Studies on the retarded recrystallization of tungsten in CIMPLE-PSI exposed under very-high target temperature and long He⁺-fluence

Abstract

Experiments are carried out in CIMPLE-PSI, to understand the recrystallization behavior of tungsten (W) exposed under very-high target temperature and ITER relevant long He+-fluence. The effect of helium bubbles on possible retardation of the recrystallization process is also studied. W samples are simultaneously exposed under He plasma and annealed by the plasma heat-load, in contrast to previously reported experiments in literature, which were carried out sequentially. Exposed samples are characterized by field emission scanning electron microscopy (FESEM), Vickers surface micro-hardness, nano-hardness and electron backscattered diffraction (EBSD). It is observed that the sample exposed to plasma under the highest temperature (1866 K) suffered acute retarded grain growth. This also contained small, unrecovered grains on the exposed surface. FESEM imaging of the cross-sections confirm relatively smaller helium bubbles still form even at the very high temperature conditions, which can impede the grain growth locally, whenever they are forming right on the grain boundaries. This results in an inhomogeneous mixture of surface grains with sizes from few micrometer to few tens of micrometer. EBSD estimates that the plasma exposed surface was only 34% recrystallized. The second sample exposed at a lower temperature (1699 K) but for three times higher fluence (ion fluence: 1.19×1027 m-2) was almost fully recrystallized, which shows retardation diminishes very fast with the duration of the exposure. Hardness measurements were undertaken to understand the variation with plasma exposure/annealing temperature and the extent of recrystallization, with three different probing length scales, spanning from few hundreds of nanometer to several micrometers. Both helium plasma exposed W samples are observed to undergo retarded softening up to a depth of few hundred nanometers from the surface, compared to when the metal may be recrystallized by simple heating, without any plasma exposure.