First-principles Study On Irradiated Defects In MAX Phase Material Ti₃AlC₂

The continuous growth of people's demand for energy and the increasingly serious environmental problems call for strengthening the research and development of new energy sources other than fossil fuel.With its many benefits,controlled nuclear fusion is considered a key direction for future energy development.However,for the intense irradiation environment in the fusion reactor,the radiation damage caused by high-energy neutrons to various materials is a great challenge to the fusion materials.On one hand,atomic cascade collisions caused by high-energy neutron incident can lead to displacement defects of lattice atoms.On the other hand,H/He impurity gas directly induced by high-energy neutrons in various materials is easily captured by crystal defects produced by displacement effect,and further form hydrogen or helium bubbles.These defects will eventually lead to embrittlement,swelling and creep of material,which will greatly shorten the service life of material.Therefore,the realization of fusion will depend strongly on how the material behaves under the intense radiation field.Ti₃AlC₂ is considered as a promising candidate for high-temperature structural materials in future nuclear reactors because of its excellent properties.In this paper,the spatial distribution of displacement per atom(DPA)when Ti₃AlC₂ was used as the backplane material and the sample was studied by combining with the radiation damage problem of the backplane material during the operation of Compact Materials Irradiation Facility(CMIF).Based on the first-principles calculations,we have studied the intrinsic point defects and the occupancies of impurity atoms,the interaction between impurity atoms and vacancy defects,as well as the evolution of clusters in Ti₃AlC₂under irradiation.By studying the intrinsic point defects in Ti₃AlC₂,we found that Al monovacancy(V_Al),Al divacancy(2V_Al–Al),and the Al-C divacancy(2V_Al–C)are most easily formed in all vacancies.The interactions between multiple vacancies are weak.The formation of vacancy is relatively independent and not affected by other vacancies.In addition,all self-interstitial atoms tend to migrate to the vicinity of the Al plane.For the antiposition defect of Ti₃AlC₂,the formation energies are usually relatively low when other atoms occupy the positions of Al atoms,indicating that the Al plane is still a relatively active region in Ti₃AlC₂ structure,which is easily affected by irradiation defects.The configurations of H–m V(m=0,1,2)complexes have been studied to assess the energetically favorable sites for H atoms.Within pre-existing V_Al or 2V_Al–Al,the most favorable site for H atoms is the I_tetr-2 site,but the H atom tends to occupy the I_oct-4 site within 2V_Al–C.H clusters trapped in a primary Al vacancy can promote the formation of vacancy and prefer to form platelet-like bubbles in the Al plane,while H clusters trapped in a primary C vacancy have higher probability to form spherical ones.Vacancies serve as trapping centers to induce H atoms to segregate on their internal surface by providing an optimal H-embedding isosurface of charge density.The divacancy exhibit stronger H trapping ability than monovacancy,where 2V_Al–Al and2V_Al–C could capture up to seven and six H atoms,respectively.Meanwhile,the He–2V_Al–Al complex could only capture four H atoms to form H–He hybridized bubbles,and He impurities effectively suppress further aggregation of H atoms.In addition,the interaction between impurity oxygen atom and H/He/vacancy irradiation defects in Ti₃AlC₂ is studied.The formation energy and occupation of O atoms within different defects are discussed.It is found that O atom preferentially occupies the hexahedral interstitial site(I_hex-1)in bulk Ti₃AlC₂,whereas,O atom preferentially occupies the tetrahedral interstitial site(I_tetr-2)within pre-existing vacancy.The appearance of C vacancy could greatly reduce the formation energy of O atom and make O atom more inclined to occupy the center of C vacancy.Then the capturing process of O/H clusters in Ti₃AlC₂ is studied.Vacancy could capture more O atoms than H/He atoms,where the V_Al and 2V_Al-Al could hold up to fifteen and eighteen O atoms,respectively.Meanwhile,the O could also promote the formation of vacancy.On the other hand,O atoms could quickly combine with Al to form Al₂O₃ protective layer on the surface of Ti₃AlC₂,which inhibits further oxidation inside the material.In addition,the H-O exhibits repulsion interaction,but strong attraction occurs in the He-O interaction.Therefore the O atom has an inhibitory effect on the formation of H cluster,while O could bind more He atoms to form a large number of He bubbles.Besides,the O impurity greatly reduces the trapping ability of vacancy to H atoms,and O and He have a synergistic interaction for inhibiting the aggregation of H clusters.The calculation results in this paper provide microscopic explanations of the behavior of Ti₃AlC₂ under irradiation and high temperature oxidation from the atomic scale.All of these provide the necessary physical parameters for higher scale simulation studies to evaluate the performance of Ti₃AlC₂ as a structural material for the fusion reactor,so as to better design the structural materials based on MAX phase.