Doped Tungsten With Annealing Behavior Research

The influences of different annealing conditions on recovery and recrystallization of doped tungsten wire were studied. The influencing factor contains annealing atmosphere, temperature and time. Microstructural evolution, which taken place during recovering, primary recrystallization and secondary recrystallization, and the influence of doped K on these changes, were observed. According to those observations, the mechanism of primary recrystallization and the mode of secondary recrystallization were presented. Hardness bend performance changes during annealing were assessed. It followed that:(1) if Fe element exist in the annealing atmosphere, it will make the doped tungsten wire recrystallization at800℃. If the annealing atmosphere contains C, tungsten wire get brittle after annealing at1200℃. Increasing the O content in annealing atmosphere will decreasing C content, which can eliminate the brittleness caused by C. the N2content in atmosphere will make tungsten wire recrystallization at1500℃.(2) when annealed in vacuum, the doped tungsten showed excellent recrystallzation resistance. There were little microstructural changes when it was annealed below1200℃. TEM observation demonstrate that nothing changed except for the decreasing of dislocation density and the coarsening of subgrain. After annealed between1200℃～1600℃, doped tungsten wire recrystallised. After1700℃, secondary recrystallization happed in the surface of tungsten wire, while it remained the fibrous grains in the center of wire. After1800℃, secondary recrystallization happened overall in the tungsten wire.(3) Hardness experiment demonstrated that hardness of tungsten wire changed a little before secondary recrystallization, because before secondary recrystallization, microstructure of tungsten wire was consisted by small subgrain, which made the fine-grained strengthening. Bending experiment demonstrated that if there were no impurities existed in annealing atmosphere, primary recrystallization won’t make the tungsten wire brittle because the fibrous structure was retained after primary recrystallization. (4) the doped K existed in tungsten wire in the shaped of rod and line, which split into K bubbles during annealing. The recovery and recrystallization nucleation stage during annealing were delayed as a result of the pinning effect of K bubbles, especially annealing below the1600℃. When tungsten wire was annealed above1600℃, some vertical grain boundary can get rid of the pinning effect of K bubbles to migrate.(5) microstructure of doped tungsten wire was consisted by strip-shaped dislocation cells, which changed into strip-shaped subgrains during annealing, then subgrain coarsening happed with the aid of interfacial tension, by which the primary recrystallization nucleation happened. EBSD experiment demonstrated that subgrain rotation accompanying the coarsening process, the rotation rod was the wire axle. By rotation, high angle boundary was formed.(6) Combining the experimental results and actual condition, the stress removing criterion of Solenoid was proposed:first stage of shape fixing:1450min,25min. secondary stage of shaped fixing:1500℃,1.5h.