Activated Recrystallization of Non-Sag Tungsten Wire
The objective of the present investigation was to explore the mechanism of activated recrystallization of heavily drawn doped (non-sag) tungsten wire material. The recrystallization can be induced at low temperatures by the presence of some metals, such as palladium, nickel, iron and others, on the wire surface, which is important as it can seriously deteriorate the mechanical properties of non-sag tungsten wires, when the wires serve as reinforcement material in metal matrix composites. This investigation, which included the recrystallization nucleation mechanism of doped tungsten wire, the short-circuit paths for activator diffusion into tungsten wire, the activated recrys- tallization mechanism of non-sag tungsten wire, and the precipitation of activators in recrystallized tungsten wire, was carried out using atom probe field ion microscopy, optical metallography and electron microscopy.
TEM observations about the initial stage of recrystallization of 0.18 mm doped tungsten wire showed that the formation, through dislocation rearrangement, of new grains with a relative misorientation could play a critical role in the recrystallization process. The impeding effect of potassium bubbles both on dislocation rearrangement and boundary migration resulted in the abnormal recrystallization behaviour of the heavily drawn doped tungsten wire. In addition, it was found that the non-<110> orientation of recrystallized tungsten grains could originate either from the grains with a non-<110> orientation in the starting material or from the newly nucleated grains. These observations could explain the recrystallization features of tungsten wire which the current theory could not account for.
The activated recrystallization was studied both for nickel plated and palladium plated 0.18 mm cold drawn doped tungsten wire. A recrystallized structure formed in both types of wires after annealing at 1100°C for 100 hours, but their structural features were different. The recrystallization behaviour caused by palladium presence was very similar to that of undoped tungsten wire, whereas the behaviour of nickel plated wire was similar to the general recrystallization of doped wire, except for the much lower recrystallization temperature. This indicated that palladium was more effective as an activator than nickel was.
It was shown, using APFIM analysis, that grain boundaries and lattice dislocations could act as fast paths for diffusion of activators in tungsten wire during annealing. The presence of the large potassium bubbles and the segregation of activators on potassium bubbles indicated that the effect of activators was to neutralize the impeding influence of potassium bubbles on dislocation movement and boundary migration.
Two types of nickel rich precipitates were found. One was a Ni(W) solid solution, which formed on the bubbles at grain boundaries, and another type, with Ni4W structure, was found on the potassium bubbles connected with dislocations. Palladium rich precipitates were present only in triple junctions. Their structure could not be established.