The D022 Precipitation Strengthening Behavior of Cocrfe0.25ni1.75nb0.2 High Entropy Alloy by Laser Additive Manufactured

The effect of coherent D022 nanoprecipitates on the tensile behavior of the Nb bearing CoCrFe0.25Ni1.75 based high entropy alloy (HEA) was investigated in this study. The thin-walled bulk samples without defects were fabricated by laser additive manufacturing (LAM). The Ni3Nb phase was in situ formed in the process of lase-directed energy deposition (LDED). The samples were homogenized treatment at 1200℃ for 2 h and aging temperature at 650℃ for 24 hrs and 48 hrs. The microstructure transforms from as-deposited dendrites to the heat-treated large-sized equiaxed crystal. The nanoscale olive-shaped D022 precipitates with a volume fraction of 12.8% are distributed in the FCC matrix, and significantly improve the tensile strength of the HEA (i.e., σYS ~ 976.7 MPa, σUTS ~ 1133.1 MPa, εTE ~7.2 %). The strength improvement of the aged HEA is attributed to the contribution of four hardening mechanisms. Although the appearance of the D022 precipitates improved the strength substantially, with the extension of aging time, the reduction of plasticity was induced by the size growth of the precipitates. The above research provides a reference for laser additive manufacturing of HEAs.