Application of TEM to Study the Changes in Subsurface Defects in Tungsten Samples as a Function of Annealing Temperature

Full text: In a nuclear fusion reactor, hot and dense DT plasma is confined using a combination of magnetic fields in a toroidal shaped vacuum vessel. Interaction of this plasma with the vacuum vessel wall materials is one of the very important areas of interest as plasma-wall interactions will decide the operational lifetime of the reactor in terms of plasma as well as material stability. The choice of the wall-material hence becomes an important factor and high atomic number materials such as tungsten,W, and its alloys are currently identified as candidate materials due to their relatively low H-isotope affinity. However, high energy neutrons and particles produced in the fusion reaction can introduce subsurface defects in tungsten, which may lead to H-isotope trapping through these defective sites. In order to understand the effect of these defects, it is critical first to identify them. The defects concerned here, such as dislocations, are like bulk features of the materials and cannot be identified using surface characterization equipment such as scanning electron microscope (SEM). Transmission electron microscope (TEM) is one of the very few instruments which can identify these meso-scale subsurface defects. In the work discussed here, we had used a 300 kV TEM to identify these defects in W samples. TEM microscopy of the as received W samples (cold rolled) was carried out. Grains were observed to be elongated and the dislocation density is very high. Later the W samples were subjected to annealing at various temperatures ranging from 773 K to 1838 K. The annealing was carried in a vacuum furnace under a reducing atmosphere of Ar and H₂ mixture. A base pressure of 10^-5 mbar was obtained before the Ar-H₂ mixture was introduced. The effect of annealing temperature on the changes in defect distribution and restructuring was studied using TEM. Defect density is observed to reduce with increase in annealing temperature (below recrystallization temperature), though there is not much change in grain size. However, above recrystallization temperature, the grain size was observed to change from elongated to regular shape while the defect density was reduced. Present work also explains in detail about the sample preparation procedure adopted for preparing the W samples for TEM analysis. (author)