Study On Microstructure And Oxide Nanoparticles In ODS Steel

In recent years,the next-generation nuclear fission technology and fusion reactor has been vigorously promoted by many countries due to the increasing demand for green energy.To deal with exceptionally harsh environment in the core of nuclear reactor,oxide dispersion strengthened(ODS)steel has been considered as one of the most promising candidate for cladding materials owing to the excellent high-temperature properties and irradiation resistance.In this paper,ODS steels with good microstructure and mechanical properties were successfully fabricated by combining wet-milling and spark plasma sintering(SPS)methods,offering a practical basis for large-scale production of ODS steel by wet-milling.By altering the alloy compositions of ODS steels,the effect of the addition of Al elements and rare-earth metal oxides La₂O₃and CeO₂on the microstructure,oxide nanoparticles and mechanical properties have been systematically studied by means of EBSD,STEM,and XPS techniques,and the mechanism behind the relationship between microstructure and properties was investigated.(1)Al element addition can effectively refine the second-phase particles and increase the particle number density in ODS steels prepared by wet-milling.Furthermore,the increased Al content can also promote the particles refinement.The presence of Al element can preferentially react with O to form fine-sized Al-O particles,avoiding the formation of coarse Cr-O and Cr-Ti-O particles,and thus optimizing the size distribution of the second-phase particles.With the refinement of particles,the increased Al content can largely reduce the matrix grain size,inhibit the bimodal grain size distribution and consequently homogenize the grain size.Based on the dispersion and grain refinement strengthening mechanism,the tensile strength was significantly increased.In the 15Cr3Al-ODS steel,the tensile strength and elongation is 775.3MPa and 15.1%,respectively.(2)Oxide nanoparticles with the size ranging from 5nm to 50nm were observed in TEM images of 15Cr1Al-ODS steel.The observed Y-Ti-O and Al-O particles are identified as pyrochlore Y₂Ti₂O₇and hexagonal Al₂O₃respectively,the space groups belong to Fd-3m and R-3C,respectively.(3)Al addition can significantly improve the corrosion resistance of ODS steels.An Al₂O₃-rich layer is formed during the electrochemical corrosion process,which prevents the inner alloy matrix from being corroded by HNO₃solution and reduces the passive current(i_pass).The corrosion resistance can be further improved as the Al content is increased.(4)By introducing TiFe₂hard phase into ODS steels,the average grain size of matrix can be refined,and the tensile strength as well as microhardness are improved effectively.Oxide particles composed of La,Ti and Ce,Ti elements are observed in the La₂O₃and CeO₂added ODS steels,respectively,and these La-Ti-O and Ce-Ti-O particles can be identified as orthorhombic LaTiO₃and CeTiO₃structure with the same space group Pbnm.