First-principles Study on the Precipitation and Interface Behavior of Carbides in Steel
Author:Xiong Hui Hui
Supervisor:zhang heng hua
The strength and toughness of high strength low alloy steel could be improved by adding the small-content of micro elements,through the strength mechanisms of the precipitation of second particles and the grains refinement.In recent years,the precipitation of carbides including TiC and NbC in microalloyed steel and their grain refinement have been extensively investigated.However,most researches were experimentally conducted at the macroscopic or mesoscopic scale,the study for precipitation behaviors of carbides at the atomic or even electronic scale has been fewly reported.The second phase precipitation in steel and its strengthening are originated from the interactions of interfacial atoms between second phase and matrix.Moreover,the changes of interfacial properties take root in the differences of size,electronic structure and crystal lattice of alloying elements,Fe and C atoms.Therefore,the investigations of these problems are of great significance to carbide precipitation and its grain refinement.In this thesis,the first-principles calculation and experimental methods were used to study the above-mentioned problems.Firstly,the adsorption behaviors of Fe on the surface of carbonitrides were investigated,which could illuminate the microscopic nature of the heterogeneous nucleation of molten steel on particles during the early stage of solidification.Secondly,the interfacial properties of NbC/TiN and NbC/TiC interfaces were analyzed to reveal the mechanisms of NbC heterogeneous nucleation on TiN and TiC in microalloyed steel.And then,the influence of different alloying elements on the potential of ferrite heterogeneous nucleation on TiC and NbC particles were also investigated.Finally,the combination of DFT(density functional theory)simulation and isothermal precipitation experiment was performed to reveal the effect of molybdenum on the precipitation behavior of NbC in ferrite and the microstructure of matrix.These results can provide theoretical basis for further improvement of carbide precipitation and its grain refinement.The main results obtained are as follows:(1)The supercell models of Fe adsorption on the(001)surface or(A1-xmx)C(A=Nb,Ti,m=Mo,V)and A(C1-xNx)(001)were established to study the adsorption behavior of Fe on the surface of carbonitrides.The results show that,when the Ti(Nb)atoms in TiC(NbC)are partly replaced by Mo or V atoms,the formed complex carbides exhibit larger Fe nucleation capacity.The system of Fe adsorption on the(Nb0.5Mo0.5)C(001)surface has the largest adsorption energy(Wads)and the smallest Fe-C bond length.Furthermore,Fe on the A(Nb0.5Mo0.5)C surfaces alloyed by Cr and Mn have the largest Wads compared with the others,one reason is that the coordination number of adsorbed Fe atom is increased due to the formation of Fe-Mo,Fe-Cr,Fe-Mn and Fe-C bonds;the other one is the orbital hybridizations between Fe,Cr and Mn atoms is observed in the range of-2.5 eV-2.5 eV.For the A(C1-xNx)complex carbides,both Ti(C0.5N0.5)and Nb(C0.5N0.5)with CNNC structure have relatively strong Fe adsorption capacity.Moreover,the segregation of Mn on Nb(C0.5N0.5)(001)surface and Co on Ti(C0.5N0.5)(001)surface can significantly promote the nucleation of y-Fe.(2)For the NbC/TiN and NbC/TiC interfaces,different terminations and stacking sequences were taken into account,and the interfacial properties of sixteen NbC/TiN interfaces and sixteen NbC/TiC interfaces were calculated.The results show that NbC(111)/TiN(111)interfaces with C-Ti bonding are the most stable,followed by the ones with Nb-N,Nb-Ti and C-N bondings,respectively.For the NbC/TiN interfaces,the NbC(111)/TiN(111)interface-Ⅲ with C3-Til termination has the lowest interfacial energy at the whole range of Ti and C chemical potentials,thus the NbC heterogeneous nucleation on TiN preferentially occurs with the orientation relationships of(111)NbC//(111)TiN.For the NbC/TiC interfaces,NbC heterogeneous nucleation on TiC preferentially occurs with the orientation relationships of(111)NbC//(111)TiC at low Nb and high C chemical potential,while NbC(100)grows on pre-existing TiC(100)at high Nb and low C chemical potential.These findings agree well with the results of isothermal precipitation experiment.(3)The structures of ferrite/TiC and ferrite/NbC interfaces alloyed by different transition metal elements were constructed and their properties were calculated,and the effects of alloying elements on the ferrite heterogeneous nucleation on TiC and NbC surface were also investigated.Based on the lowest energy principle,the Zr,V,Cr,Mn,Mo,W and Nb tend to segregate at the ferrite/TiC interface,while Cr,V and Ti segregate easily ferrite/NbC interface.Additionally,Cr,Mo and V can improve the bonding strength and stability of both a-Fe/TiC and a-Fe/NbC,which is beneficial to the nucleation of γ-Fe.The interaction mechanism among interfacial atoms shows that Cr,Mo and V can enhance the bonding strength of atoms at the interface and thus reduce the interface energy.(4)When the Nb atom in NbC lattice is substituted by Mo atom,the mismatch of between a-Fe and(Nb,Mo)C is decreased as the increasing site fraction of Mo atom.Furthermore,the results of DLP/NNBB model and first-principles calculation show that it can also decrease the interracial energy of a-Fe/(Nb,Mo)C and critical nucleation energy of(Nb,Mo)C.Therefore,molybdenum is in favor of NbC precipitation in ferrite.The experimental results indicate that the Nb-Mo steel exhibits decreasing grain and increasing dislocation density with increasing Mo content.Moreover,the grain of Nb steel is gradually coarsened with increasing holding time,but the addition of Mo can restrains grain growth due to its superior coarsening resistance.Furthermore,the addition of Mo increases precipitation amounts of nano-sized carbides in ferrite,and thus obtains the smaller(Nb,Mo)C particles after replacement of Nb by Mo in the NbC lattice.Meanwhile,(Nb,Mo)C has stronger coarsening resistance compared to NbC.