Super austenitic stainless steel,possessing an austenitic microstructure with a face-centered cubic crystal structure,has been extensively applied in harsh service conditions such as seawater,oilfields,cooling water systems,flue gases,chemical applications,and nuclear reactions due to its excellent corrosion resistance and good mechanical properties.The high Cr and Mo contents ensure its unique corrosion resistance,especially in reducing and chloride-containing solutions.And the addition of Mo significantly improves the pitting corrosion resistance and the crevice corrosion resistance of stainless steels.However,the higher Cr and Mo content is likely to cause the precipitation of brittle second phases,such as sigma,chi and laves,which reduces the hot workability and corrosion resistance of stainless steels.Previous experiments and theoretical calculations show that B preferentially segregates at grain boundaries over the Mo atoms to inhibit the precipitation of?phases enriched in Cr and Mo.However,the effects of B and Mo on the passivation film have not been explored,and the properties of passivation film play an important role in the corrosion resistance of stainless steels.Therefore,the first-principles calculations based on density functional theory were employed to study the interaction between B and Mo atoms at the metal/oxide interface,analyze their segregation behavior and their effects on the interfacial adhesion,and explain it according to the electronic characteristics of the interface structure.In order to build a reasonable interface structure,the appropriate calculation parameters were chosen through the optimization of Fe unit cell and Cr2O3 unit cell,and then O-terminal Fe(111)/Cr2O3(0001)interface structures with five different atomic coordination were built.It is found that the top Cr1 interface has the largest work of separation and the smallest interface energy,which indicates that this interface has higher interfacial adhesion and thermodynamic stability,and this interface structure was selected as the basic configuration in focus for the following study.According to the composition of super austenitic stainless steel S31254,13 Cr atoms were substituted for Fe atoms in the substrate uniformly to construct Fe-Cr/Cr2O3 interface.It is found that Cr atoms can spontaneously segregate to the interface,which can make the passivation film self-healing.With the increase of the extent of segregation,the interfacial adhesion is continuously strengthened.There is no perfect interface without defects in reality.Therefore,we studied the effects of different vacancy defects(Fe vacancy,Cr vacancy,O vacancy and O-int vacancy)on the property of the interface.These vacancy defects are easy to locate at the interface,which has a lower formation energy,but the Cr vacancy is more likely to exist on the surface of oxide layer.No matter whether there are vacancy defects or not,it has to overcome a larger energy barrier for the diffusion of a single Mo atom into the oxide layer,while B can promote the diffusion of Mo into the oxide layer to improve the Mo content in the passivation film.And Mo can improve the interfacial adhesion,especially when Mo is located on the surface of the oxide layer.Through the analysis of electronic characteristics,it is found that when the separation work of interface is larger,there are higher charge densities between the interfacial metal atoms and interfacial oxygen atoms,which indicates a strong interaction among these atoms.Combining with the electron localization function(ELF),it is found that Cr-O and Mo-O bonds of the oxide layer are highly ionic and somewhat covalent.Compared with Cr atoms,Mo in the oxide has a higher chemical valence due to more electrons of Mo atoms transfer to O atoms.When the Mo atom is in or near the oxide layer,it can be oxidized prior to Fe atoms to improve the compactness of the passivation film.It can be seen from the density of states that O atoms can form strong chemical bond with Cr atoms or O atoms,and the interface has higher electrochemical stability with the Mo at the interface or on the surface of the oxide layer than pure interface without B and Mo. |