First-principles Study Of Cementite And Precipitation Behavior Of Copper-rich Phase In Ferritic Steels

Cementite is one of the most important precipitates in ferritic steels. Its existingforms and states directly influence the mechanical properties of ferritic steels. Theperformance of ferritic steels will change as the changes in the structure, compositionand properties of cementite due to the addition of alloying elements. Cu is a commonelement in many steels. Precipitation of Cu-rich clusters is a common precipitate inferritic steels and also causes embrittlement of reator pressure vessel (RPV) steel. Inthis paper, we study some basic properties of cementite precipitate and Cu-richclusters in order to understand the strength and embrittlement of ferritic steels. Detailsare as follows:1. We study the partition behaviour of Ti and V between ferrite and cementite,and also study the stability and mechanical properties of alloyed cementite.In ferritic steels, Ti partitioning to cementite is energetically favorable inferromagnetic (FM) and non-magnetic (NM) states. Ti prefers to substitute the generalFe position (8d) in FM state and substitute the special Fe position (4c) in NM state. Vpartitioning to cementite is energetically unfavourable in FM state, but V partitioningto cementite is energetically favorable in NM state. V prefers to substitute the8d Fepositions in both FM and NM states. Cementite is metastable compound. Alloyingelements Ti and V are able to enhance the stability of cementite in both FM and NMstates. Cementite with Ti and V substitions are mechanically stable in FM state andmechanically unstable in NM state. Ti and V partitioning can enhance the resistanceto shear strain, the stiffness and the stability against shear of FM cementite.2. A comparative study of the containing Cu solid solution and Cu-rich zone withCu solute atoms precipitation in early phase in ferritic steel.Since Fe and Cu are immiscible, Cu solute atoms easily precipitate in the ferritesteels. Here we study the change of mechanical properties of Cu-rich region withlower the amount of Cu in the initial stage of precipitation. We calculate the thermodynamic and elastic properties of the three Fe-Cu solid solution alloys(Fe127Cu1, Fe119Cu9, Fe111Cu17, the mole fraction of Cu atoms are0.78%,7.03%,13.28%resoectively). The results show that the chemical bonds of three Fe-Cu solidsolution alloy are weaker than that of bcc Fe and the ability of mixture of the twophases is also decline with the increasing of the content of Cu through the formationof the binding energy and enthalpy calculations. In the three models, Fe127Cu1is mostprone to form. Pure Fe, Fe127Cu1and Fe119Cu9are mechanically stable and Fe111Cu17is mechanically unstable. Fe127Cu1has the more resistance to shear strain, the morestiffness and the more stability against shear than pure Fe. Fe119Cu9and Fe111Cu17show opposite results. With Cu content increasing, their performances decline moreobviously. Both Fe and Fe-Cu solid solution alloy belong to ductile materials...