Irradiation-Induced Nanoprecipitation in Ni-W Alloys

The evolution of Ni-W alloy thin films subjected to Kr irradiation at room temperature and subsequent annealing at 1123 K (850 °C) was studied by X-ray diffraction and transmission electron microscopy. Irradiation resulted in significant increase in grain size, from ~20 nm in the as-grown state to over 300 nm after irradiation and annealing. The compositions selected for the study, 18 and 23 at. pct W, resulted in the formation of an ordered Ni₄W matrix after annealing. Remarkably, in the Ni-23 at. pct W films, irradiation followed by annealing induced the precipitation of two families of Ni₂W₄C carbides, large blocky ones at grain boundaries, and intragranular nanocarbides, ~5 to 20 nm in size and with a high number density, 9.0 × 10²²m⁻³. In contrast, only blocky Ni₆W₆C carbides formed in control specimens directly subjected to annealing. The intragranular Ni₂W₄C nanocarbides displayed an orientation relationship with the Ni₄W matrix, and they appear to be effective traps for implanted Kr ions, since nanobubbles formed on their periphery. The results suggest that non-equilibrium processing can be used to nucleate nanocarbides in the grain interiors of Ni-W alloys, and that this may improve alloy properties, including radiation resistance.