First-principles Study Of Crystal Defect In Face-centered Cubic Metals

Due to the excellent comprehensive properties,some face centered cubic(FCC)metals,such as Aluminum(Al),Copper(Cu)and Nickel(Ni),have been extensively used in aerospace,automotive,defense and electronics fields.However,it is very difficult to produce twins in FCC because of the high staking fault energy(SFE)and the twin stress,which greatly limits their wider application.The new development trend of materials requires a thorough understanding of the deformation mechanism.It is helpful to understand the deformation mechanism by calculational material design.Especially,it can overcome the limitation of experimental conditions and equipment,and be used to simulate the process that is difficult to observe in some experiments,with the advantages of high efficiency and convenience.To be directed against the practical problems of stacking faults(SFs)and twins in FCC metals,the influence of alloying atoms on SFs and twins are described systemically in this paper,by combining the first principle calculation,interaction model,Fermi Dirac distribution,generalized stacking fault energy(GSFE)and the P-N dislocation model.First of all,based on the analysis formula of GSFE surface,the periodic supercell model,which is suitable for the hexagonal crystal,is successfully applied to compute the GSFE of FCC crystal.Then,the advantages and disadvantages of the vacuum model,Alias shear model and periodic supercell model are compared.The results show that the calculation results of the periodic supercell model are the most accurate and reliable.Furthermore,the structure of the screw and edge dislocations are investigated by combine the 2D P-N model and the GSFE surfaces of Al,Ag,Cu and Ni obtained through the periodic supercell model.The calculated results indicated that all the dislocations could be divided into two partial dislocations.The screw dislocations have a few stable states,while the edge dislocations have only one stable state.The dislocation width of the edge dislocations in the four kinds of crystals is larger than that of the screw dislocations,which indicates that the decomposition of the edge dislocation in FCC crystal is easier than the screw dislocation.But the Peierls energies and Peierls forces of the screw dislocations are larger than those of the edge dislocations.Then,three types of impurities,including intrinsic defects,substitutional solutes and interstitial solutes were considered to study their effects on the SFEs of Al alloys.It is shown that most of the intrinsic defects can hardly change the SFEs of aluminum,whereas only tetrahedral self-interstitials can significantly lower the extrinsic SFE with a relatively high concentration at 200K.Substitutional solutes such as Ge,Y,Sc,Sr and interstitial solutes such as C,N,H,can drastically lower the SFEs of A1 at low temperatures.Our study suggests that at circumstances the possibilities to introduce twins in A1 materials can be increased significantly,since their SFEs can be greatly lowered by the impurities.The physical mechanism of lowering the SFEs in those Al solid solutions was interpreted by their electronic structures around the SFs and the solutes or impurities.Next,the effect of 31 kinds of alloying elements on the deformation twin energy(DTE)of A1 alloy in terms of their twin boundary segregations was investigated.It is found that although the solubilities of alloying elements K,Cs,Er,Rb,Sr,Fe,Yb,Ba,La,Ca and Y in A1 alloy are very small,there exists a strong interaction between these elements and the TBs,reducing the DTE greatly at low temperatures.With increasing temperature,alloying elements tend to distribute evenly and the degree of segregation near TBs weakens,resulting in their influence on DTE gradually reduced.The interactions between TB and some alloying elements such as Mg,Zn,Sc and Ag are not very strong,but their solubility in Al alloy is relatively large,therefore,they can greatly reduce the DTE.Moreover,the reductions of DTE are still large at high temperatures.After that,based on the interaction energies of 18 substitutional atoms and 5 interstitial atoms with SFs,the SFEs of Cu solid solutions at various temperatures and solute concentrations were studied by using first-principles calculations.The results show that the interactions can extend to the second layer with respect to the SFs.Substitutional atoms Sn,Al,Zn,P,Si,Ge as well as interstitial atoms Be,C,N,O,H have large attraction energies with the stacking fault(SF);whereas,substitutional atoms Ti,Mn,Cr are repelled by the SF,and the rest atoms have small interaction energies.Furthermore,the effects of Fermi-Dirac distribution and uniform distribution of solute atoms on the SFE of Cu alloys were investigated based on these interaction energies.It is found that the SFE of Cu can be drastically reduced by Sn,Al,Zn,P,Si,Ge,C,N and O atoms with relatively high concentration at low temperatures.Finally,on the basis of the interaction energy model,we evaluated the effects of a single alloying atom(i.e.,Mn,Al,Si,C and N),as well as its aggregates,including the Mn-X dimer and Mn2-X trimers(X=Al,Si,C and N)on the SFE of the austenitic steel via first-principles calculations.Given low concentrations(<10 wt.%)of alloying atoms,dimers and trimers,theoretical calculations reveal that alloying atom Mn causes a decrease in the SFE,whereas Al,Si,C and N significantly increase the SFE.Combination with other alloying atoms to form the Mn-X dimer exerts an effect on SFE that,to a certain extent,is close to that of the corresponding single X atom.The interaction between Mn2-X and the stacking fault is stronger than that of the corresponding single X atom,inducing a significant increase in the SFE of austenitic steel.The theoretical results we obtained demonstrate that the increase in SFE in high-Mn steel originates from the synergistic effect of Mn and other trace alloy atoms.In this thesis,the dislocation,SF and twins were studied systemically by improving the theoretical model,then the possibility of the introduction of twins in the high SFE FCC alloys is predicted theoretically,which provides a theoretical basis and guidance for the subsequent experimental verification.