Ion Irradiation Characterization Studies of MAX Phase Ceramics

The family of layered carbides and nitrides known as MAX phase ceramics combine many attractive properties of both ceramics and metals due to their nanolaminate crystal structure and are promising potential candidates for application in future nuclear reactors. This thesis reports on the background, design, and analysis of an experiment focused on determining the effects of energetic heavy ion irradiations on polycrystalline samples of titanium silicon carbide 312, titanium aluminum carbide 312, and titanium aluminum carbide 211. The irradiation conditions consisted of ion doses between 10 and 30 displacements per atom at temperatures of 400 and 700 degrees Celsius, conditions relevant to application in future nuclear reactors, and a relatively un-explored regime for this new class of materials known as the MAX phase. Following irradiation, a comprehensive analysis of radiation response properties was compiled using X-ray diffraction, nanoindentation, scanning electron microcopy, and transmission electron microscopy. In all cases, the materials remain fully crystalline though atomic collisions induce significant damage and disorder into the layered crystalline lattice. X-ray diffraction and nanoindentation show this damage is manifest in anisotropic swelling and hardening at all conditions and in all materials, with the aluminum based MAX phase exhibiting significantly more damage than their silicon counterpart. In all three materials there is little damage dependence on dose, suggesting saturation of radiation damage at levels below 10 displacements per atom, and a high correlation between residual damage and irradiation temperature, with significantly less damage at higher temperatures, suggesting radiation defect annealing. SEM surface analysis showed significant grain boundary cracking and loss of damage tolerance properties in the aluminum based MAX phase irradiated at 400 degrees Celsius, but not in the silicon counterpart. TEM analysis of select samples suggest that interstitials are highly mobile while vacancies are immobile and that all three materials are in the so-called point defect swelling regime between 400 and 700 degrees Celsius. All results are consistent with previous work involving traditional and MAX phase ceramics. Results show that the aluminum MAX phase are not fit for application near 400 degrees Celsius and that the silicon MAX phase is overall more damage tolerant.