A Transmission Electron Microscope and Atom-probe Tomographic Study of the Microstructure of a Ni-Mo Based "Hastelloy" Alloy after 10 MeV Electron Irradiation

Ni-Mo based "Hastelloy" alloys are structural materials for the accelerator-driven molten-salt reactor, in which they are subjected to intense radiation damage. This causes a complete alteration of the microstructure. Since atomic collisions are responsible for inducing, modifying and activating the kinetic processes which alter the microstructure, high energy electron or proton irradiation is suitable to simulate the microstructural changes. A "Hastelloy" alloy with the nominal composition Ni-11.7Mo-6.7Cr-1.5Fe-0.8Al-0.5Ti-0.5Mg-0.5Nb-0.15Si-0.05Y (wt.%) was investigated in this study. The material was homogenized at 1100degC for 1 h and aged at 675degC for 50 h. Scanning electron microscopy (SEM) studies demonstrate decoration of the grain boundaries by large Ni-Ti-Si precipitates with a high content of Y (up to 30 at.%). Local-electrode atom-probe (LEAP) tomographic analyses reveal additional small precipitates with a cylindrical morphology. The composition of these precipitates is 73.8Ni-14.8Al-3.7Ti-3.6Mo-1.7Cr (at.%) and small amounts of Mn, Fe, Si and Nb. This composition corresponds to an intermetallic phase with an A₃B structure. Additionally transmission electron microscopy (TEM) analyses show that the precipitates have a mean diameter of 22 nm and are coherent with the fee matrix. The number density of these precipitates is about 2 times 10 ²¹ m^-3. Electron diffraction patterns from these precipitates exhibit superlattice reflections corresponding to the ordered L1₂ structure. For studying the microstructural changes during irradiation, the alloy was irradiated with 10 MeV electrons at 650degC for 700 h. The dose was 2 times 10^-3 dpa (displacement per atom). The large precipitates at the grain boundaries remain unchanged by the electron irradiation. The small precipitates with the cylindrical morphology, however, are no longer visible. Instead, new small precipitates with an irregular morphology are o- bserved. The number density of these new precipitates is of the same order as that of the A₃B precipitates before irradiation. The L1₂ diffraction spots disappear completely, instead diffuse diffraction spots are observed at lang1 1/2 0rang _fcc, which are attributed to short range order (SRO). An ordering transformation from L1₂ to lang1 1/2 0rang_fcc requires a change of the atom positions. SRO with lang1 1/2 0rang_fcc superstructure reflections have been found in Ni₃Mo or in Ni₄Mo precipitates in Ni-Mo alloys after thermal treatment. However, the content of Mo in the investigated "Hastelloy" alloy is significantly lower than that of these alloys. In both cases the ordering transformation requires changes of atom positions, which are induced by thermally activated transport or by radiation-induced diffusion, respectively. In the present case the irradiation damage may lead to the formation of metastable phases