XXVII. On the strength of tungsten single crystals and its variation with temperature

Abstract

By heating drawn tungsten wires to a high temperature, large crystals are developed with random orientation of crystal axes, many of which occupy locally the complete volume of the wire. If these wires are subjected to a suitable tensile stress, some of these large crystals will deform by a process of slip; and one will eventually fracture. The orientation of the crystal axis in this crystal is such that two sets of slip planes are symmetrically inclined to the direction of stress, and the crystal fractures in the form of a wedge symmetrical to the stress. This affords a very simple means of studying the deformation of single crystals under conditions involving a minimum distortion of the crystal lattice. Measurements are made on the strength of such crystals in wires of various diameters and at temperatures varying from 700° K. to the melting-point. It was found that the time required for fracture—which may be regarded as an inverse measure of the mean rate of deformation—is approximately an exponential function of the load and the temperature; that for a given rate of deformation the load is proportional to the cross-sectional area; and—apart from transition temperatures—is a linear function of the temperature. Discontinuities occur in the load-temperature curves at temperatures approximately 850°, 1600°, and 2606° K. Variations in other physical properties are found to occur at the same temperatures, which leads to the view that these changes represent “transformation points” of a new type. Some new measurements on the thermal expansion of tungsten are recorded.