Helium bubbles retard recrystallization in tungsten by limiting subgrain growth

• We reveal He bubbles retarded recrystallization in tungsten at 1573 K using quasi-in-situ EBSD analysis. • We demonstrate He bubbles limited subgrain growth as a viable mechanism of retarding recrystallization. The tungsten plasma facing components (PFCs) in fusion devices must withstand high heat loads. Recrystallization sets in under such conditions and embrittles tungsten, hence motivating its mitigation. He plasma exposure, a natural consequence of the deuterium–tritium reaction, has been shown to retard recrystallization, the mechanism of which was generally attributed to the Zener pinning of grain boundaries during the grain growth stage of recrystallization. However, neither the Zener pinning effect was quantified nor its role in the nucleation stage of recrystallization was considered, limiting our understanding and exploitation of this beneficial phenomenon. Here, we approach this problem by splitting the conventional annealing experiment into finer steps using a powerful electron beam at 1573 K. After each 15 min annealing of the He plasma exposed tungsten specimen, the same area was examined by electron backscatter diffraction (EBSD) to detect recrystallized grains and transmission electron microscopy (TEM) to observe subsurface He bubbles. We found that near-surface recrystallization was retarded at 1573 K. Surprisingly, site-specific TEM imaging revealed that recrystallization was retarded even when the grain boundaries of the recrystallized grains were not pinned by He bubbles, which directly contradicts previous assumptions. Additionally, the pressure balance on the grain boundaries suggested that grain growth was unfavorable, even in the absence of the Zener pinning pressure. Instead, recrystallization and its suppression were dominated by nucleation, as supported by EBSD analysis of the nucleation rate. A model based on He bubbles limited subgrain growth was developed and quantitatively agreed with the recrystallization fraction trend at 1573 K. Overall, we demonstrate He bubbles limited subgrain growth as a viable mechanism of retarding recrystallization. Moreover, it is a promising model for identifying a regime of retarded recrystallization for tungsten PFCs in fusion devices.