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Type Amouyal, Yaron
  Publication Thermodynamics and kinetics of grain boundaries in ultra fine grained copper produced by severe plastic deformation Volume Book Whole
Pages 2007
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  Summary Language 148 Series Editor  
Abbreviated Series Title Reducing the average grain size of polycrystalline metals and alloys is a traditional way of increasing their strength. Moreover, many other attractive properties can be achieved by reducing average grain size: low-temperature superplasticity, improved magnetic properties, and homogeneity of physical properties. 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. Its main principle is pressing a metal billet through an angular channel, a process that involves extremely large shear deformations forming dislocation cell structure at submicron scale. Subsequent pressings result in the formation of ultra-fine grains (UFG) with high-angle grain boundaries (GBs). Many unusual properties of materials produced by ECAP are attributed to nonequilibrium grain boundaries. 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 is, therefore, providing an adequate experimental proof for the concept of non-equilibrium GBs by measurements of GB diffusivity and energy in copper and copper alloys subjected to ECAP. The diffusion of 63Ni radiotracer in Cu and Cu-Zr alloy was studied using the serial-sectioning method. The diffusion annealings were performed in the temperature range 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 former is very close to that of relaxed GBs in coarse-grained Cu. Based on the analysis of the activity profiles, we proposed a hierarchical microstructure model of the UFG Cu-Zr alloy studied. In this model, a cellular skeleton of “fast” GBs with the characteristic cell size in the micrometer range is embedded in a network of “slow” GBs formed by sub-micrometer grains. This model allowed a quantitative processing of the measured activity profiles. The Arrhenius parameters of the GB diffusivities for the “slow” and “fast” GBs were determined, indicating a 3-4 orders of magnitude difference in respective pre-exponential factors. The measured radiotracer penetration profiles in pure ECAP-ed Cu exhibited a bimodal shape similar to that observed in the Cu-Zr alloy. In contrast to the Cu-Zr alloy, the pure Cu exhibited recrystallization during all thermal annealings. The explicit expression describing the kinetics of recrystallization in ECAP-ed Cu was obtained. A model that considers diffusion in UFG polycrystal undergoing recrystallization was developed. Its main assumption is that diffusion flux is allowed in the UFG phase only, while the recrystallizing grains “freeze” the concentration of solutes existing in the UFG matrix before it was consumed by recrystallizing grain. Application of this model enabled us deriving the slow-diffusion coefficients from the experimentally measured penetration profiles. The Arrhenius parameters of the GB diffusivities for the “slow” and “fast” GBs were determined, indicating about 3 orders of magnitude difference in respective pre-exponential factors. The relative energies of GBs in ultrafine grain copper obtained by ECAP were determined using the thermal grooving technique. The dihedral angles at the roots of GB grooves formed after annealings at 400 °C for 15 min and at 800 °C for 2h were determined with the aid of AFM. The average relative GB energies in the ECAP-ed samples annealed at 400 and 800 °C are 0.48 ± 0.11 and 0.27 ± 0.07 , respectively. Theoretical estimates of the relaxation time of non-equilibrium GBs indicated that little relaxation should occur after annealing at 400 °C, while full relaxation is expected after annealing at 800 °C. It was shown that the measured difference in GB energies can be correlated with the presence of two types of GBs in the same sample exhibiting very different diffusivities. We associated the fast-diffusion paths with unusually high GB diffusivities, and the high-energy GBs observed by AFM with the non-equilibrium GBs that were formed during ECAP. The volume fraction of such boundaries is small and they are separated by an extensive network of normal (i.e. exhibiting usual GB diffusivities and energies characteristic for annealed coarse grain polycrystals) GBs. These findings provide a solid experimental foundation for the concept of non-equilibrium GBs.
  Series Issue ISSN Senate of the Technion Israel Institute of Technology  
Medium Ph.D. thesis
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no NU @ karnesky @ 9840
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