Microstructural evolution and transformation mechanisms of W and W-Y2O3 matrices under ultra-high temperature for nuclear thermal propulsion

Tungsten (W)-based cermet fuels have garnered significant interest in nuclear thermal propulsion (NTP) systems due to their high melting point, thermal properties, and compatibility with hydrogen (H 2 ) propellants. The W–Y 2 O 3 composite matrix prepared by spark plasma sintering (SPS) exhibits greater densification and suppressed grain growth compared to pure W. Since NTP systems operate at temperatures above 2300 °C, the microstructural evolution under elevated temperatures is crucial to fuel performance. This study investigates the high temperature effects on the microstructure of pure W and W–Y 2 O 3 matrices fabricated by SPS. The matrices were subjected to temperatures ranging from 1800 °C to 2300 °C. Below 2300 °C, Y 2 O 3 particles effectively inhibited W grain growth through the Zener pinning effect, thereby increasing the proportion of low-angle grain boundaries (LAGBs). At high temperatures, H₂ diffuses into the matrix, reacting with uranium dioxide (UO 2 ), which contributes to fuel loss. The fine grains increase grain boundary density, extending H₂ diffusion paths, while LAGBs help mitigate harmful H₂ accumulation. However, at 2300 °C, W diffuses into Y 2 O 3 , causing the Y 2 O 3 grains to grow and leading to the formation of pores and cracks. This weakens the Zener pinning effect, promoting abnormal grain growth of W, which ultimately results in an increase in high-angle grain boundaries (HAGBs) and accelerates the diffusion of H 2 . The study identifies three stages of W diffusion into Y 2 O 3 : initial enrichment at Y 2 O 3 grain boundaries, formation of W-depleted/W-enriched island-like structures within Y 2 O 3 grains, and the formation of a Y 6 WO 12 core-Y 2 O 3 shell structure at 2300 °C, providing valuable insights for optimizing Y 2 O 3 composites for NTP applications.