Microstructural stability of an Ni-Mo based Hastelloy after 10 MeV electron irradiation at high temperature

The microstructural changes of a Hastelloy alloy under 10 MeV electron irradiation at 650 °C for 700 h to a total fluence of 2 × 10 ^-3 dpa (displacements per atom) were investigated by transmission electron microscopy and by 3- dimensional atom-probe tomography utilizing a local-electrode atom-probe tomograph. Transmission electron microscopy analysis before irradiation reveals two types of precipitates: (i) spheroidal with a mean diameter of 22 nm; and (ii) ones with an irregular morphology of a few nm in diameter. The spheroidal precipitates (i) are coherent with the fcc matrix and their number density is ̃3 ×10²¹ m^-3. Electron diffraction patterns from these precipitates exhibit superlattice reflections corresponding to the L12 ordered structure. The chemical composition as measured by atomprobe tomography, is about 75 at.% Ni with additions of Al, Ti and Mo. The other type of precipitate (ii) is mainly enriched in Ni and Mo and its number density is ca. 3.6 · 10²² m^-3. The diffuse diffraction intensity maxima arising from these precipitates are observed at <11/20>fcc, which are attributed to compositional short-range order. After electron irradiation, the Ni -Al-rich precipitates (i) exhibiting L12 superlattice reflections have disappeared completely, while the diffuse intensity maxima from the Ni -Mo rich clusters (ii) are still visible. The results are discussed with respect to the influence of the electron irradiation on the morphology and structural changes of the ordered precipitates.