Effect of nano-Y2O3 on microstructure of laser cladding cobalt-based alloy coatings

Cobalt-based alloy coatings with and without nano-Y₂O₃ particles produced by a 5 kW CO₂ laser on Ni-based superalloy were introduced. Solidified microstructure, phase compositions and distribution of nano-particles were investigated by using optical microscope, SEM, TEM and XRD. The results showed that metastable phases, such as γ-Co and Cr₂₃C₆ existed in cobalt-based alloy coatings. Another two phases (Y₂O₃ and ?-Co) were found by adding nano-Y₂O₃. Without nano-Y₂O₃, rapid directional solidified microstructure of columnar dendrite appeared. Fine and short dendritic microstructure and columnar to equiaxed transition (CET) occurred by adding nano-Y₂O₃ particles. With the increase in amount of nano-Y₂O₃, fully equiaxed crystallization appeared and the formation mechanism was analyzed. The results also showed that the sub-microstructure of the coatings changed from dislocation to stacking fault by adding nano-Y₂O₃.