Effect Of Mn And Ni On Precipitation Strengthening Of Cu-rich Phase In Steel

The formation of Cu-rich nanoprecipitates in Cu-containing steels during thermal aging can provide a substantial increase in strength by the precipitation strengthening.It is known that precipitation strengthening is strongly dependent on the precipitate size,number density and distribution.The Cu-containing steels usually contain Mn and Ni to improve the mechanical property,the presence of these alloying elements can induce solid solution strengthening and grain refinement strengthening and also can affect the precipitation strengthening of Cu-rich phase.In this paper,the content of C in steel was reduced to less than 0.005 wt.%,Mn and Ni were added to Cu-containing steels.The precipitation evolution and strengthening effects of precipitates in the Fe-Cu-based steels were studied by the Vichers hardness,tensile test,optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),atom probe tomography(APT)and first-principles simulation,to further understand the effects of Mn and Ni on the precipitation strengthening of Cu-rich phase.The main results can be summarized as follows:(1)Comparing Fe-Cu with Fe-Cu-Mn steels and Fe-Cu-Ni with Fe-Cu-Ni-Mn steels,it is found that the addition of Mn element accelerates the process of precipitation strengthening,which is due to the addition of Mn accelerate the nucleation,growth and coarsening rate of the Cu-rich phase.DFT calculations reveal that the reason why Mn increases the nucleation rate is that Mn can increase the formation possibility of atomic vacancy and SIA.Both atomic vacancy and SIA can serve as nucleation sites for Cu precipitates,therefore the nucleation centers of Cu precipitate in Mn-containing steels at the very initial stage of aging should be much more than that in Mn-noncontaining steels.The formation energy of a Frenkel pair in Mn-containing steels is much lower than that in Mn-noncontaining steels,indicating that the formation of Frenkel pairs in the former is much easier than that in the latter.Thus,the formation possibility of Frenkel pairs is responsible for the increase of the evolution rate of Cu precipitates in Mn-containing steels at the stage of over aging.(2)Comparing Fe-Cu with Fe-Cu-Ni steels,there is no significant difference in processes of precipitation strengthening.The hardness of the Fe-Cu-Ni steel in the as-quenched state is higher than that of the Fe-Cu steel due to the solid solution strengthening and grain-size refinement induced by Ni.TEM results show that the addition of Ni has little effect on the structural transformation of the Cu-rich phase.At the early aging stage,the addition of Ni reduces the nucleation rate of Cu-rich clusters,therefore the number density of Cu-rich clusters in Fe-Cu-Ni steel is lower than that of the Fe-Cu steel,thus the increment of hardness of Fe-Cu-Ni steel is lower than that of Fe-Cu steel.At the later stage of aging,the precipitation strengthening effect provided by Cu-rich phase is similar in the two steels.(3)Comparing Fe-Cu-Mn with Fe-Cu-Mn-Ni steel,the addition of Ni has little effect on the grain size of the matrix of Fe-Cu-Mn steel,the hardness of Fe-Cu-Mn-Ni steel is higher than that of Fe-Cu-Mn steel due to the solid solution strengthening induced by Ni.At the initial aging stage,the addition of Ni reduces the chemical driving force in Fe-Cu-Mn-Ni steel,thus reducing the nucleation rate of Cu-rich clusters and the precipitation strengthening effect of Cu-rich phases.After the peak ageing treatment,Ni atoms are enriched at the precipitate/matrix interface close to the matrix side,and promote the segregation of Mn atoms at the interface close to the Cu-rich precipitate,forming the core-shell structure with the core of Cu-rich precipitate,thereby reducing the rate of growth and coarsening of the Cu-rich precipitate and the decline rate of hardness of Fe-Cu-Mn-Ni alloy.