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

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 1998-12, Vol.29 (12), p.2933-2939
• Main Authors: BEWLAY, B. P, BRIANT, C. L, MURRAY, M. L
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical interdiffusion; diffusion barriers; Condensed matter: structure, mechanical and thermal properties; Diffusion in solids; Exact sciences and technology; Metals. Metallurgy; Physics; Transport properties of condensed matter (nonelectronic)
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-998-0200-4

The present article describes the effects of molybdenum contamination on the microstructure of lamp-grade potassium-doped tungsten wire after exposure to temperatures > 2500 deg C. Molybdenum is generally used as the mandrel material in the coiling of single and double coil lamp filaments. During high-temperature beat treatments used during filament manufacture, significant amounts of molybdenum can diffuse into the tungsten. In this study, tungsten-molybdenum diffusion couples were prepared and heat treated at temperatures of 2500 deg C and above to generate Mo contamination of the tungsten. After dissolution of the Mo from the diffusion couple, additional high-temperature heat treatments of the tungsten were performed to simulate lamp burning; equiaxed grains and excessive potassium bubble growth were observed in the tungsten. Explanations for these microstructural changes are discussed. Electron beam microprobe data were also obtained to characterize tungsten-molybdenum interdiffusion, and a Boltzmann-Matano analysis was performed to evaluate the contribution of the concentration dependence of the interdiffusion coefficient to the measured diffusion profiles.