The relationship of microstructure to mechanical properties of Al-, Si-, K-doped tungsten wire

Tungsten wire, commercially doped with aluminium, silicon, and potassium, was obtained from six different manufacturers with a diameter of 0.0063 cm. The tensile strength at 1620 °C, a measurement of the quality of the wire used by the General Electric Company, was determined and found to be different for each wire. Additional samples of wire were annealed at 1620 °C under the same conditions employed for the tensile test. To determine the microstructure, these samples were thinned for transmission electron microscopy along with the wires in the as-drawn condition. The widths of from 75 to 125 subgrains were measured for each sample. In addition, in the annealed samples, the spacing of the strings of potassium bubbles was also determined. It was found that the 1620 °C tensile strength was related to the increase in width of the subgrains during the 1620 °C anneal and that the amount of subgrain widening was determined by the spacing of the strings of bubbles. Hence, the 1620 °C tensile strength was related to the bubble string spacing; the closer together were the strings, the higher was the 1620 °C tensile strength. It was also shown that the grain morphology of the wire, as measured by the stretch or sag of a filament during a stress test, is also determined by the spacing of the strings of potassium bubbles.