has accordingly led to a more intensive study of twinned crystals produced by this method, which forms the subject of a separate paper by the authors. Quite recently Miss Elam has succeeded in producing single-crystal test-pieces of copper, silver and gold direct from the liquid, by a modification of Bridgman’s method. These crystals are quite free from strain and contain no twins.