Microstructural evolution in single tungsten fiber-reinforced tungsten composites during annealing: recrystallization and abnormal grain growth

Tungsten is considered as plasma-facing material in future fusion reactors. Due to the high operation temperatures, the occurrence of microstructural restoration processes as recovery, recrystallization and grain growth will unavoidably alter the desired microstructure obtained after processing and impede otherwise beneficial mechanical properties.

Potassium-doping proved to be efficient in hindering or at least delaying such phenomena. Tungsten fiber-reinforced tungsten composites with drawn tungsten wires embedded in a tungsten matrix show a certain pseudo-ductility. Cylindrical single fiber composites with a single potassium-doped drawn tungsten wire in a chemically vapor deposited tungsten matrix, with or without a rare-earth oxide interlayer, were investigated as model systems.

Individual specimens were annealed at 1400°C up to four weeks and changes in their microstructure tracked by electron backscatter diffraction. In the as-processed condition, the tungsten matrix showed large wedge-shaped grains stretching radially from either the interface between matrix and wire or the oxide interlayer to the outer surface with very small grains in the vicinity of the interface/interlayer.

Upon heating, two zones with larger grains compared to the as-processed condition developed: in the wire regions close to its perimeter primary recrystallization led to formation of new grains with orientations deviating slightly in alignment of their crystallographic directions with the wire axis compared to the drawn wire.

In the matrix, abnormally grown grains consumed the as-deposited microstructure in the vicinity of the interface/interlayer. Without an interlayer, abnormally growing grains from the matrix invaded the recrystallizing wire progressively consuming the wire. Both erbia and yttria interlayers efficiently prevented abnormally growing grains from invading the wire (except at few occasions where the oxide interlayer presented imperfections).

In this manner, the presence of an interlayer becomes crucial for retaining an interface essential for the pseudo-ductility of the composite.