Void formation in tungsten lamp filaments

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Abstract
Microstructural changes occurring during the annealing of tungsten halogen lamp filaments have been investigated using both optical and scanning electron microscopy. In tungsten wires that had completely recrystallised linear arrays of voids were observed approximately parallel to the axis of the wire, the void size being smaller the more extensive the deformation. With progressive annealing the voids increased in size and in the larger wires void coalescence caused the formation of a small number of relatively large voids, preferentially located at the grain boundaries. The mechanical stability of the lamp filaments would appear to be related to the spacings between both the linear arrays and the individual voids. At elevated temperature the voids appeared to contain gas which may have originated from the dopant materials or impurities in the tungsten. The observations also provide further support for the postulate that the doping mechanism in lamp filament tungsten depends on the formation of fine voids which retard recrystallisation and also preferentially inhibit grain boundary movements.