Investigating bubble formation in dispersion-strengthened tungsten alloys using in-situ TEM irradiation

The divertor region of tokamak reactors experience an extremely harsh environment, including the intense irradiation flux, the very high heat flux and the high temperature. The leading material of choice is tungsten because of its very high melting point. However, pure tungsten itself accumulates damage quickly from particle irradiation, so new tungsten alloys need to be developed to withstand such an environment. Dispersion-strengthened tungsten (DS-W) is a class of tungsten alloys strengthened by transition metal carbides such as TiC, ZrC, and TaC. Initial studies have found that DS-W alloys show better long-term use in highly damaging environments as they possess increased recrystallization temperature and improved ductility. Helium (He) bubble formation is a serious threat to materials within the divertor region. Here we investigated bubble formation in DS-W using in-situ He irradiation in a transmission electron microscope (TEM). The W-TaC sample was irradiated at 950°C with a He flux of 3*10¹² cm ^-2 s ^-1. Our analysis showed He bubble formation was suppressed by the carbides within the DS-W alloys. During the irradiation, the bubble area number density began at 226 /µm² at 994 s into irradiation and gradually increased to a maximum density of 689 /µm² at 2117 s, and then falling to 544 /µm² at 2329 s into the irradiation. This in-situ TEM videos showed He bubbles preferred to nucleate and grow along the grain boundaries in the W matrix, and bubbles did not appear to form within the TaC dispersoids or at the dispersoid-matrix interfaces.