The glide of single crystals of sodium and potassium

Abstract

The data on the glide of elements of single crystals are particularly scarce for metals of the cubic body-centred form, only tungsten (Goucher 1924), α -iron (Taylor and Elam 1926; Gough 1928; Fahrenhorst and Schmid 1932) and β -brass (Taylor 1928; Elam 1936) having been so far investigated. Metals of this crystal structure are, however, of particular interest, since, while the glide direction for the pure metals is, according to most workers, always the most closely packed, i. e. the [111] direction, the glide plane is not, as it generally is in other structures, the most closely packed plane. In general, with these body-centred cubic crystals there is a certain indefiniteness about the glide plane. For α -iron, according to Taylor and Elam, slip does not, except for particular orientations of the shear stress with respect to the crystallographic axes, occur on any particular crystallographic plane, but rather in rod-like elements. According to the later work of Elam, while the slip bands sometimes agree with the (110) plane, they do not in general do so, nor is [111] invariably the glide direction. Gough resolves his deformation into slip on pairs of planes: Fahrenhorst and Schmid decide for the (123) plane as “most probable” but their experiments do not definitely establish this plane. In the case of β -brass Elam decides that no simple crystallographic distortion can explain all the results, and extends this conclusion to α -iron. The question is, then, a complicated one. Our results with sodium and potassium show a simpler state of things, and have led to the conclusion that slipping does here take place in the [111] direction, and on, or very near to, a crystallographic plane, the (123) plane, but it must be pointed out that our method of manufacture of single crystal rods led, for some reason not yet fully clear, to crystals in which the [111] direction was generally near to a plane containing the axis. As regards twinning the results are inconclusive. A further limitation of the generality of our results is that, so far, on account of difficulties of manipulation at low temperatures, the measurements have been carried out at one temperature only. It is hoped, however, in the future, to extend the work to other temperatures.