In situ He⁺ irradiation of the double solid solution (Ti_0.5,Zr_0.5)₂(Al_0.5,Sn_0.5)C MAX phase: Defect evolution in the 350–800 °C temperature range

Thin foils of the double solid solution (Zr_0.5,Ti_0.5)₂(Al_0.5,Sn_0.5)C MAX phase were in situ irradiated in a transmission electron microscope (TEM) up to a fluence of 1.3 × 10¹⁷ ions⋅cm^-2 (∼7.5 dpa), using 6 keV He⁺ ions. Irradiations were performed in the 350–800 °C temperature range. In situ and post-irradiation examination (PIE) by TEM was used to study the evolution of irradiation-induced defects as function of dose and temperature. Spherical He bubbles and string-like arrangements thereof, He platelets, and dislocation loops were observed. Dislocation loop segments were found to lie in non-basal-planes. At irradiation temperatures ≥ 450 °C, grain boundary tearing was observed locally due to He bubble segregation. However, the tears did not result in transgranular crack propagation. The intensity of specific spots in the selected area electron diffraction patterns weakened upon irradiation at 450 and 500 °C, indicating an increased crystal symmetry. Above 700 °C this was not observed, indicating damage recovery at the high end of the investigated temperature range. High-resolution scanning TEM imaging performed during the PIE of foils previously irradiated at 700 °C showed that the chemical ordering and nanolamination of the MAX phase were preserved after 7.5 dpa He⁺ irradiation. The size distributions of the He platelets and spherical bubbles were evaluated as function of temperature and dose.