On novel Ni-rich phase formed during aging of an additively manufactured ultra-high strength CoFeNi alloy

The precipitation mechanism of strengthening phase of a body-centered-cubic (BCC) structured CoFeNi multi-principal element alloy produced by additive manufacturing was studied by transmission electron microscopy (TEM) and atom probe tomography (APT). The hardness of CoFeNi alloy achieved the peak value of 554.79 ± 9.86 HV after aging for 160 h at 400 °C. The CoFeNi alloy exhibited an ultrahigh compressive yield strength and ultimate compressive strength up to 1.8 GPa and 2.3 GPa respectively, whilst maintains a strain of 18.9 %. The microstructure of the alloy exhibited nanoscale lamellar Ni-rich phase with high stability homogeneously distributed in BCC matrix after aging at 400 °C. The novel Ni-rich phase containing about 50 at. % Ni with a hexagonal structure is the mainly precipitation strengthening phase, and has coherent orientation relationship with the BCC matrix in the bcc∥Niand 111bcc∥{0001}Ni. Based on the experimental results, the precipitation mechanisms of the Ni-rich phase were discussed in detail. The current work provides a possible new direction to design high-strength alloy by introducing the novel Ni-rich phase. •An ultra-high strength CoFeNi alloy was prepared by additively manufacturing.•Novel Ni-rich phase precipitated in the alloy without additional alloying element.•The structure and composition of the Ni-rich phase are stable during aging treatment.