Helium-driven element depletion and phase transformation in irradiated Ti3SiC2 at high temperature

Abstract

Future nuclear reactors and advanced power generators require materials with good stability and damage tolerance under harsh conditions, including high temperatures and high-dose radiation. Ti3SiC2 MAX phase has good physical properties and mechanical strength. It can remain crystalline under serious microstructure damage due to the nanolaminate structure. In this study, the effects of helium in irradiated Ti3SiC2 at up to 1100 °C were investigated by microstructural and chemical composition analysis. The concentrated helium can grow into large bubbles without significant confinement or capture by the nano-laminated layers. A new hexagonal to fcc phase transformation mechanism, driven mainly by the evolution of the helium bubbles accompanied by Si diffusion and depletion, is found and investigated. Si interstitials are forced to move out from the peak helium region by the helium evolution and segregate at the outermost surface, forming a thin Si-O layer, at 1100 °C. The formation of the fcc phase is the result of chemical compositional changes and local compressive stress contributed by He bubbles.