Forced atomic mixing during severe plastic deformation: Chemical interactions and kinetically driven segregation

Shear mixing of the ternary alloy system Ag–Cu–Ni during ball-milling and high-pressure torsion was investigated to elucidate the effects of chemical interactions on phase formation. First, ball-milling of pure Ni with homogeneous Ag67Cu33 alloy powders at room temperature (RT) was studied for average Ni atomic concentrations of 4%, 9%, 15% and 25%. Additional samples with an average composition of Ag50Cu25Ni25 were ball-milled at ∼−15 °C or subjected to high pressure torsion at ∼−125 °C. X-ray diffraction and atom probe tomography measurements showed that Cu largely transferred from the Ag–Cu alloy phase to the Ni-rich phase at all temperatures, but that Ag and Ni did not significantly intermix. The Cu concentration in the steady state, moreover, was surprisingly higher in the Ni-rich phase than in the Ag-rich phase, and it was further enriched at the interphase boundary, even at −125 °C. High-resolution transmission electron microscopy revealed that the sizes of the Ni/Cu precipitates and the grain size of the Ag-rich matrix were reduced to a few nanometers during RT or cryo-ball-milling, which is much finer than those observed after ball-milling of Cu–Ag or Ni–Ag binary powders. These findings illustrate that chemical effects can play an important role in phase formation during severe plastic deformation, but they also show that other kinetic factors can influence the final microstructure as well.