Abstract
In order to make clear the non-sag nature of tungsten lamp filament its recrystallization behaviour was studied by means of microscopic scrutiny and X-ray diffraction.
Filament having sag nature begins the recrystallization in a preferred orientation related to initial fibre direction at temperature about 1000°C. This orientation is destroyed at about 1300°C, leading to an ordinary non-preferential polygon structure.
The subsequent grain growth is very slow even at considerably higher temperatures. On the contrary, with non-sag wires this oriented recrystallization is maintained up to 2000∼2100°C, followed by the abrupt coarsening, in which the coarsened crystals encroach directly the remaining fibre structure. Thus, the coarsened crystals tend to occupy the whole cross-section of the wire, and the filament becomes finally to form a successive chain consisting of single crystals.
This phenomenon was interpreted as the inhibition dependent coarsening advocated by Beck and others in 1949. This inhibiting action in tungsten filament is most likely caused by the minute quantities of doped materials.
