Microstructural stability of nanostructured Cu-Nb-W alloys during high-temperature annealing and irradiation

The microstructural evolution of dilute Cu-based ternary Cu-Nb alloys during high-temperature annealing was investigated using a combination of transmission electron microscopy, X-ray diffraction and kinetic Monte Carlo (KMC) simulations. The experiments showed that, during annealing, the Cu ₉₀Nb₁₀ binary alloy undergoes extensive coarsening at 600 °C, with precipitate sizes increasing to >40 nm. Additions of just 1.5 at.% W to this alloy, however, dramatically suppresses the growth; the average precipitate size in the ternary alloy now increases to only ∼10 nm at 600 °C, and only to 18 nm at 700 °C. On annealing at still higher temperatures, the precipitate size then, surprisingly, decreases. The precipitate size distribution at the higher temperatures, moreover, is bimodal. It is also observed that irradiation has no effect on the microstructure of the ternary alloys above 600 °C. KMC simulations indicate that the saturation of the average precipitate size, followed by its decrease as the annealing temperature is increased and the development of a bimodal size distribution can be explained by competition between thermodynamic and kinetic effects during precipitation in this ternary alloy.