Molybdenum-tungsten interdiffusion and the influence on potassium bubbles in tungsten lamp wire

The present article describes the effects of molybdenum contamination on th e microstructure of lamp-grade potassium-doped tungsten wire after exposure to temperatures above 2500 degrees C. Molybdenum is generally used as the mandrel material in the coiling of single and double coil lamp filaments. D uring high-temperature heat treatments used during filament manufacture, si gnificant amounts of molybdenum can diffuse into the tungsten. In this stud y, tungsten-molybdenum diffusion couples were prepared and heat treated at temperatures of 2500 degrees C and above to generate molybdenum contaminati on of the tungsten. After dissolution of the molybdenum from the diffusion couple, additional high-temperature heat treatments of the tungsten were pe rformed to simulate lamp burning; equiaxed grains and excessive potassium b ubble growth were observed in the tungsten. Explanations for these microstr uctural changes are discussed. Electron beam microprobe data were also obta ined to characterize tungsten-molybdenum interdiffusion, and a Boltzmann-Ma tano analysis was performed to evaluate the contribution of the concentrati on dependence of the interdiffusion coefficient to the measured diffusion p rofiles.