Influence of Nitrogen on Microstructure, Mechanical Properties and Martensitic Phase Transformation of Co–26Cr–5Mo–5W Alloys by Selective Laser Melting

Abstract

The relationship between the microstructure, mechanical properties and martensitic phase transformation of N-containing Co–26Cr–5Mo–5W alloys by selective laser melting (SLM) is studied. The high-resolution transmission electron microscope and X-ray diffraction observations show that two phases (e and γ phase) co-exist in N-free alloys. In contrast, a significant decrease of e phase is observed in N-containing alloys. It is believed that the stabilization of γ phase in N-containing alloys results from the lattice distortion and Si-rich fine-distributed precipitates that block the motion of dislocations. Both the ultimate tensile strength and 0.2% proof strength can be significantly improved by nitrogen addition. Interestingly, the elongation slightly increases as well. They are ~1385 MPa, ~1140 MPa, and ~18.4% for 0.08 N-containing Co–26Cr–5Mo–5W alloys, respectively. It is clear that nitrogen addition during the SLM processing could be a promising strategy to fabricate Co–Cr–Mo–W alloys with an excellent combination of strength and ductility by suppressing face-centered cubic (fcc) → hexagonal close-packed (hcp) martensitic phase transformation.