Effect of Annealing Temperature on Pitting Resistance of CoCrFeMnNi High-entropy Alloy Fabricated by Laser Additive Manufacturing

Equiatomic CoCrFeMnNi high-entropy alloy (HEA) has good corrosion resistance and radiation resistance, and possesses high strength and excellent toughness at cryogenic temperature, which makes it a potential application material for advanced nuclear reactors. As one of the most widely applied additive manufacturing (AM) technologies, selective laser melting (SLM) provides a new idea for the green preparation of HEA, reducing the production cycle and the manufacturing cost. However, the corrosion resistance of the CoCrFeMnNi HEA prepared by laser additive manufacturing still lacks systematic study. The work aims to investigate the effect laws of microstructure evolution on the pitting resistance of the SLMed CoCrFeMnNi high-entropy alloy in NaCl solution after annealing at various temperature. Spherical CoCrFeMnNi HEA powder with an average diameter of 26.0 μm fabricated by gas-atomized was used in this study. The high-density CoCrFeMnNi HEA with a relative density of 99.4 % was fabricated by a SLM machine (Hanbang HBD-150D) under pure argon gas protection. The manufacturing process was set as: checkerboard scanning strategy, laser power of 180 W, scanning speed of 670 mm/s, hatching space of 70 μm and layer thickness of 50 μm. Then, the SLMed specimens were subject to annealing for 4 hours at 700 ℃, 900 ℃ and 1 100 ℃ respectively, followed by cooling in a furnace in an argon atmosphere. The phase of the HEA before and after annealing was analyzed by X-ray diffraction (XRD, Japanese Rigaku D/max2500pc). The microstructure was examined by optical microscopy (OM, Zeiss Axiovert 40MAT) and scanning electron microscopy (SEM, Tescan MIRA3 XMU). The pitting resistance of the SLMed HEA in 3.5 wt.% NaCl solution was analyzed by kinetic potential polarization and electrochemical impedance spectroscopy (EIS). The composition of the passivation film of the HEA formed in the NaCl solution was investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The phase of the CoCrFeMnNi HEA before and after annealing was single-phase face-centered cubic, which indicated that the HEA had good phase stability at elevated temperature. However, the microstructure of the HEAs had undergone a significant transformation after annealing. The microstructure of the SLMed HEA consisted of melt pools, columnar crystals, and cellular subgrains due to the rapid heating and cooling in the manufacturing process. As the annealing temperature increased, the melt pool boundary and subgrain structure gradually disappeared and the grains gradually grew. Annealing treatment significantly affected the pitting resistance of the HEAs. With the increase of annealing temperature from 700 ℃ to 1 100 ℃, the corrosion current density of the HEAs decreased firstly and then increased. The corrosion current density of the sample annealed at 700 ℃ decreased by 97 % compared with that of the as-built sample. The HEA passive film mainly consisted of metal oxides and hydroxides. The cation ratios of the Co+Cr+Ni to Mn+Fe in the passivation film of the samples prepared by SLM and annealed at 700 ℃ were 1.38 and 1.61, respectively, and the thickness of the Cr intrinsic oxide layer in the passivation film was 5.43 nm and 5.75 nm, respectively. The sample annealed at 700 ℃ has the best pitting resistance. With the increase of annealing temperature, the pitting resistance of high-entropy alloy firstly increases and then decreases. Cellular subgrain is beneficial to hindering the expansion of pitting and promoting the formation of stable passivation film. The annealing at 700 °℃ provides the best pitting resistance by removing part of the melt pool boundary while maintaining the cellular subgrains, preventing further pitting pit expansion and improving the protection ability of passivation film. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.