The effect of Nb on the tensile properties and thermal fatigue behavior of Stellite 21 alloy coatings

In this study, the effects of Nb addition on the microstructure, tensile properties, and thermal fatigue behavior of the Stellite 21 coatings were systematically investigated. The results reveal that the microstructure of the Stellite 21 coatings primarily consists of columnar and cellular grains, with γ-Co (fcc) as the dominant phase, accompanied by minor amounts of ε-Co (hcp), M 23 C 6 carbides, and M 7 C 3 carbides. The addition of Nb leads to its solid solution into the γ-Co matrix, while the precipitation of ε-Co and NbCr 2 increases with higher Nb content, resulting in enhanced dislocation density and solid solution strengthening. However, the introduction of Nb alters the phase composition of the coating, and the simultaneous and adjacent precipitation of NbCr 2 and ε-Co phases induces stress concentration, leading to the formation of thermal cracks during the laser cladding process. Tensile tests demonstrate that the addition of Nb improves the strength of the Stellite 21 coatings due to solid solution strengthening, but the formation of the brittle NbCr 2 phase reduces the ductility of the coatings. The NbCr 2 phase primarily precipitates within the grains, making the grain interiors more brittle. When the coatings are subjected to thermal cycling during thermal fatigue testing, the brittle NbCr 2 phase tends to act as crack initiation sites, thereby degrading the thermal fatigue performance. This study demonstrates that while the addition of Nb enhances the strength of Stellite 21 coatings, it compromises their ductility and thermal fatigue resistance.