First Principles Investigation Of Point Defect-related Properties In Cr₂AlC

In recent years,the MAX phases,combining the merits of metals and ceramics,have attracted considerable attention for oxidation resistance,thermal and mechanical properties under extreme environments.In this work,the point defect related properties in Cr2AlC were investigated by first-principles calculations,the unique properties enable Cr2AlC to be a promising candidate for materials in extreme environments,such as nuclear reactors.In the second chapter,we briefly introduce the theoretical basis and calculation methods of this work.First,we introduce the density functional theory(DFT).Then,we introduce the exchange and correlation potential,the Hartree-Fork approximation and Pseudopotential plane wave method.At last,we introduce the methods of defect formation energy and defect migration energy.In the third chapter,we have investigated intrinsic defects including mono-vacancy and anti-site defects in Cr2AlC.The results indicate that the C vacancy could be easily formed compared with Cr and Al vacancies.The formation of vacancies make the configuration of material collapsed,C vacancy makes biggest volume changes.Vacancy also reduced the conductivity of Cr2AlC,and it is seen that the covalency of Al and Cr atoms increased due to C vacancy.Cr2AlC is hardening under radiation damage conditions.Based on the calculation of migration energy,it is concluded that the migration energy of Al vacancy is lowest,which explains why Cr2AlC has great oxidation resistance,due to Al atoms easily migrate to the surface of material to form oxide.The calculation of anti-site defect shows that Al-Cr defect could be easily formed,and anti-site defect enhanced the conductivity of Cr2AlC.In the fourth chapter,we have investigated impurity defects in Cr2AlC,we mainly studied three types of impurities:H?He?O.It is seen that vacancies tend to trap H and O atoms due to their lower formation energies than that of He atom.Impurities atoms occupy Cr vacancy are unstable,they have a tendency to migrate,especially O atom migrates into Al layer forming Al-O bond.Substitution defects decrease the stiffness of material.The result of interstitial defects shows that there are two stable interstitial site:octahedral interstitial site is surrounded by three A1 atoms and three Cr atoms,and tetrahedral interstitial site is located in center of three A1 atoms and one Cr atom.Since the impurity atoms tend to occupy the octahedral interstitial sites,it indicates that the Cr-Al layer spacing is relatively large and bonding is weak,so the presence of impurity atoms has less effect on the unit cell,and thus the formation energy is lower.It is seen that when H atom occupies tetrahedral interstitial position,it has a tendency to migrate to the adjacent tetrahedral or octahedral interstitial sites.O atom has a tendency to migrate to A1 layer,whenever it is in position of tetrahedron or octahedron.The migration mechanism of O atoms gives a explanation which Cr2AlC has a good oxidation and corrosion resistance.He atom easily occupy the octahedral interstitial site to reach stability.In the fifth chapter,we summarize the whole work,and make a plan for the future work.