Segregation Of Alloying Elements At NbC/fcc-Fe Interface And Boron Effects

Super austenitic stainless steel has excellent corrosion resistance,and it is widely used in flue gas desulfurization,garbage incineration,seawater desalination and petrochemical industries.For high-molybdenum super austenitic stainless steel with a Mo content of 6% to7%,in some cases,it can be comparable to iron-nickel-based alloys such as C276 and titanium alloys with excellent corrosion resistance.On this basis,in order to make the austenitic stainless steel have more excellent mechanical properties,precipitation strengthening is the most effective way of strengthening,and the yield strength and tensile strength of the material can be significantly improved through the dispersed precipitation phase.At present,there have been many experimental and theoretical studies on the analysis of the Nb C precipitate phase and the characteristics of the Nb C/bcc-Fe and Nb C/fcc-Fe interfaces by adding Nb to the stainless steel.However,there are few studies on the segregation behavior of alloying elements,especially Mo,at the Nb C/fcc-Fe interface.Mo is an easily segregated element,and it is easy to form ? phase.If it is precipitated at the Nb C/fcc-Fe interface,for super austenitic stainless steel is unfavorable.Relevant studies have shown that a large amount of precipitation of ? phase causes the steel to easily crack during the subsequent hot working process,causing performance damage.In related studies,the addition of a small amount of boron is also used to control the grain boundary,which makes the distribution of alloy elements more reasonable and improves the performance of the material.If a certain amount of carbide forming elements are added to the super austenitic steel,the phase boundary between carbide and austenite will inevitably be formed.Then,the segregation behavior of alloying elements at this phase boundary is particularly important.In this paper,first-principles calculation methods were used to build Nb C/fcc-Fe interface and Nb N/fcc-Fe interface models.The segregation behavior of alloying elements at the Nb C/fcc-Fe interface and Nb N/fcc-Fe interface has been systematically studied from the atomic level.The effects of alloying element segregation on the stability of the phase interface and the bonding strength are analyzed by the segregation energy,Griffith work of fracture,charge density,differential charge density and density of states.At the same time,the effect of boron on the segregation behavior of alloying elements at the Nb C/fcc-Fe interface and Nb N/fcc-Fe interface was studied.The calculation results show that Si does not segregate and is evenly distributed in the Fe matrix;Cu,Ni,and Mn are slightly segregated at the interface;Mo and Cr segregate at the interface,and there is a tendency to further segregate to carbonized species.After the alloying elements(Mn,Mo,and Cr)segregate at the interface,the Griffith fracture work of the interface,the charge density distribution between atoms,and the charge transfer are significantly reduced,indicating that the interface segregation of Mn,Mo,and Cr reduces the bonding ability of the interface.In addition,the interface segregation of Mo and Cr is not conducive to the corrosion resistance of the material.Therefore,the B element is doped at the interface to study the influence of B on the segregation behavior of the alloy elements at the interface.The calculation results show that B inhibits the slight segregation of Cu,Ni,Mn at the interface,and inhibits the segregation of alloying elements Mo,Cr,especially at the interface,but Mo and Cr still have a tendency to segregate into Nb C.Aiming at the tendency of Mo to further segregate into Nb C,the related properties of the composite niobium-molybdenum carbide were analyzed.Studies have shown that with the addition of Mo content,the formation energy of composite carbides gradually increases,making it difficult to form;but the interfacial bonding capacity also gradually increases.When the Mo content reaches 40%,the interfacial bonding energy reaches its maximum value and then gradually decreases.In addition,after the addition of B at the interface,the Griffith fracture work of the interface,the charge density distribution between atoms,and the charge transfer increased significantly,indicating that the bonding strength of the interface increased significantly.And when the alloying element Mo segregates to the interface,the bonding strength of the interface also increases significantly.