Deviating from the pure MAX phase concept: Radiation-tolerant nanostructured dual-phase Cr2AlC.

A dual-phase Cr2AlC material was synthesized using magnetron sputtering at a temperature of 648 K. A stoichiometric and nanocrystalline MAX phase matrix was observed along with the presence of spherical-shaped amorphous nano-zones as a secondary phase. The irradiation resistance of the material was assessed using a 300-keV Xe ion beam in situ within a transmission electron microscope up to 40 displacements per atom at 623 K: a condition that extrapolates the harmful environments of future fusion and fission nuclear reactors. At the maximum dose investigated, complete amorphization was not observed. Scanning transmission electron microscopy coupled with energy-dispersive x-ray revealed an association between swelling due to inert gas bubble nucleation and growth and radiation-induced segregation and clustering. Counterintuitively, the findings suggest that preexisting amorphous nano-zones can be beneficial to Cr2AlC MAX phase under extreme environments.

Effect of aluminium concentration on phase formation and radiation stability of Cr2Al x C thin film.

Near-stoichiometric and under-stoichiometric Cr2Al x C (x = 0.9 and 0.75) amorphous compositions were deposited onto a silicon substrate at 330 K in a layer-by-layer fashion using magnetron sputtering from elemental targets. The film thickness was found to be 0.9 µm and 1.2 µm for the near- and under-stoichiometric compositions respectively. A transmission electron microscope (TEM) heating holder was used to heat thin sample lamellae prepared using focused ion beam milling. Near-stoichiometric Cr2AlC thin films consisted of nano MAX phase after crystallization at 873 K. Under-stoichiometric Cr2Al x C (x = 0.75) thin films contained MAX phase along with nanocrystalline chromium aluminides after crystallization at 973 K. Irradiations with 320 keV xenon ions was performed at 623 K using a TEM with an in-situ ion irradiation (MIAMI) facility. Nanocrystalline films of near-stoichiometric Cr2AlC irradiated up to 83 displacements per atom (dpa) showed no observable changes. Also, irradiation of under-stoichiometric nanocrystalline thin films up to 138 dpa did not show any observable amorphization, and recrystallization was observed. This radiation resistance of near- and under-stoichiometric thin films is attributed to the known self-healing property of Cr2Al x C compositions further enhanced by nanocrystallinity.