Mechanism and kinetics of bubble formation in doped tungsten

Abstract

The mechanisms and kinetics of bubble formation that occur during annealing of tungsten doped with small amounts of K, Al, and Si compounds has been investigated by electron microscopy. Bubbles are present in the sintered ingot due to volatilization of the dope during the sintering treatment. As the ingot is worked down into wire, the bubbles present in the ingot become elongated. On annealing these elongated bubbles undergo shape changes which depend on the amount of working. Elongated bubbles with a length-to-width ratio of less than ten spheroidized while those with a length-to-width ratio greater than twenty are unstable and break up into a row of bubbles. If working has been sufficient to close up the elongated bubbles,e.g., in ribbon, 0.125 mm thick rolled from 0.9 mm diam wire, bubble formation results from the diffusion of vacancies to the volatile dopant particles.