Ab initio study of irradiation tolerance for different M_n+1AX_n phases: Ti₃SiC₂ and Ti₃AlC₂

Layered ternary M_n+1AX_n (MAX) materials are recently proposed to be promising candidates for future fission and fusion programmes because of their unique properties inherited from both ceramics and metals. However, different M_n+1AX_n materials demonstrate different behaviors when exposed to energetic neutron or ion irradiations. Based on first-principles calculations, we have investigated the irradiation tolerance of two typical M_n+1AX_n materials: Ti₃SiC₂ and Ti₃AlC₂ from two aspects. First, we make a detailed analysis on the interatomic bonding characters, which are believed to be responsible for the resistance to radiation-induced amorphization. Second, the formation energies of various intrinsic and antisite defects in these two compounds are calculated in order to elucidate their amorphization mechanism. Our results show that the absence of orbitals overlap of Al-C in Ti₃AlC₂ renders it more resistant to amorphization compared to Ti₃SiC₂. In addition, the antisite defects Al_Ti(1) and Al_Ti(2) in Ti₃AlC₂ have much lower formation energies compared to Si_Ti(1) and Si _Ti(2) in Ti₃SiC₂, which implies that the replacement of Ti with Al is easier than Si, thus providing an alternative way to accommodate the defects resulted from irradiation damage cascades. These results indicate that Ti₃AlC₂ is more irradiation tolerant than Ti₃SiC₂, in accordance with experimental observations. Our results have profound implications for the choice of appropriate MAX phase with best performance to be used in next reaction reactors.