Investigation of the microstructure of He+ ion-irradiated TiBe12 and CrBe12 using ex-situ transmission electron microscopy

Abstract

Titanium and chromium beryllides, TiBe12 and CrBe12, are materials of potential future importance as neutron multipliers for tritium breeding in nuclear fusion reactors. Beryllium experiences extremely high transmutation according to a n →2n transmutation reaction in which both tritium and helium are produced, which normally form bubbles in solids at the relevant concentration range. Neutron irradiation from the fusion plasma also introduces point defects into solids. The ensuing effect of this environment on the beryllides’ microstructure is poorly characterised, but important for understanding beryllides’ mechanical properties and their evolution in the irradiative environment inside a fusion reactor. This study is intended to initially determine and describe the microstructural features that occur in TiBe12 and CrBe12 when He and fast-particle-induced point defects have been introduced at fusion reactor neutron breeder relevant temperatures. In this study, beryllide samples were implanted with 300 kV He at a range of temperatures between 387 and 900 °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 X-ray spectroscopy (EDX/EDS) and precession diffraction mapping.Nanometre-scale bubbles grew in both TiBe12 and CrBe12 at 600 °C and larger (100+ nm) bubbles, some faceted, grew at 900 °C. Some bubbles in CrBe12 were lined with Cr, with some of the Cr oxidised. TiBe12 developed planar faults, on {110} planes at 600 °C and below but on to {111} at 900 °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 the two types R=12〈011〉 and R=12〈110〉; the present study also found faults that did not match either previously found type. CrBe12 also developed planar faults but the appearance of these was quite different from the typical striped appearance of planar stacking faults and their nature remains unknown. Oxide particles from manufacture were found in both beryllides, most prominently in CrBe12.