Microstructure evolution of fine-grained tungsten coated layer under high temperature for nuclear thermal propulsion

Abstract

Nuclear thermal propulsion (NTP) is a promising candidate for deep space exploration, with cermet fuel offering safety and efficiency due to advantages like fission product retention, hydrogen compatibility, and high strength. The W coated layers of cermet fuel help minimize fuel loss but must withstand temperatures above 2500 K during operation. This study investigates the microstructure and property evolution of W coated layers at high temperatures. Fine-grained W coated layers were prepared using WCl₆ as a precursor, followed by high temperature tests between 1500 and 2300°C. The effects of temperature on phase composition, microstructure, and mechanical properties were analyzed. The (110) crystal plane remained the preferred orientation throughout testing. The grain structure evolved from fine columnar grains to equiaxed grains, and then to large columnar grains, with rapid growth from sub-micron to approximately 20 microns. At elevated temperatures, bubbles formed inside the grains and at the grain boundaries. As the temperature increased, bubble numbers decreased, sizes grew, and bubbles migrated toward the boundaries. At 1900°C, bubbles only existed at grain boundaries, and by 2300°C, bubbles disappeared. As the temperature increased, the relative chlorine (Cl) content and average dislocation density in the W coated layers decreased. The study proposes a mechanism for bubble formation and migration, where Cl impurities in the W coated layer diffuse into defects, forming submicron bubbles. The radial growth of columnar grains causes stress transitions from compressive in the interior to tensile at the exterior, promoting bubble diffusion along grain boundaries toward the surface.