Nanomaterials by Severe Plastic Deformation: NanoSPD5

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Authors: Rolf Berghammer, Wei Ping Hu, Arman Hasani, Günter Gottstein
Abstract: An aluminum alloy with two different precipitation states was processed by Equal Channel Angular Pressing up to 16 passes. The thermal stability of the ultra-fine grain structure was examined during annealing at 300°C up to 1 h. As the present precipitates are coarse they don’t contribute to strengthening where as soluted elements improved the strength of the material. Additionally the soluted elements formed dispersoids upon annealing and thereby retarded softening.
Authors: Roberto B. Figueiredo, Terence G. Langdon
Abstract: Disks of an AZ31 magnesium alloy were processed by High-Pressure Torsion (HPT) at 463 K to different numbers of rotations. The grain structure was evaluated along the cross-section of the disks using optical microscopy. Significant heterogeneities in the average grain size were observed in areas of the disks which were located at similar distances to the center but at different distances from the surface. Moreover, different grain structures were observed in neighboring areas and shear bands occurred at several locations in the disks. Microhardness tests revealed differences in the strength of the material as a function of the distance to the surface. An analysis of the grain structure and hardness distribution suggests the occurrence of flow localization in HPT processing.
Authors: Wei Ping Hu, Si Yuan Zhang, Xiao Yu He, Zhen Yang Liu, Rolf Berghammer, Günter Gottstein
Abstract: An aged Al-5Zn-1.6Mg alloy with fine η' precipitates was grain refined to ~100 nm grain size by severe plastic deformation (SPD). Microstructure evolution during SPD and mechanical behaviour after SPD of the alloy were characterized by electron microscopy and tensile, compression as well as nanoindentation tests. The influence of η' precipitates on microstructure and mechanical properties of ultrafine grained Al-Zn-Mg alloy is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy.
Authors: Daichi Akama, Z. Horita, Kenji Matsuda, Shoichi Hirosawa
Abstract: This research investigates simultaneous strengthening by grain refinement and fine precipitation in age-hardenable Al-Mg-Si alloys containing an additional element of either Ag, Cu, Pt or Pd. The alloys were solution-treated and processed by high-pressure torsion (HPT) at room temperature under a pressure of 6 GPa. They were aged at a temperature of 373 K for up to a total period of 6.7 hours. Vickers microhardness was measured after selected periods of aging and the microstructures were observed by transmission electron microscopy. It was found that, in all alloys, the grain sizes after HPT were refined to 300-400 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent ageing treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying compositions.
Authors: Yu Zeng Chen, Andreas Herz, Reiner Kirchheim
Abstract: Based on a novel defactants (defect acting agents) concept (R. Kirchheim, Acta Materialia 55 (2007) 5129 and 5139), a novel method of understanding and synthesizing NC material was proposed by introducing defactants (segregating solute atoms) into the materials to ease the formation of grain boundaries (GBs) and enhance the formation ability of nanocrystalline (NC) structures. The iron-carbon system was chosen as a model system where carbon acts as the so-called defactant. Iron powders mixed with different amount of graphite were ball milled to prepare NC iron-carbon alloys with different carbon concentrations (C0). After ball milling, the as-milled powder with relatively low carbon concentration was annealed at a certain temperature to achieve saturation of GBs by carbon atoms. The microstructures of the powders were investigated by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. The mean grain sizes (D) of the powders were determined by analyzing TEM dark field images and X-ray line profiles. The results indicated that once the saturation of GBs is achieved, D of the NC iron-carbon powders will be strongly dependent on C0 and will follow a simple mass balance of carbon in a closed system, i.e. D=3ΓgbVm/(C0-Cg) with Cg the carbon concentration in grains, Γgb the grain boundary excess, and Vm the molar volume of iron. Based on the experimental results, the formation of NC iron-carbon alloys was treated in detail within the framework of the defactant concept. The increase of C0 significantly reduces the formation energy of GBs, leading to a substantial decrease of D.
