Chemical mixing and self-organization of Nb precipitates in Cu during severe plastic deformation

Severe plastic deformation (SPD) of Cu, Ag and Nb ternary alloys with average Nb concentrations of 5, 10 and 15 at.% and an Ag concentration of 10 at.% was studied using high resolution scanning electron microscopy, atom probe tomography and X-ray diffraction. Ball-milled powders were compacted, annealed at 600 or 950 °C and subsequently strained by high pressure torsion at room temperature. Ag redistributed homogeneously in the Cu matrix during SPD, but the Nb solubility increased by less than 1%. The Nb precipitates, moreover, self-organized into a patterned microstructure, whereby their sizes all fell within a narrow distribution, centered on ∼20 nm. This size was independent of the initial microstructure for a given alloy and insensitive to the Nb concentration. The size was also insensitive to the method of shearing. Since Nb diffusion is negligible in Cu at room temperature, these results establish that SPD alone can induce compositional patterning in highly immiscible alloys. We discuss the mechanisms responsible for this behavior.