The mechanical behaviour of single crystals of certain face-centred cubic metals

Abstract

Single crystal rods of gold, silver and nickel have been prepared from the melt by a modified cooling method. The three metals differ in that the gold is very pure and has no surface contamination; the silver is very pure and the surface normally shows contamination; the nickel is markedly less pure. These three features find expression in the results. The stress-glide relationship has been investigated over the whole range to fracture, special attention being paid to the region of small glide. At very small glides the results show certain variations attributed to metastable conditions. For pure metals at low temperatures a region of easy glide has been established, which is absent in nickel. Considerations of the variation of hardening with temperature at constant glide show that, at moderate glide, rise of temperature at low temperature promotes hardening, but rise of temperature at high temperature leads to decrease of hardening. From these results it is concluded that in metals without notable impurity glide is due to two distinct processes, first the initiation of glide planes, which temperature agitation tends to check, and secondly to glide proceeding on planes already initiated, which temperature agitation tends to promote. The surface contamination of silver single crystals which takes place by exposure to air at room temperature raises the critical shear stress somewhat, but has a much more marked effect on the hardening after some tens of per cent of glide has taken place, which is attributed to diffusion of the impurity atoms, probably oxygen, taking place during the travel of the dislocations. This is distinct from normal diffusion, which is negligible at room temperature. The results on the effect of surface contamination explain abnormalities previously noted with silver. The close connexion of asterism and hardening is exhibited in the results obtained with gold and silver. In particular, silver in the region of easy glide shows very small asterism, even at large glide. The cube root of the breaking stress of all metals in single-crystal form has been found to show a linear relationship with temperature.