Authors: Sai Yi Li, Hao Li
Abstract: An experimental characterization of texture evolution during equal channel angular extrusion (ECAE) of pure copper was conducted up to 8 passes considering an extended range of processing routes. These routes are featured by 0°, 45°, 90°, 135°, and 180° rotation about the billet longitudinal axis after each pass, and were designated as R0, R45, R90, R135, and R180, respectively. They were implemented using new die designs with the cross-section of the die channels as a 24-sided regular convex polygon and with die angle (Φ) of 90° and 120°, respectively. X-ray diffraction measurements show that for both die sets, the textures developed via the different routes all show orientation concentrations along fibers with the {111} planes parallel to the macroscopic simple shear plane and <110> directions parallel to the macroscopic simple shear direction, yet the locations and orientation densities of the main texture components vary significantly with the pass number and the processing route. After 4 to 8 passes, the texture is found to be the weakest via route R180 for both die sets, and strongest via R0 or R45. For a given route and pass number, the texture developed with Φ = 120° is generally weaker than its counterpart with Φ = 90°. These results thus confirm the general tendencies of texture development in face-centered cubic metals with {111}<110> slip as the dominant deformation mechanisms, albeit in a wide range of processing route or deformation history.
Authors: Saleh N. Alhajeri, Megumi Kawasaki, Nong Gao, Terence G. Langdon
Abstract: Disks of a commercial purity aluminium Al-1050 alloy and Al-1%Mg alloy were processed by high-pressure torsion (HPT) at room temperature for up to a maximum of 5 turns under a pressure of 6 GPa. Following processing, hardness measurements were recorded across the surfaces of the disks. These measurements showed low values of hardness at the center and high values near the edges of the disks and the hardness increased in both alloys with increasing numbers of turns. The evolution of homogeneity in hardness was rapid in Al-1050 compared to the Al-1%Mg alloy. After 5 turns of HPT under a pressure of 6 GPa, the hardness was fully homogeneous across the total surface of the Al-1050 disk whereas there was a region of lower hardness around the center of the Al-1%Mg disk. The results reveal the significant difference between both alloys where the higher rate of recovery in the Al-1050 alloy leads to a rapid evolution of the hardness homogeneity.
Authors: Sergiy V. Divinski, K. Anantha Padmanabhan, Gerhard Wilde
Abstract: Systematic radiotracer diffusion studies on metals present in severely deformed, ultra-fine grained (UFG) states have revealed the existence of ultra-fast transport paths, which include the so-called “non-equilibrium” grain boundaries and other defects including excess free volume. Under certain experimental conditions percolating porosity is produced even in a ductile metal like pure copper. This result indicates the importance of the cavitation phenomena in severe plastic deformation under those conditions. It is well known that micro-cracking can take place in metals rather early, if the local maximum shear stress equals or exceeds the shear yield stress of the material. However, the growth and propagation of these cracks will be postponed till very late in the deformation process because of the intrinsic ductility of metals, the effect of the superimposed hydrostatic component of the stress system and/ or concurrent dynamic recovery/ recrystallization, when the latter two are present (which is likely to be the case, if the severe plastic deformation operation is successful). That is, the stage in which crack growth and propagation is present represents a material state in which the scope for further deformation is exhausted and fracture processes have taken over. Using these and similar ideas, the load required for equal channel angular pressing, the change in the slope of the Hall-Petch plot with decreasing grain size and the theoretical limit for the smallest grain size attainable in a metal subjected to a severe plastic deformation (SPD) process are predicted and checked against experimental results.
Authors: Gennady A. Salishchev, N.D. Stepanov, A.V. Kuznetsov, Sergey V. Zherebtsov, Oleg R. Valiakhmetov, A.A. Kuznetsov, Sergey V. Dobatkin
Abstract: Evolution of micro- and macrostructure and mechanical properties of oxygen-free copper after MAF at room temperature was studied. MAF included sequential upsetting and drawing with total cycles number equal to 20 and maximum strain ≈50. MAF causes the formation of homogenous UFG structure with a grain/subgrain size of 0.3 m and fraction of high angle boundaries 50%, but macrostructure is heterogeneous. Rough shear macrobands areas of different orientation are observed. MAF results in significant strengthening from 280 MPa to 445 MPa, but samples remain very ductile even after large strains. Mechanisms of UFG structure formations during MAF are discussed.
Authors: N.D. Stepanov, A.V. Kuznetsov, Gennady A. Salishchev, Georgy I. Raab, Ruslan Valiev
Abstract: Commercial purity copper was subjected to ECAP and subsequent cold rolling. Structure and mechanical properties were studied using EBSD analysis, TEM and tensile tests. Effect of ECAP number passes on grain size and fraction of high angle boundaries after cold rolling was investigated. Rolling results in grain refinement and HABs fraction increase the more ECAP number passes. UTS increases significantly after rolling. Increase of strength is accompanied by loss of plasticity. Evolution of microstructure and mechanical properties is discussed.

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