Potassium bubble growth in doped tungsten

Abstract

Potassium filled submicroscopic bubbles play a decisive role in determining the metallurgical properties of doped tungsten wires. Besides the influence of bubbles on the recrystallization behaviour and the high temperature creep strength, the bubbles may also form the nuclei for the generation of large cavities or voids which influence the fracture behaviour and lamp failure. The most accepted mechanism of bubble coarsening is stress assisted bubble growth; however, this needs as prerequisite a critical bubble size dependence on the stress system of the tungsten coil. It will be shown that a possible mechanism for the coarsening until the critical size is the diffusion controlled transport of potassium provided there are corresponding diffusion paths, as for example grain boundaries or dislocations. In addition, the new model also explains the formation of voids in doped tungsten wire and gives the basis for a material limited failure mechanism in halogen lamps.