Effect of Y₂Ti₂O₇ nanoparticles on inclusions in CLAM steel

As the preferred material for the first wall of fusion reactors, China’s low-activation martensitic (CLAM) steel has several advantages; however, its high-temperature (>550 ℃) strength is not enough, and the helium produced by fusion can easily form a thick helium bubble and gather at the boundary, which leads to helium embrittlement; thus, the low-activation ferrite/martensite steel cannot effectively function in the fusion reactor working environment. Previous studies have shown that adding nano-sized oxide strengthening phase into CLAM steel can significantly improve the high-temperature strength and irradiation resistance of the steel, and Y₂O₃, Al₂O₃, or ThO₂ are commonly used as strengthening phases. Moreover, it has been found that adding Ti will result in a better strengthening effect. In this study, CLAM steel with the addition of Y₂Ti₂O₇ nanoparticles was fabricated using a vacuum induction furnace. Afterward, the effect of Y₂Ti₂O₇ nanoparticles on inclusions in CLAM steel was investigated via scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and a universal testing machine experiment; then, the mechanical properties of CLAM steel were analyzed. The results show that Y₂Ti₂O₇+Fe nanoparticles are successfully added to CLAM steel. The inclusion size of CLAM steel is 0.5−1.5 μm. The inclusion morphology is near-spherical, and the inclusion composition is Y–Ti–O–Mn–C–Ta–W–V–Cr–Fe; thus, the inclusion is characterized as a compound inclusion, mainly because Ta and V are strong carbide-forming elements and some Y₂Ti₂O₇ particles may agglomerate. When the Y₂Ti₂O₇ content is 0.5%, the inclusions in the steel modify into composite inclusions of rare-earth oxides, and the steel strength is 1356 MPa, while the elongation and section shrinkage are 13.44% and 63.15%, respectively. Moreover, second-phase particles also exist in the fracture dimples. The particles are spherical, less than 1 μm and have a complex composition, mainly Y–Ti–O–C–Ta–W phase.