Novel nanoscale precipitate structures in ion-irradiated Cu_1-xV_x alloys: Cherry-pit formation

The nanoscale precipitation induced by 1.8 MeV ion irradiation in Cu_1-xV _x thin films, with x ≈ 0.09 and 0.91, was studied by atom probe tomography. For the Cu₉₁V₉ alloys, irradiation in the range of 300-500 °C led to steady-state compositional patterning of V-rich nanoprecipitates. V nanoprecipitates larger than ∼10 nm in diameter, moreover, contained Cu-rich cores, resulting in an unusual cherry-pit nanostructure. The number of these pits within one precipitate increased with the precipitate size, but with the volume fraction of pits within a given precipitate remaining roughly constant, from ∼1.5 to 5%. Similar irradiations performed on V₉₁Cu₉ also resulted in an enhanced precipitation reaction, but with smaller Cu-rich nanoprecipitates, <3 nm in diameter, and no cherry pits. These results are rationalized using recent atomistic simulations that have explored the conditions for stabilizing by ion irradiation cherry-pit nanostructures in immiscible A-B alloy systems, such as Cu-V.