The effect of wire history on the coarsened substructure and secondary recrystallization of doped tungsten

Abstract

This article reports a study of primary and secondary recrystallization in tungsten wire. Samples with two different processing histories were annealed in the electron emission microscope, and the recrystallization process was followed. The fibers produced by drawing were first observed to widen and break up into shorter lengths. As the temperature was raised, the secondary recrystallization occurred in a characteristic, stepwise motion, with the secondary grain moving from one position to the next and then remaining pinned at the new position, sometimes for the entire length of the test. It was found that the temperature at which secondary recrystallization occurred depended on the heating rate. If a slow heating rate was used, the temperature at which secondary recrystallization occurred would be higher. This result was interpreted to mean that the slower heating rate allowed more strain to be annealed out of the wire before secondary recrystallization occurred and thus lowered the driving force for this process. The secondary recrystallization temperature could not be correlated with the primary grain structure or differences in the potassium bubble distribution in the wire. The primary recrystallized grain structures of the two wires were also different, and this difference, too, was attributed to differences in the amount of stored energy in the wire at the start of the annealing. It was also shown that even though the bulk potassium content of the two wires was the same and the bubble distributions in the two wires were similar, the bubble distributions in the ingots were different.