The Stability And Migration Of Vacancy And Impurity Atoms?H/He? In Ti₃AlC₂: First Principles Calculations

Nano-layered Ti₃AlC₂ as one of typical MAX phases,due to the unique characteristics of both ceramics and metals?such as high modulus,high melting point,low thermal expansion coefficient,good thermal properties and excellent high temperature oxidation resistance?,are considered as promising candidates for fuel cladding or coating materials in future generation ? reactor.However,long-term elevated temperature and high dose of irradiation can induce various induce various changes in the mechanical and physical properties of structural material.For example,it has been recognized that a large amount of vacancies,interstitials,impurity atoms,and clusters.This paper is investigating the stability and migration properties of vacancy,impurity atom?H/He?,and clusters based on first principles calculations in Ti₃AlC₂.Our results show that Al vacancy and C vacancy are easily to form in Ti₃AlC₂,while two type of non-equivalent Ti vacancies exhibit higher formation energies in the same irradiation condition.The results are in good agreement with the previous experimental ones,which have shown the selective loss of A element at high temperature in MAX phase.Meanwhile,the Al layer can be successfully exfoliated from Ti₃AlC₂ to yield so-called two dimensional “MXenes” in room temperature.For impurity atoms?H/He?,we are focused on the calculations of the formation energy of four interstitial sites?Ihex1?Ihex2?Ioct?Itetra?located at high-symmetry stable configurations.It has showed that the formation energy of impurity H atom is negative,which indicates it can incorporate at the four interstitial sites spontaneously.Moreover,the impurity H and He atoms favorably occupy the octahedral site?Ioct?located in Ti-Al layer with the corresponding formation energy of-3.31 e V and 2.94 e V,respectively.Therefore,the site of Ioct is considered to be the most stable interstitial site.Besides,the obtained results show that the formation energy of HV cluster is generally lower than He V cluster at the same condition,which can confirm that the vacancy is likely to trap H atom.On the other hand,in order to understand the microscopic evolution behaviors of vacancy and impurity H and He atoms in Ti₃AlC₂,the migration mechanism of these point defects are studied using first principles methods at atomistic level.It is suggested that Al vacancy can easily migrate in Ti₃AlC₂ with the lowest barriers,0.95 e V.For the migration of H and He atoms,it is interesting that the migration of H atom shows a remarkable anisotropy along [-1,0,0]?[0,1,0]?[0,0,-1] direction.Furthermore,it is more likely for a single interstitial H atom to migrate along the path of 1?c?5 in [0,0,-1] direction,rather than jump via the straight trajectory 1?5.While the similar phenomenon is not observed for the migration of He atom.Finally,the obtained calculation results will provide a theoretical guideline to the microscopic evolution behavior of H/He bubbles.