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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.

Using special quasirandom structures (SQS's), we perform first-principles calculations studying the metastable bcc NiCu and FeCu alloys which occur in Fe-Cu-Ni alloy steels as precipitated second phase. The mixing enthalpies, density of state, and equilibrium lattice parameters of these alloys are reported. The results show that quasi-chemical approach and vegard rule can well predict the energetic and structural properties of FeCu alloys but fail to yield that of NiCu. The reason rests with the difference of bond energy variation with composition between NiCu and FeCu alloys induced by competition between ferromagnetic and paramagnetic state. Furthermore, the calculated results show that the energetic and structural properties of these alloys can well explain the local composition of the corresponding precipitates in ferrite steels., Comment: Comput. Mater. Sci.(2011)