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     

Porosity and anomalous recrystallization behavior in doped tungsten wire

An anomalous recrystallization behavior has been observed in 0.010 in. diam K?Si?Al doped tungsten wire. Recrystallization to relatively equiaxed grains occurred only in the core at temperatures of 1400° to 1600°C, but not at lower temperatures or higher temperatures up to 1900°C. The anomalous recrystallization reaction was suppressed when the wire was preannealed at 1700° to 1900°C, but not by preanneals at temperatures between 500° and 1100°C. Critically shadowed electron fractographs indicated that linear arrays of microporosity (≈100Å diam) were responsible for the stabilization of the fibrous structures, Recrystallization occurred when these strings of pores were absent.     

Sintering of very fine molybdenum and tungsten powders

Conclusions Vacuum annealing at a temperature above 900°K enables the specific surfaces of very fine loose tungsten and molybdenum powders to be varied in a wide range. The vacuum sintering of compacts pressed from very fine (particle size less than 0.05m tungsten and molybdenum powders is accompanied by severe cracking. In the hot pressing of very fine Mo and W powders produced by the pyrolysis of carbonyls in a stream of high-temperature plasma, a specimen density close to theoretical is reached at 1600°K i.e., at a temperature not less than 400°K lower than the sintering temperatures of powders of particle size more than 1 m. Sintering lowers the amounts of carbon and oxygen in Mo and W by more than half compared with the starting condition.Translated from Poroshkovaya Metallurgiya, No. 1(229), pp. 47–51, January, 1982.     

Splitting of tungsten wire in the knife-edge compression test

As-drawn tungsten wire has been known to split and rolled tungsten sheet to delaminate if mishandled. The mechanism of splitting of tungsten wire in a knife-edge compression test was studied for a number of tungsten wires of different diameter, composition, processing history, and origin. The micro-structure of these wires was examined by optical, scanning, and transmission electron microscopy. Grain boundary composition was determined by Auger electron spectroscopy. A correlation between the number of transverse boundaries in the microstructure and the split load in the knife-edge compression test was observed; the greater the number of transverse boundaries, the lower the load required to induce a split. This relationship held for all wires, regardless of the history of the wire. It was concluded that transverse grain or subgrain boundaries are responsible for initiating splits in the knife-edge compression test. The mechanism of splitting is discussed.     

Properties and structure of tungsten wire produced from rolled rods

Powder Metallurgy and Metal Ceramics -     

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.     

The high temperature creep behavior of doped tungsten wire

The creep behavior of 0.018 cm diam doped tungsten wire has been studied over a range of stress from 30 to 90 MPa and temperature from 2400 to 2800 K. Grain aspect ratio (gar) had a strong influence on creep and rupture of the recrystallized wires, and separated the creep behavior into two regimes with a transitional gar of about 11 between the two. The low gar regime showed lower strength and characteristics typical of grain boundary sliding. In the high gar regime, properties were independent of gar, and evi-dence is presented to show that creep is governed by dislocation-bubble dispersion strengthening. Formerly with the Lamp Business Division of General Electric Company, E. Cleveland, Ohio     

钨矿细泥选矿工艺发展

随着对钨细泥特点的不断研究,浮选、重选、磁选、浸出等单一工艺不能满足对钨细泥选别指标的要求,多种选别方法结合的联合工艺应运而生。文中简述了各种流程的特点以及针对钨细泥选别在选厂调研中发现的问题,对未来钨细泥的研究方向以及设备应用进行展望。     

钨矿细泥选矿工艺发展

随着对钨细泥特点的不断研究,浮选、重选、磁选、浸出等单一工艺不能满足对钨细泥选别指标的要求,多种选别方法结合的联合工艺应运而生.文中简述了各种流程的特点以及针对钨细泥选别在选厂调研中发现的问题,对未来钨细泥的研究方向以及设备应用进行展望.      

钨细泥选矿工艺现状

近年来由于离心选矿机、螺旋溜槽、高梯度磁选机等应用于钨细泥回收的高效设备的不断改进与合理利用,钨细泥回收技术已逐渐趋于成熟.根据钨细泥自身特征的不同,钨细泥回收主要选择采用重、磁、浮等单一或几种工艺联合流程,文中对其进行了综述比较,概述了各自的适用范围和特点.     

The influence of microstructure on fracture of drawn tungsten wire

The microstructures and longitudinal fracture resistances of 0.635 mm diam lamp-doped and undoped tungsten wire were examined in the as-drawn condition and after anealing at temperatures between 600 and 1500°C. A variety of experimental techniques were employed, including Auger Electron Spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and a newly developed mechanical testing technique. The longitudinal fracture mode was intergranular for all wires and a second phase was observed on the grain boundaries of all doped wires. High concentrations of the dopant element potassium were present on the fracture surfaces of doped wires and experimental evidence was obtained which suggests they may be due to postfracture surface diffusion. Doped wires demonstrated increasing amounts of structure coarsening up to 1500°C whereas large equiaxed grains were formed in undoped wires annealed at 1300 and 1500°C. The longitudinal fracture resistance of undoped wire was unaltered by annealing at 1050°C and below, but decreased dramatically after annealing at 1300 and 1500°C. In contrast the fracture resistance of doped wire decreased after annealing at 1050 and 1300°C, but increased after annealing at 1500°C. Fracture resistance is discussed in terms of microstructure and fracture surface chemistry. A. W. FUNKENBUSCH, formerly Research Metallurgist with General Electric Refractory Metals Product Department     

Density changes accompanying the annealing of wrought thoriated tungsten wire

Density measurements were made on 0.762 mm W-l wt pct ThO₂ wires which had been annealed at temperatures between 1000°C and 2000°C for times up to 120 min. Selected specimens were examined using optical and scanning electron microscopy. The results of this study indicate that the low, as-worked wire density is a result of large cracks associated with the ThO₂ particles. Time-temperature annealing regimes, significantly below those necessary for recrystallization of the wire, result in partial healing of the cracks. This crack healing process can be followed qualitatively by observing longitudinal fracture surfaces in the scanning electron microscope and quantitatively by changes in the density of the wire. The apparent activation energy for the crack healing process was found to be 57.8 kcal/mole. The results suggest that crack closure is a result of both shear and grain boundary diffusion transport processes. It is concluded that the presence of cracks can influence the morphology of the recrystallized grains.