Structural and chemical characterization of Cu-Ag and Ni-Ag nanocomposites synthesized by high-energy ball-milling

Mechanical alloying is widely used to synthesize nanostructured materials. In the case of alloys with positive heat of mixing, it is also sometimes possible to force a solid solution by low temperature milling (e.g., Cu-Ag), or to stabilize a nanocomposite by intermediate temperature milling (e.g., Cu-Ag or Ni-Ag). In order to gain insight on the fundamental understanding of these driven phase transformations, and in order to be able to optimize these nanostructures, a systematic study on the steady-states reached by temperature-controlled high energy ball-milling has been undertaken. In addition to standard x-ray diffraction and scanning calorimetry measurements, field ion microscope atom probe (AP), transmission and transmission scanning electron microscopy (TEM, STEM) are used to characterize the ball-milled powders. It is shown that the Cu-Ag microstructure after low temperature milling can exhibit a pronounced texture, with a complex microstructure. At higher milling temperature dynamic recrystallization leads to small equiaxed nanograins that are randomly oriented. In the Ni-Ag case, the absence of texture in the Ni-rich phase, as well as the very small grain size, suggest that full mixing of the species cannot take place because of the differences in mechanical properties of the terminal phases.