Void growth in tungsten wire

Abstract

This paper presents a study of void growth in tungsten wire. This wire contains potassium bubbles which are used to control grain growth and morphology at elevated temperatures. Voids grow on the grain boundaries of this wire when it is operated in a lamp or heated in an uncoiled configuration. It was found that void growth was accelerated as the temperature of the wire, the stress on the wire and the amount of oxygen in the environment were increased. It was also found that in the lamps operated without an externally applied stress (i.e. other than gravitational forces), voids did not grow unless there was a population of potassium bubbles with diameters greater than 0.08 μm. Based on these observations, it is proposed that the voids grow by creep. The larger pre-existing potassium bubbles serve as nuclei for these creep voids, since the low stresses and high temperatures preclude mechanically assisted nucleation at the high temperatures of lamp operation. It is also proposed that grain boundary sliding is an important source of stress in causing the voids to grow. Model calculations are used to support these conclusions.