The Internal Friction and the Grain Size of Tungsten Filaments

Abstract

The internal friction Q^-1 of three kinds of tungsten filaments is measured at tempera-tures between a room temperature and 2900°K, using a simple apparatus newly designed. It increases with temperature, but there are no noticeable differences among its values below 1000K°. Above this temperature, however, it becomes quite sensitive to the crystal structure. Thee value of Q^-1 of a thoriated filament with polycrystalline structure reaches 4×10^-2 at 2000K°, whereas that of a nonsag wire with well developed long grains is 0.2×10^-2 at the same temperature and remains at as small a value as 0.4×10^-2 even at 2900°K. The value of a filament composed of both long and small grains is in between. A nonsag wire before the grain growth gives high Q^-1, which decreases rapidly when exaggerated grain growth takes place in the filament by suitable heat treatment. The internal friction of tungsten filaments at high temperatures mostly originates in grain boundaries and is sensi-tive to the area of boundaries or the size of grains. This phenomenon can be used as a non-destructive method either to estimate the size of grains or to follow the grain growth in the filaments.