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

The present article describes the effects of molybdenum contamination on the microstructure of lampgrade potassium-doped tungsten wire after exposure to temperatures above 2500 °C. Molybdenum is generally used as the mandrel material in the coiling of single and double coil lamp filaments. During high-temperature heat 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 °C and above to generate molybdenum contamination of the tungsten. After dissolution of the molybdenum 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.     

Van der waals approximation for potassium bubbles in tungsten

The finely dispersed potassium bubbles are responsible for the high-temperature sag resistance of lamp filaments; thus, the understanding of development of bubble structure is of practical importance. Usually, the ideal gas theory is used for describing the potassium bubbles in tungsten. Instead, we used the van der Waals approximation which can account for phase changes and describes a two-phase coexistence region. It is shown theoretically that bubbles in the practically important size range (some tenth of microns) can exist in a two-phase state. Some experimental findings are also better explained by the real gas approximation.     

Potassium bubbles in tungsten wire

Tungsten filaments for incandescent lamps almost always contain potassium bubbles. These are required to produce the interlocking grain structure that gives the tungsten lamp filament a long life. In this article, we consider the formation of the bubbles from the ellipsoids of potassium produced by deformation, the growth of these bubbles as a result of internal pressure created by the potassium vapor within the bubbles, and the reasons why these bubbles can sometimes grow to very large sizes. In each case, we present a simple model to describe the process. We finally use nucleation theory to describe the overall process of bubble growth. Formerly Visiting Scientist, Institute for Technical Physics, Hungarian Academy of Science, Budapest, Hungary     

The effect of deformation on abnormal grain growth in tungsten ingots

This paper reports a study of abnormal grain growth in tungsten ingots that had been deformed in compression at elevated temperatures. The results show that very large grains form in these samples directly from the polygonized structure when the deformed samples are annealed. A critical amount of deformation is required to begin this process. Beyond that point, the resulting grain size decreases with increasing amounts of deformation and decreasing test temperature. Abnormal grain growth occurs first in the regions of the sample that have undergone the most strain. Two factors appear to provide the driving force for the formation of these large grains. One is the elimination of grain boundary area. This effect would be present in any system undergoing regular or abnormal grain growth. The other is the elimination of grains that are more highly strained by grains that are less strained. In this way, this process is similar to the large grain growth resulting from strain annealing.     

Effect of gas bubbles on recrystallization of tungsten

Tungsten sheet deposited from WF₆ vapor was rolled at temperatures up to 1000°C and reductions in thickness up to 95 pct. Recovery and recrystallization were studied using hardness measurements and transmission electron microscopy. The temperatures for the onset of recrystallization during 1 hr anneals were 1600°C or more. Recrystallization was sluggish, requiring temperatures of 2100°C or more for completion. Transmission electron microscopy revealed that many small (< 500Å diam) gas bubbles were present on dislocations and recovered subgrain boundaries and caused scalloping of these boundaries and the advancing recrystallized grain boundaries. Coarsening of the bubbles to a relatively few larger bubbles permitted complete recrystallization. No solid inclusions were found. Annealing treatments that coarsened the gas bubbles prior to rolling caused accelerated recovery and complete recrystallization was observed at approximately 1200°C. It is concluded that small gas bubbles were the cause of the original high recrystallization temperatures and also influenced the size and shape of the recrystallized grains.     

Enhanced low-temperature sintering of tungsten

The effects of various transition metal additions on the sintering of a well-characterized, fine tungsten powder were analyzed using both isothermal and constant heating rate experiments in the temperature range 900 to 1400‡ C. Approximately four atomic monolayers of palladium on the tungsten powder surface were found to be the optimal enhancer, followed by nickel, cobalt, platinum, and iron. The addition of Cu to the tungsten had no appreciable effect on the sintering kinetics. Sintering enhancement by these transition metals is related to their periodic chart position(i.e., electron structure). An overall non-Arrhenius shrinkage temperature dependence is attributed to grain growth in the activator-treated specimens. The activation energy for tungsten densification was determined to be 430 to 450 kJ/mol, which is in general agreement with a grain boundary diffusion process.     

The high-temperature sintering of tungsten

Summary A study was made of the effect of high-temperature sintering on the shrinkage, density, porosity, chemical composition, grain size, grain density, electrical conductivity, hardness, and strength of VA tungsten bars with silicon-alkali and aluminum additions and VCh tungsten bars without additions.     

Enhanced low-temperature sintering of tungsten

The effects of various transition metal additions on the sintering of a well-characterized, fine tungsten powder were analyzed using both isothermal and constant heating rate experiments in the temperature range 900 to 1400°C. Approximately four atomic mono-layers of palladium on the tungsten powder surface were found to be the optimal enhancer, followed by nickel, cobalt, platinum, and iron. The addition of Cu to the tungsten had no appreciable effect on the sintering kinetics. Sintering enhancement by these transition metals is related to their periodic chart position (i.e., electron structure). An overall non-Arrhenius shrinkage temperature dependence is attributed to grain growth in the activator-treated specimens. The activation energy for tungsten densification was determined to be 430 to 450 kJ/mol, which is in general agreement with a grain boundary diffusion process.     

Phase transformations in reactive sintering of tungsten

Abstract

It has been demonstrated recently that tungsten (T _m = 3410±20°C) can be sintered by reactive sintering in a reductive atmosphere such as hydrogen. This alternative technique to the conventional sintering (T _s>2000°C) makes use of a small amount of aluminium addition which acts as a sintering aid and hence lowers the sintering temperature significantly (T _s1200°C). This study explores the phase transformations that take place during reactive sintering of tungsten in view of the mechanisms involved. DSC, SEM and TEM have been used for a fundamental understanding of this system.     

In situ strength evolution during the sintering of bronze powders

Powder metallurgy (PM) allows the fabrication of complex net-shaped components. Accurate design specification of these components requires precise prediction of the compact’s response to sintering process parameters. Nonuniform sintering responses such as strain gradients can result in process failures such as permanent deformation and cracks. To avoid these types of process failures without costly trial-and-error design, the most important response to predict is the compact’s strength as it evolves during the sintering process. A device and method have been developed to characterize the in situ strength evolution as a function of various sintering process parameters. The specific strength parameter investigated and modeled in this article was transverse rupture. This strength was precisely determined for 90Cu-10Sn bronze in response to various combinations of temperature, heating rate, and heating time. The consequence of this work is to identify thermal cycles that minimize distortion and otherwise improve dimensional tolerances.