Interfacial segregation and precipitation behavior of Cu-rich precipitates in Cu-bearing 316LN stainless steel after aging at different temperatures

Interfacial segregation and precipitation behavior of Cu-rich precipitates in a Cu-bearing 316LN austenitic stainless steel (316LN-Cu SS) aged at 600, 700 and 800 °C were systematically investigated. The variations in morphology, composition of Cu-rich precipitates and interfacial segregation were characterized by transmission electron microscopy (TEM), atom probe tomography (APT) and Gibbsian interfacial excess. Experimental results revealed the Cu-rich precipitates led to increase in hardness and strength of steel. The 316LN-Cu SS aged at 700 °C for 6 h possessed the optimal strength and elongation due to the proper size, number density and volume fraction of the Cu-rich precipitates in the steel matrix. The average size of Cu-rich precipitates significantly increased from 0.85 ± 0.14 nm to 5.21 ± 1.51 nm with increasing the aging temperature, while the coherent relationship between Cu-rich precipitates and γ-Fe matrix remained unaltered even for large size Cu-rich precipitate. The APT analysis confirmed that the composition of Cu-rich precipitate was enriched in Cu atoms and depleted of other elements with increasing aging temperature, while their core composition became nearly pure Cu at high aging temperatures. In addition, due to strong bonding energy, Ni was significantly segregated at the γ-Fe matrix/precipitate heterophase interface, with Gibbsian interfacial excesses related to Fe and Cu of 0.6036 ± 0.52, 2.892 ± 0.55 and 2.908 ± 0.38 atoms nm-2 after aging at 600, 700 and 800 °C, respectively. This interface segregation further caused the reductions in interfacial free energies up to 7.28, 38.9 and 43.1 mJ m-2, respectively.