|Ph.D Student||Amouyal Yaron|
|Subject||Thermodynamics and Kinetics of Grain Boundaries in Ultra|
Fine Grained Copper Produced by Severe Plastic
|Department||Department of Materials Science and Engineering||Supervisors||Professor Eugen Rabkin|
|Professor Elazar Gutmanas|
|Full Thesis text|
Reducing the average grain size of polycrystalline metals and alloys is a traditional way of increasing their strength. The recently developed technique of Equal Channel Angular Pressing (ECAP) allowed a breakthrough in decreasing the grain size of bulk materials to the sub-micrometer level. Many unusual properties of materials produced by ECAP are attributed to non-equilibrium grain boundaries (GBs). These GBs are expected to exhibit higher values of energy, higher amplitude of strain fields, larger free volume, and higher diffusivity than their relaxed counterparts. Although the concept of non-equilibrium state of GBs is theoretically well-established, its experimental foundation is still controversial. The aim of the present study was to provide an adequate experimental proof for the concept of non-equilibrium GBs.
In the present study, the diffusivity of 63Ni radiotracer in ECAP-processed Cu and Cu-Zr alloy was measured in the low-temperature range of 150 °C - 350 °C for annealing times when volume diffusion is frozen and only short-circuit diffusion occurs. The microstructure studies by Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and Focused Ion Beam (FIB) microscopy indicated that alloying with Zr is essential for stabilizing the ECAP-processed alloys against grain growth and recrystallization. In all samples studied the experimentally-acquired diffusion profiles exhibited two distinct slopes, which are associated with "slow"- and "fast" diffusion paths. The diffusivity of these "slow" diffusion paths is very close to that of relaxed GBs in coarse-grained Cu. Further AFM studies aimed at estimating the relative energy of GBs yielded a clear distinction between high- and low-energy GB populations appearing in the as-ECAPed and relaxed samples, respectively.
We associated the fast-diffusion paths observed in the radiotracer experiments, as well as the high-energy GBs observed by AFM, with the non-equilibrium GBs being formed during ECAP. The volume fraction of such boundaries is small and they are separated by an extensive network of normal (slow-diffusion, low-energy) equilibrated GBs. These findings supply experimental foundation for the concept of non-equilibrium GBs.