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Nanolaminated M n+1 AX n phases as candidate materials for next generation nuclear reactor applications show great potential in tolerating radiation damage. However, different M n+1 AX n materials behave very differently when exposed to energetic neutron and ion irradiations. Based on first-principle calculations, the radiation tolerance of two M 3 AX 2 and four M 2 AX phases were studied in this work, covering all the M n+1 AX n phases previously investigated with experiments. We have calculated the formation energies of Frenkel pairs and antisite pairs in these materials. The improved radiation tolerance from Ti 3 AlC 2 to Ti 2 AlC observed by experiments can be understood in terms of different Al/TiC layer ratio as the A atomic plane in the nanolaminated crystal M n+1 AX n accommodates radiation-induced point defects. The formation of M A -A M antisite pair in M n+1 AX n materials would provide an alternative way to accommodate the defects resulted from radiation damage cascades, whereas this ideal substitution channel does not exist for Cr 2 GeC due to its pronouncedly higher M A -A M antisite pair formation energy. To further elucidate their radiation damage tolerance mechanism, we have made a detailed analysis on their interatomic M-X, M-A, and X-A bonding characters. Criteria based on the bonding analysis are proposed to assess the radiation tolerance of the six M n+1 AX n materials, which can be further applied to explore other M n+1 AX n phases with respect to their performances under radiation environment. †

Nanolayered structural metallic ceramics, MAX phases, possess unique and highly attractive properties, including excellent radiation tolerance for some of them, whereas little is known about the detailed process of irradiation-induced structural transitions. In this study, the microstructural transformations and the stabilities of V 2 AlC and Cr 2 AlC induced by 1 MeV Au + ions irradiation over a wide range of fluences were investigated by grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). GIXRD analyses show different processes of phase transitions and amorphization tolerance under irradiation between these two MAX phases, which are consistent with the selected area electron diffraction (SAED) results and the high-resolution observations. TEM observations reveal that the nanolamellar structures are disturbed and respective phase transitions occur at relatively low fluences, with the formation of stacking faults. As the fluence increases, Cr 2 AlC becomes completely amorphous, while V 2 AlC are gradually transformed into facecentered cubic (fcc) structure from the original hexagonal close-packed (hcp) structure without amorphization, indicating that V 2 AlC is more tolerant of irradiation than Cr 2 AlC. Based on the phase contrast images and the electron-diffraction pattern (EDP) simulation of the microstructures, mechanisms of the phase transitions of V 2 AlC and Cr 2 AlC are proposed and the difference of the irradiation tolerance between them is discussed.

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