Shear induced mixing and self-organization in immiscible alloys during severe plastic deformation.

摘要

Metals and alloys processed by severe plastic deformation (SPD) techniques are gaining widespread interest for applications where bulk nanostructured materials are desired. This dissertation research focuses on understanding the underlying mechanisms and evolution of the microstructure in Cu-Ag, an immiscible alloy, during SPD. The two key features of SPD processing that will be presented are shear induced chemical mixing of alloying elements at room temperature and self-organization at elevated temperatures. Shear induced mixing mechanism in a moderately immiscible system is often attributed to the glide of dislocations but the details of how such dislocation glide effects mixing remains an unresolved issue. Self-organization in an immiscible system is a result of the competition between shearing induced mixing and thermally induced phase separation. Understanding the effects of temperature and shearing rate during processing by SPD is limited which hinders the validation of relevant models. In this thesis, a combination of high energy ball milling (BM) and high pressure torsion (HPT) experiments are performed on a model Cu-Ag system between room temperature and 400 °C at strain rates ranging from 0.1 to 6.25 s-1. Characterization of the shear induced mixing and self-organization is carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). The experimental results support the prediction of a 'superdiffusive' model for shear induced mixing in Cu-Ag system, where the dissolution rate of particles is controlled by the rate of dislocation glide across interfaces. Moreover, the steady state Ag sizes during self-organization at elevated temperatures exhibited limited strain rate dependence and low apparent activation energy (0.39 eV). At 400 °C, shear enhanced diffusion is observed at highest deformation rates. The results suggest that shear induced vacancies play a dominant role during self-organization.