Investigation of second phase particles in Aks-doped tungsten by TEM, SEM and atom probe microanalysis

Abstract

Tungsten wire used in the production of coiled lamp filaments is usually stabilised against creep by“AKS Doping”. In this process, small additions of Al, K and Si compounds are made to the original tungsten oxide powder prior to reduction, sintering and fabrication. The final drawn wires contain long strings of ultra-fine second phase material, from which rows of stable voids develop when the material is annealed at high temperatures. These voids stabilise the tungsten microstructure by impeding the lateral migration of grain boundaries. It has recently been shown that the voids contain potassium, but the role of the other dopant elements remains unclear. Small solid alumina particles have been found after dissolving doped wires, and mullite crystals have also been reported, but these observations seem inadequate to explain the apparent interaction between the various dopant species in commercial wires. A number of observations of very small solid particles in doped material have also been reported by Field-Ion Microscopy (FIM), e.g. by Meyrick and the present authors but no direct microanalytical investigation has previously been reported.