Study On The Nucleation Mechanism Of Intragranular Ferrite Induced By Mg-Ti-Oxide Inclusions

High heat input welding technology is an important means to improve the shipbuilding efficiency of large vessels.However,with the increase of welding energy input,welding heat affected zone（HAZ）temperature increases,high temperature residence time,and slow post-weld cooling rate,austenite grain coarsening occurs at the HAZ.The oxide metallurgy technology proposed in the 1990s can induce intragranular acicular ferrite（IAF）nucleation by micron-sized inclusions of a certain composition,which can refine the microstructure and improve the strength and toughness.The Mn-depletion zone（MDZ）mechanism is an important factor in the induction of Ti-containing inclusions for IAF nucleation.However,there have been limited deep microscopic studies conducted on this mechanism,which has hindered the further development of oxide metallurgy technology.In this paper,first-principles calculations were used to systematically investigate the nucleation and growth mechanism of ferrite on Mg-Ti-oxide surfaces（Mg Ti O₃ and Mg Ti₂O₅）.The calculation results were further verified using solid-state pressure bonding experiments.The main research contents and results are as follows:（1）Based on the manganese-depletion zone mechanism,the process of absorption of Mn solute atoms in the surrounding matrix by oxide inclusions in steel was divided into three steps.The Mn dissociation energy,cation vacancy formation energy,formation enthalpy,and binding energy were calculated separately to study these three steps.As a result,the limiting factor for Mn absorption by oxide inclusions is the formation of cation vacancies in oxide inclusions.In Mg-Ti oxides,the valence electrons of Mg atoms are hardly involved in bonding and have less electron transfer compared to Ti atoms.Therefore,the covalent and ionic nature of the Mg-O bond is much weaker,and it is easy to break to form Mg vacancies and promote the absorption of Mn atoms.（2）The calculated results were verified by using solid-state pressure bonding experiments.The results showed that Mg Ti O₃ had the strongest ability to induce ferrite nucleation.The thicknesses of the ferrite layer induced by Mg Ti O₃,Mg Ti₂O₅,Ti₂O₃,Ti₃O₅,and Al₂O₃ were 26.01±0.53,21.55±1.26,20.87±0.82,14.51±0.55 and 0μm,respectively.The ability of the five oxides to induce ferrite nucleation decreases sequentially,which is consistent with the order of the MDZ width formed at the interface from wide to narrow.In order to correlate the microscopic bonding features with the experimental results,the results of the overlap population,MDZ width and ferrite layer thickness of five oxides were fitted to construct a constitutive model of the intrinsic relationship between them.（3）The physical properties of theα-Fe/Mg Ti O₃ interface were investigated by taking Mg Ti O₃,which is the most effective in inducing ferrite nucleation,as an example.The most stable interfacial matching relationship isα-Fe（110）/Mg Ti O₃（001）obtained by calculating the planar disregistry under different interfacial matchings.Fe atoms are more easily stacked as[（Mg-O）-Fe]-1 interfaces,which are analyzed by calculating the interfacial energy and adhesion work.The[（Mg-O）-Fe]-1 interfacial stability is the result of the joint action of Fe-Mg metal bonds and Fe-O ionic bonds at the interface by calculating the electronic structure.On this basis,the mechanism of nucleation and growth of intragranular acicular ferrite on the surface of Mg-Ti oxides was summarized and generalized.