Abstract
A study of the development of porosity in the temperature range of 3000° to 3350°C in several grades of tungsten wire has been performed. In addition to measurements of the kinetics of pore growth and pore number density by light microscopy, examinations on heat-treated wire by means of scanning electron microscopy and electron fractography were accomplished and revealed submicron-sized pores principally arranged in linear arrays. The observations were evaluated in terms of various theories of pore growth. While the phenomena encountered were complex and overlapping, it seems probable that initial pore growth is by plastic deformation due to volatilization of dopant. The constancy of the number of growing microscopic-sized pores with time suggests that coalescence processes are not rate controlling. Since an activation energy of 110 kcal per mole was measured for pore growth in doped tungsten, vacancy diffusion from the surrounding matrix to the pore under internal pressure was considered. A simplistic analytic treatment yielded a 1/5 power-time dependence in comparison to the linear relation observed.
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