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The m e c h a n i s m s and k i n e t i c s of bubble f o r m a t i o n that o c c u r d u r i n g an n eal i n g of tungsten doped with s m a l l a m o u n t s of K, A1, and Si co m p o u n d s has b e e n i n v e s t i g a t e d by e l e c t r o n m i c r o s c o p y . Bubbles a r e p r e s e n t in the s i n t e r e d ingot due to v o l a t i l i z a t i o n of the dope during the s i n t e r i n g t r e a t m e n t . As the ingot is w o r k e d down into w i r e , the bubbles p r e s e n t 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 m m thick rolled from 0.9 m m diam wire, bubble formation results from the diffusion of vacancies to the volatile dopant particles.

TUNGSTEN filaments in incandescent lamps require a high resistance to sag at the operating temperatures of 2000~ to 3000~ Nonsag filaments have been produced for nearly half a century with the addition of dopants, consisting of small amounts of K, Al, and Si (usually in the form of oxides) introduced as aqueous solution into tungsten oxide powder prior to reduction to metallic tungsten by hydrogen. ~ The effect of doping on the sag resistance is remarkably strong as evidenced by the fact that over 90 pct of the dopant is volatized during sintering.2'3 In spite of the commercial cial and scientific interest in this doping process, extensive research has failed to deduce unequivocally either the physical state of the dopants in the filaments or the reasons why the dopants confer nonsag properties. Various conjectures exist in the literature on the mechanisms of doping. Early observations had led to the speculation that the primary function of the volatile dopant is to keep the ingot porous during sintering so as to allow the escape of the less volatile impurities which would not have been removed otherwise.4 Other investigators have proposed that the dopant exists as a second-phase in the form of thin tubes parallel to the wire axis ,s-v and in one investigation the dopants were assumed to remain in solid solution interacting with grain boundaries upon annealing.S However, replica electron microscopy of polished and etched wire failed to reveal the presence of second-phase impurities or porosity,4 and there is no experimental evidence in support of the latter hypothesis. Recent studies by transmission electron microscopy have unambiguously disclosed stringers of the dopant in annealed doped wire, several hundred Angstroms in diameter.I~ Two interpretations exist: one is that they are particles of mullite,I~ and the other is that they are bubbles developed by the volatilization of the dopant upon annealing,n'~e In the present paper, additional evidence is presented in support of