The response of Ti₃SiC₂ MAX phase to ion energy dissipation during energetic irradiations at different temperatures

The energy dissipation pathways of electronic and nuclear energy loss (S_e and S_n, respectively) during ion irradiations are investigated to evaluate their influence on the damage evolution in Ti₃SiC₂ [titanium silicon carbide] MAX phase. Interest in the unusual properties of M_n+1AX_n [MAX] phase ceramics for high radiation environments has led to numerous neutron and ion irradiation studies; however, an understanding of the response to energy deposition from high-energy ions is necessary for a comparison between the two irradiation environments. Two novel experiments were developed to compare variation in energy dissipation at constant damage dose: 1) a series of high energy self-ion irradiations at room temperature, 500°C [degrees Celsius] and 880°C, and 2) a sequential irradiation study. Grazing incidence x-ray diffraction and transmission electron microscopy were utilized to characterize structural variation as a function of depth, and integral to this evaluation was the novel adaptation of a method to measure fine changes in lattice strain in hexagonal polycrystals. The results reveal a threshold in Se, above which the inelastic energy dissipation is a major contributor to the damage evolution in the system, exhibited by increased lattice strain and the formation of a FCC [face centered cubic] phase. High temperature in situ irradiation conditions appear to mitigate resulting damage effects, leading to more uniform lattice strain and significant reduction in FCC phase formation. Further, separating the energy loss pathways, the S_n elastic collision cascades are demonstrated to produce Ti and Si anti-site point defects and eventually a structural shift to the FCC phase embedded in the hexagonal structure. While S_e above the threshold adds heat and additional point defects, through a thermal spike and its subsequent shockwaves, there is competition between the dissipation pathways, defined by their ratio. When S_e/S_n is low, the formation of the FCC-type phase is enhanced by the additional defects and energy in the system, but as S_e/S_n increases, in-cascade annealing by the thermal spike, while insufficient to anneal preexisting point defects, suppresses the formation of the FCC phase, thus drastically increasing the relative lattice strain in the hexagonal structure and eventually saturating it.