Nitrogen-doping assisted local chemical heterogeneity and mechanical properties in CoCrMoW alloys manufactured via laser powder bed fusion

CoCrMoW alloys with different nitrogen (N) additions (0, 0.05, 0.1, and 0.2 ​wt%) were prepared via laser powder bed fusion (LPBF). The effects of N content on the microstructure and mechanical properties were investigated. The results indicate that the LPBFed CoCrMoW alloy with 0.1 ​wt% N addition (0.1 ​N alloy) shows the best combination of mechanical properties with a yield strength of ∼983 ​MPa and an elongation of ∼19 ​%. Both the LPBF process and the N addition impose great effects on suppressing the γ to ε martensitic transformation, resulting in a decrease in the width and amount of ε laths/stacking faults. Besides, the N addition promotes the segregation of elements Mo, W, and Si along the cellular sub-grain boundaries (CBs), forming fine and discontinuous precipitates rich in Mo, W and Si along the CBs in the 0.1 ​N alloy, but dense and continuous (Mo,W)₅Si₃ precipitates along the CBs in the 0.2 ​N alloy. The (Mo,W)₅Si₃ precipitates with a tetragonal structure were observed and characterized for the first time in the Co–Cr based alloys. The negative mixing enthalpy between the non-metallic elements N, Si and the metallic elements Mo, W, Cr, and the rapid solidification induced segregation of high melting point elements such as Mo and W along CBs during LPBF process, synergistically contribute to the chemical heterogeneity in the alloys. The pure FCC matrix, the slightly increased segregation of Mo, W, Si elements and fine precipitates along the CBs contribute to the good combination of strength and elongation of the 0.1 ​N alloy. However, though pure FCC phase was present in the 0.2 ​N alloy, the dense and continuous (Mo,W)₅Si₃ precipitates along CBs acted as nucleation sites for cracks, deteriorating the elongation of the alloy. Overall, it is possible to tune the mechanical properties of the LPBFed CoCrMoW alloy by adjusting the local chemical heterogeneity.