Characterization of ferroniobium topologically close-packed phase inclusion in nickel-based 152 cladding of dissimilar metal welds

Highlights: • The TCP-phase inclusions originate from large-size ferroniobium supplements in 152 welding electrode flux. • The TCP-phase inclusion consists of Fe₇Nb₆ (μ phase) and Fe₂Nb (Laves phase). • The average hardness of inclusion core (Fe₇Nb₆, 17.89 GPa) is over 4.5 times as high as 152 cladding matrix (3.91 GPa). • The ferroniobium TCP-phase inclusions are of higher cracking susceptibility than 152 cladding. - Abstract: Ferroniobium topologically close-packed (TCP) phase inclusions were found in the nickel-based152 nickel-based cladding of dissimilar metal weld joint in pressurized water nuclear reactor. A systematical investigation on the morphologies, microstructures and mechanical properties was conducted by using multi-scale techniques. The formation causes of the TCP-phase inclusion and its possible influences on crack initiation and propagation during long-term operation are also discussed. The ferroniobium TCP-phase inclusions originate from large-size ferroniobium supplement particles in 152 welding electrode flux during welding. The inclusion with core has a homogeneous kernel and a continuous transition zone between the core and eutectic zone, and the transition zone consists of columnar grain zone and isometric grain zone. The inclusion core is confirmed as single-phase rhombohedral Fe₇Nb₆ (μ phase), and columnar grain zone in the transition zone is also mainly Fe₇Nb₆, while the isometric grain zone and the Nb-rich phase in the eutectic zone are both mainly hexagonal Fe₂Nb (Laves phase), the other phase in the eutectic zone is austenite. The Fe₂Nb is formed by eutectic metallurgical reaction between inclusion core and 152 cladding. The average hardness of inclusion core (17.89 GPa) is over 4.5 times as high as cladding matrix (3.91 GPa). The TCP-phase inclusion is of higher cracking susceptibility than 152 cladding, and acts as the initiation of brittle cracks during in-situ tensile test. The ferroniobium TCP-phase inclusions are of higher cracking susceptibility than 152 cladding.