Study On Surface Properties And Alloying Behavior Of Cementite From Density Functional Calculations

Cementite is one of the most important carbides, and is the most commonstrengthening phase in carbon steels and white cast irons. The addition of alloyingelements will change the internal structures of steels, and then will have a direct impact onthe phase stability, deformation and fracture characteristics, physical and chemicalproperties. The changes of internal structures are originated from the interaction betweenalloying elements and iron, carbon as well as other alloying atoms. Furthermore, thesechanges take root in the differences of electronic structure, size, and crystal lattice ofalloying atoms.In this thesis, density-functional theory was adopted to research the surface structuralcharacteristics, stability and electronic properties of cementite with orthorhombic structure(θ-Fe3C). By constructing polar (001) surfaces of pure cementite, the atomic arrangementduring cementite growth along some direction was explored, and the electronic propertiesof polar (001) surfaces were calculated. The progress of alloying elements integrating incementite was simulated with the method of cementite surface adsorbing and substitutingalloying elements for cementite surface atoms. The main results obtained are as follows.Three most common cementite surfaces among the orientation relationships betweencementite and ferrite were cleaved from relaxed pure cementite:(001),(010) and (100)surface. And then the three surface structures were optimized precisely. The calculationresults reveal that the (001) surface with the highest surface roughness and the strongestsurface Fe-C bonding is the most stable, and the surface stability of pure cementitegradually decreases from (001) and (010) to (100).The polar (001) surface was reconstructed, and then eight kinds of terminatedsurfaces without macroscopic dipole moment were built. The optimization results showthat the relaxation energy of one surface is associated with the exposure level of surfacecarbon atoms, in other words, the surface possesses a low relaxation energy if there is notthe carbon atom exposed in the surface layer. The cleavage energy of one surface is linkedwith the split position in the bulk cell, that is to say, the deeper the position of the segmentation line located at the bulk unit cell is, the higher the cleavage energy of thesurface cleaved from that bulk cell is. Based on the lowest energy principle, the presentstudy predicted that when cementite grows along its [001] crystal orientation: if the initialatom is carbon, cementite will grow in accordance with CFe3-terminal surface structure; ifthe initial atom is iron, cementite will grow in accordance with CC-Fe3Fe3polar surfacestructure.When the alloying elements Cr and Mn are partitioned to the cementite phase, foradsorbing on the surface of minimal area (the (001) surface), the ability of Cr atom isstronger than that of Mn atom, and Cr atom locates at the5-fold site (one surface C atomand four Fe atoms) after the structure was optimized. When Cr and Mn atoms continue topenetrate the (001) surface layer of cementite, the ability of Cr atom is still stronger thanthat of Mn atom substituting for the surface Fe atom.Based on the lowest energy principle, the surface stability of alloyed cementiteFe2MC (M=Cr/Mn/Co/Ni) was predicted. The surface stability of Fe2MC (M=Cr/Co/Ni)gradually decreases from (001) and (010) to (100), while the (100) surface of Fe2MnC ismore stable than the (010) surface. The stability (from the strongest to the weakest) ofalloyed cementite surfaces doped with different atoms (Cr/Mn/Co/Ni) follows the order:Fe2NiC>Fe2CoC>Fe2MnC>Fe2CrC. All the bulk phase and surfaces of Fe2MC(M=Cr/Mn/Co/Ni) have magnetic characteristic as those of pure cementite do. Theelectronic structure shows that the bonds of Fe2MC surfaces are mixtures of metallic,covalent and ionic bond, and the covalent bonds are dominant.The surface stability of FeM2C (M=Cr/Mn/Co/Ni) gradually decreases from (001)and (100) to (010). The stability (from the strongest to the weakest) of alloyed cementitesurfaces doped with different atoms (Cr/Mn/Co/Ni) follows the order:FeNi2C>FeMn2C>FeCo2C>FeCr2C. All the bulk phase and surfaces of FeM2C(M=Cr/Mn/Co/Ni) have magnetic characteristic. The bonds of Fe2MC surfaces are alsomixtures of metallic, covalent and ionic bond, and the covalent bonds are also dominant.