Interfacial segregation at Cu-rich precipitates in a high-strength low-carbon steel studied on a sub-nanometer scale

The local composition of small, coherent Cu-rich precipitates with a metastable body-centered cubic structure in a ferritic (alpha)-Fe matrix of a high-strength lowcarbon steel was studied by conventional atom-probe tomography. The average diameter, , of the precipitates is 2.5 (plus-minus) 0.3 nm at a number density of (1.1 (plus-minus) 0.3) * 10^24 m^-3 after direct aging at 490 °C for 100 min to a near-peak hardness condition, yielding a value of 84 Rockwell G. Besides Cu, the precipitates contain 33 (plus-minus) 1 at.% Fe and are enriched in Al (0.5 (plus-minus) 0.1 at.%). Nickel and Mn are significantly segregated at the (alpha)-Fe matrix/precipitate heterophase interfaces. The Gibbsian interfacial excesses relative to Fe and Cu are 1.5 (plus-minus) 0.4 atoms nm^-2 for Ni and 1.0 (plus-minus) 0.3 atoms nm^-2 for Mn. The reduction of the interfacial free energy, calculated utilizing the Gibbs adsorption isotherm, is 16 mJ m^-2 for Ni and 11 mJ m^-2 for Mn.