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Precipitation hardening is one of the most prevailing methods to strengthen alloy steels.The nature of Cu precipitation reaction is of fundamental scientific interest and technologically important.The nanometer sized Cu-rich precipitates and carbides in quench - tempered HSLA steels are studied at atomic scale by atom probe tomography ( APT).The results indicate that Cu-rich precipitates greatly correlate with carbides in aspect of distributional characters,i.e.,the two phases are prone to co-precipitate on the dislocations and interfaces ( low angle boundaries of martensite laths).These crystallographic defects have a significant effect on the sizes,morphology and composition of Cu-rich precipitates with Ni and Mn segregation shell.The Ni-rich clusters but Ni- Mn clusters are detected,which act as preferential nucleation sites for Cu-rich precipitates.Besides,the Ni segregation at the Cu-rich precipitate /matrix heterophase interface contribute to fast growth of the Cu-rich precipitates.This can be attribute to high diffusion rate of Cu and Ni at these sites.It is concluded that Ni additions facilitate the Cu precipitation and indirectly contribute to strength of HSLA steels.