Irradiation of TiBe12 and CrBe12 with He+ ions and ex-situ transmission electron microscopy to investigate ion damage as a proxy for neutron irradiation damage in nuclear fusion

Abstract: Ti and Cr beryllides are materials of potential importance as neutron multipliers for tritium breeding in nuclear fusion reactors. The n-to-2n transmutation reaction also produces tritium and helium, which form bubbles. Neutron irradiation from fusion plasma also introduces point defects. The effect on beryllides' microstructure is important for understanding their mechanical properties and evolution in the fusion reactor environment. This study determines and describes the microstructural features that occur in TiBe12 and CrBe12 when He and fast-particle-induced point defects are introduced at fusion reactor neutron breeder relevant temperatures. Beryllide samples were implanted with 300kV He at temperatures between $387-900^{\circ}C$, sectioned down through the implantation surface with a focused ion beam post-irradiation, and the resulting microstructures examined using transmission electron microscopy, electron-dispersive spectroscopy (EDS) and precession diffraction mapping. Nanometre bubbles grew in both TiBe12 and CrBe12 at $600^{\circ}C$; larger (100 nm and over) bubbles, some faceted, grew at $900^{\circ}C$. Some bubbles in CrBe12 were lined with Cr, some oxidised. TiBe12 developed planar faults, on ${110}$ planes at $600^{\circ}C$ and below but on ${111}$ at $900^{\circ}C$. Faults were preferentially associated with large bubbles. The displacement vectors of faults on the ${110}$ planes had some commonality with previous studies that found displacement vectors of families $R=\langle 011\rangle$ and $R= \langle 110\rangle$; the present study also found faults that matched neither previously found type. CrBe12 also developed planar faults with an appearance quite different from the typical striped appearance of planar stacking faults; their nature remains unknown. Oxide particles were found in both beryllides, most prominently in CrBe12.