Strain effect on the defect formation and diffusion in Ti2AlC and Ti3AlC2: A first-principles study

Abstract

Mn+1AXn phases have the prospect of being the candidate materials as coating and fuel cladding in advanced nuclear systems. It is well known that nuclear materials are subjected to harsh environments such as high temperature, high pressure, and neutron radiation et al. during their application. Consequently, defects and strains inevitably arise in Mn+1AXn phases used as nuclear materials. In this work, by using a first-principles method, the strain effects on the defect formation and diffusion in Ti2AlC and Ti3AlC2 were studied by imposing an equibiaxial in-plane strain along the a and b (denoted as ab) axes. The formation energies of Ti vacancies are substantially larger than those of Al and C vacancies. As the imposed equibiaxial strain goes from compressive to tensile state with strain ranges from −4% to 4%, the formation energies of Ti and C vacancies increase, while the formation energy of Al vacancy decreases. The formation energies of the antisite defect TiAl-AlTi, the substitutional defect AlTi, and the substitutional defect TiAl all increase with the applied biaxial strain. In both Ti2AlC and Ti3AlC2, the process of Al atom diffusing to the Ti vacancy is energetically favorable, conversely, the process of Ti atom diffusing to the Al vacancy is energetically unfavorable. Therefore, the diffusion of Al atom is expected to play an important role in the microstructural evolution under irradiation environments. According to our research about the atomic diffusions at different strains, atomic diffusion across the atomic layer can be slowed down under the compressive strain along the ab axes. As a consequence, the MAX materials show better microstructural stability and improved radiation resistance. That is, the irradiation resistance of the MAX materials can be improved when they are applied under a certain compressive strain in nuclear systems.