Retarded recrystallization of helium-exposed tungsten

In ITER and future tokamaks, recrystallization has been identified as an important issue which may reduce the strength of tungsten plasma-facing component and deteriorate its thermal shock resistance. In this study, isothermal annealing of un-exposed and helium-exposed tungsten was performed to investigate the effect of helium plasma exposure on the recrystallization kinetics. Rolled tungsten samples with a helium plasma fluence of 1.4 x 10(26) m(-2) were annealed at temperatures ranging from 1273 K to 1973 K for 1 h. It was found that helium plasma exposure influence both the recrystallization stage (for 1423 K < T < 1573 K) and the grain growth stage (T > 1523 K). The results suggested that the retarding effect is caused by the impediment of high-angle grain boundaries migration by helium clusters and bubbles. Retarded recrystallization was observed at a depth up to a few micrometers beneath the surface. Present results demonstrate that helium plasma exposure plays an important role when qualifying the tungsten divertor performance under heat loading conditions.