Splitting of tungsten wire in the knife-edge compression test
- PDF / 4,244,159 Bytes
- 10 Pages / 594 x 774 pts Page_size
- 75 Downloads / 181 Views
I.
INTRODUCTION
II.
IN the processing of tungsten from ingot to wire, splitting and breaking of the wire may occur during wire drawing. Splits may remain hidden until the wire is bent, coiled into the form of a filament, or stretched.t Breaking, of course, requires restarting the wire-drawing operation. Splitting may be induced by bending or by a compressive force exerted at the periphery of the wire. Since splitting is detrimental to the wire drawing and coiling operations, there is a need for a test to determine the split propensity of tungsten wire. One such test may be the knife-edge compression test which can cause splits to occur in tungsten wire. 2 Understanding the cause of splitting in this test may lead to an understanding of the factors that cause tungsten wire to split during wire drawing or bending and coiling. Since splitting is at least partly intergranular, it may be related to the microstructure of the wire or to grain boundary contamination or both. The microstructure is determined by the detailed processing steps in converting the pressed and sintered ingot into wire. Impurities which may affect grain boundary strength may be those deliberately added (aluminum, silicon, and potassium) or they may be incidental impurities, including the interstitial impurities oxygen, nitrogen, and carbon. In this study, we have examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy, the microstructures of tungsten wires of different diameter, composition, and processing history. In addition, scanning Auger electron spectroscopy has been employed to determine the composition of freshly fractured surfaces to find a possible relationship between grain boundary composition and the propensity for splitting. The information obtained was related to splitting propensity defined as the mechanical force required to cause a split to form in the wire samples in the knife-edge compression test. It was observed that there was a direct correlation between the resistance of the wire to splitting and a particular feature of the microstructure, namely, the number of transverse grain or subgrain boundaries. The results indicate that impurities are not a major factor in splitting. J.L. WALTER and C . L . BRIANT, Metallurgists, and E. E KOCH, Associate Staff, are all with General Electric Company, Corporate Research and Development, P. O. Box 8, Schenectady, NY 12301. Manuscript submitted August 17, 1981. METALLURGICALTRANSACTIONS A
EXPERIMENTAL PROCEDURE
The tungsten wires used in this study were supplied by the Refractory Metals Department of the General Electric Company, Cleveland, Ohio. All of the wires were processed from standard ingots doped with potassium, aluminum, and silicon to control the microstructure of the filaments; this doping provides the desired large, elongated, interlocking grains which prevent lamp filament failure by grain boundary offset. 3,4 The split resistance of the wires in a knife-edge compression test was measured. 2 In this test, two knife edges with ra
Data Loading...
