  We characterise the influence of the segregation behaviours of two typical
alloying elements, aluminium and nickel, on the interfacial cohesive properties
of copper-rich precipitates in ferritic steels, with a view towards
understanding steel embrittlement. The first-principles method is used to
compute the energetic and bonding properties of aluminium and nickel at the
interfaces of the precipitates and corresponding fracture surfaces. Our results
show the segregation of aluminium and nickel at interfaces of precipitates are
both energetically favourable. We find that the interfacial cohesion of copper
precipitates is enhanced by aluminium segregation but reduced by nickel
segregation. Opposite roles can be attributed to the different symmetrical
features of the valence states for aluminium and nickel. The nickel-induced
interfacial embrittlement of copper-rich precipitates increase the
ductile-brittle transition temperature (DBTT) of ferritic steels and provides
an explanation of many experimental phenomena, such as the fact that the shifts
of DBTT of reactor pressure vessel steels depend the copper and nickel content.
