1) Mechanical alloying and amorphization were studied in Cu-Nb-Ag in situ composite wires using TEM and atom probe tomography. 2) Results showed nanoscale mixtures of Cu, Nb, and Ag formed through mechanical deformation. High dislocation densities and amorphization were observed at some interfaces. 3) The mechanisms of mechanically-induced mixing and amorphization are discussed, involving increased vacancy concentrations from plastic deformation enabling diffusion, atomic-scale interface roughening, and amorphization in regions of heavy mixing and high dislocation densities. Further studies on these mechanisms and effects on superconductivity are outlined.