Atomistic insights into the radiation response of MAX phases Ti3AlC2 and Ti3SiC2

Determining the spatial rearrangements of point defects during the initial stages of ion radiation is essential for understanding the mechanisms of radiation damage and resistances in materials. However, direct observation of point defect migrations at the atomic scale in irradiated materials remains experimentally challenging. In this study, ab initio molecular dynamics simulations were utilized to study the low-energy recoil events in two representative MAX phases Ti3AlC2 (TAC) and Ti3SiC2 (TSC), enabling us to uncover similarities and differences in the dynamic processes of atomic rearrangement in these two materials. Our results demonstrate that the ATi2 (A = Al or Si) block plays a dominant role in accommodating the radiation-induced point defects with low formation energies, such as C, Ti, and A interstitials, as well as Ti-A antisites. Most importantly, a novel recombination mechanism for the A Frenkel pairs (FPs) has been identified, which is driven by the synergistic rotation of three interstitial A atoms. Comparative analysis demonstrates that the differences in the radiation response between these phases originate from the distinctively different properties of the A atomic layers. TSC exhibits significantly larger threshold displacement energies than TAC when Ti and A atoms are involved in forming antisite defects or A FPs. Due to the larger open region of AlTi2 block, TAC prefers an indirect pathway for the replacement of Al with a Ti1 atom along the [0001] direction which differs from TSC. In addition, we reveal that the higher radiation resistance of Si[0001] compared to Al[0001] is attributed to the formation of a TSC-specific bound Si FP distributed along the z-direction which stabilizes the early encountered intermediate defective state in TSC. Based on climbing image nudged elastic band calculations, the recoiling mechanisms for ions were proposed, together with the energy barriers for ion migrations, to provide deep insights into these discrepancies.