Identification of tantalum precipitates in CLAM steel

Abstract Tantalum is an essential alloying element in reduced-activation ferritic/martensitic (RAFM) steels being a promising candidate structural material for fusion reactor. Tantalum has an important influence on the microstructure and mechanical properties of RAFM steels by forming tantalum precipitates. The tantalum precipitates in CLAM steel, a type of RAFM steels, normalized for 30 min at 980 °C and tempered for 90 min at 760 °C were investigated using transmission electron microscope. In addition to tantalum-rich M(C,N) carbonitrides and MC carbides with a face centered cubic crystal structure and a typical metallic element composition of about 80Ta, 5-10Cr, 2-5Fe, and 4-5V in atomic pct, tantalum-rich M3(C,N)2 carbonitrides and M3C2 carbides, which are not known to have been reported previously in steels including RAFM steels, were also identified in the steel. Tantalum-rich M3(C,N)2 and M3C2 precipitates have a typical metallic element composition of about 55-60Ta, 25-30Cr, 5-15Fe, and 1-2V in atomic pct, and a spherical morphology, as well as small sizes ranging from 15 to 40 nm in diameter. The experimentally observed tantalum-rich M3(C,N)2 and M3C2 precipitates exhibit a relatively small size, and appear to have a smaller amount compared with the tantalum-rich M(C,N) and MC precipitates observed. The effect of identified precipitate phases on the creep properties of the steel was also discussed.