First-principles Study On Heterogeneous Nucleation Of Primary Austenite In Steel By Adding La

Previous studies have found that the addition of rare earths can refine the solidification structure of steel,and it is generally accepted that high melting point rare earth inclusions play a major role in the heterogeneous nucleation of the primary phase,and the utility of heterogeneous nucleation is mostly evaluated using the two-dimensional lattice disregistry model.However,the results of different scholars are not uniform.In contrast to the two-dimensional lattice disregistry model,the edge-to-edge matching(E2EM)crystallographic model proposed in recent years focuses on the matching of the atoms of two phases at the interface,rather than determining the degree of lattice matching between the two phases.In this thesis,firstly,using the E2EM model,the atomic matching between the high melting point inclusions that may be formed by adding La to steel and austenite(γ-Fe)is calculated,and the orientation relationship between the inclusions phase of La and γ-Fe is predicted.The heterogeneous nucleation potency of incipient austenite on high melting point La-inclusions particles is explored from a crystallographic perspective.Based on the crystallographic study,the interfacial energy and bonding characteristics between the La-inclusions andγ-Fe were calculated on the atomic scale using the first-principles based on density functional theory,and the potency of high melting point La-inclusions as heterogeneous nucleation site for incipient austenite was investigated from an energetic point view to provide an energetic basis for the crystallographic calculations.The edge-to-edge matching crystallographic calculation shows that the minimum of the interatomic spacing misfit between La2O3 and γ-Fe along the matching direction is 6.69%and the minimum of interplanar spacing mismatch is 7.16%,and the orientation relationships between them are predicted to be:[5413]La2O3//[101]γ-Fe&(1012)La2O3//(111)γ-Fe and[5413]La2O3//[211]γ-Fe&(1012)La2O3//(111)γ-Fe.The minimum of the interatomic spacing misfit and the interplanar spacing mismatch between LaS and γ-Fe are 10.63%and 2.04%,respectively,and the orientation relationship between them is predicted to be:[100]LaS//[101]γ-Fe&(022)LaS//(111)γ-Fe和[110]LaS//[100]γ-Fe&(222)LaS//(002)γ-Fe.From the point of view of crystallography,La2O3 and LaS can be used as heterogeneous nucleation cores of primary austenite,and the atomic mismatch between La2O3 and γ-Fe is lower,and the effect of heterogeneous nucleation is higher.First-principles calculations show that the La2O3(1012)/γ-Fe(111)interface in which the atomic ratio of La and O is larger than its stoichiometric ratio can be used as the heterogeneous nucleation interface of La2O3/γ-Fe interface,whenΔμLa is in the range of-0.96eV and-0.45eV.When ΔμLa is in the range of-1.24eV and-0.81eV,the LaS(222)/γ-Fe(002)interface of the La atomic terminal may be a heterogeneous nucleation interface.Compared with LaS(222)/γ-Fe(002)interface,La2O3(1012)/γ-Fe(111)interface can be used as heterogeneous nucleation interface in a wider range of La chemical potential,indicating that La2O3 is more effective as γ-Fe heterogeneous nucleation.This is consistent with the results of crystallographic calculation,and can provide a reliable energy basis for the crystallographic calculation of high melting point La inclusions as the heterogeneous nucleation core of primary austenite.