Showing papers in "Materials Transactions in 2019"

High-Pressure Phase Transformations under Severe Plastic Deformation by Torsion in Rotational Anvils

Abstract: Numerous experiments have documented that combination of severe plastic deformation and high mean pressure during high-pressure torsion in rotational metallic, ceramic, or diamond anvils produces various important mechanochemical effects. We will focus here on four of these: plastic deformation (a) significantly reduces pressure for initiation and completion of phase transformations (PTs), (b) leads to discovery of hidden metastable phases and compounds, (c) reduces PT pressure hysteresis, and (d) substitutes a reversible PTwith irreversible PT. The goal of this review is to summarize our current understanding of the underlying phenomena based on multiscale atomistic and continuum theories and computational modeling. Recent atomistic simulations provide conditions for initiation of PTs in a defect-free lattice as a function of the general stress tensor. These conditions (a) allow one to determine stress states that significantly decrease the transformation pressure and (b) determine whether the given phase can, in principle, be preserved at ambient pressure. Nanoscale mechanisms of phase nucleation at plastic-strain-induced defects are studied analytically and by utilizing advanced phase field theory and simulations. It is demonstrated that the concentration of all components of the stress tensor near the tip of the dislocation pileup may decrease nucleation pressure by a factor of ten or more. These results are incorporated into the microscale analytical kinetic equation for strain-induced PTs. The kinetic equation is part of a macroscale geometrically-nonlinear model for combined plastic flow and PT. This model is used for finite-element simulations of plastic deformations and PT in a sample under torsion in a rotational anvil device. Numerous experimentally-observed phenomena are reproduced, and new effects are predicted and then confirmed experimentally. Combination of the results on all four scales suggests novel synthetic routes for new or known high-pressure phases (HPPs), experimental characterization of strain-induced PTs under high-pressure during torsion under elevated pressure. [doi:10.2320/matertrans.MF201923]

High-Pressure Torsion Deformation Induced Phase Transformations and Formations: New Material Combinations and Advanced Properties

Abstract: Heavy plastic shear deformation at relatively low homologous temperatures is called high-pressure torsion (HPT) deformation, which is one method of severe plastic deformation (SPD). The aim of the paper is to give an overview of a new processing approach which permits the generation of innovative metastable materials and novel nanocomposites by HPT deformation. Starting materials can be either coarse-grained multi-phase alloys, a mixture of different elemental powders or any other combination of multiphase solid starting materials. After HPT processing, the achievable microstructures are similar to the ones generated by mechanical alloying. Nevertheless, one advantage of the HPT process is that bulk samples of the different types of metastable materials and nanocomposites are obtained directly during HPT deformation. It will be shown that different material combinations can be selected and materials with tailored properties, or in other words, materials designed for specific applications and the thus required properties, can be synthesized. Areas of application for these new materials range from hydrogen storage to materials resistant to harsh radiation environments.

Grain Boundaries and Diffusion Phenomena in Severely Deformed Materials

Abstract: The present knowledge on grain boundary-related phenomena specific for severely plastically deformed materials is reviewed and critically analyzed in detail. Severe plastic deformation is shown to introduce specific metastable states of the grain boundaries which are characterized by enhanced diffusion rates, high-density of specific structure elements and large (still localized) elastic strains. An intrinsic heterogeneity of the deformation-induced modifications is revealed and examined on different scales. Relations between the existence of deformation-modified grain boundaries, specific microstructure features and resulting properties are highlighted. [doi:10.2320/matertrans.MF201934]

Strain Induced Segregations in Severely Deformed Materials

Abstract: Severely deformed materials intrinsically contain a large density of crystalline defects like dislocations or boundaries. The interactions between solute atoms or impurities with these defects play a key role in the grain refinement mechanisms as they affect dynamic recovery. This short review manuscript focusses on grain boundary segregations resulting from severe plastic deformation. The important contribution of atom probe tomography for the quantitative characterization of such segregations in various metallic alloys is at first highlighted. Then, a special emphasis is given on the physical mechanisms leading to strain induced segregations and on the connection that is sometimes observed with dynamic precipitation during severe plastic deformation. The last section is devoted to the influence of such grain boundary segregations in ultrafine grained alloys on the mechanical properties and on the thermal stability. [doi:10.2320/matertrans.MF201919]

Quantitative Analysis on Light Elements Solution Strengthening in Pure Titanium Sintered Materials by Labusch Model Using Experimental Data

Abstract: Solid solution strengthening effect by oxygen (O) and nitrogen (N) atoms of α-titanium (Ti) materials was quantitatively evaluated using Labusch model by consideration of the experimental data. When using Labusch model to predict solid solution strengthening improvement, an application of the isotropic strains by solute elements is generally assumed to estimate Fm value, which is the maximum interaction force between the solute atoms and dislocations. It is, however, difficult to exactly calculate Fm value for α-Ti materials with O and N solute atoms because the anisotropic strains are induced in α-Ti crystal with hcp structure by these elements. In this study, Fm value was experimentally derived from the relationship between 0.2% yield stress and solute elements (O and N atoms) content of Ti sintered materials. As a result, the strengthening improvement was proportional to c/Sf (c: soluted atom content, Sf: Schmid factor), and its factor of proportionality of Ti-O and Ti-N materials was 4.17 × 10 and 3.29 × 10, respectively. According to this analysis, it was clarified that Fm value of Ti-O and Ti-N materials was 6.22 × 10 and 5.21 × 10, respectively, and then the estimated strengthening improvement by using these values was significantly agreed with the experimental results of Ti sintered materials with O and N solution atoms.

Bulk-state reactions and improving the mechanical properties of metals through high-pressure torsion

Abstract: This report presents an overview of recent studies demonstrating a bulk-state reaction involving mechanical bonding through the application of high-pressure torsion (HPT) processing on two dissimilar engineering metals. This processing approach was developed by revising the sample set-up and applying the simple procedure of alternately stacking two different metal disks using several different metal combinations. Thus, this report describes the development in microstructure after the bulk-state reactions and the mechanical properties of the HPT-induced Al­Mg, Al­Cu, Al­Fe and Al­Ti alloy systems. A microstructural evaluation confirmed the capability of the HPT procedure for the formation of heterostructures across the disk diameters in these processed alloy systems. Tribology tests and hardness values together with density measurements demonstrated an improved wear resistance and an exceptional specific strength in these alloy systems. The bulk-state reaction by HPT demonstrates a considerable potential for the bonding of dissimilar metals and the fabrication of unique metal systems. [doi:10.2320/matertrans.MF201908]

Texture Evolution in Severe Plastic Deformation Processes

Abstract: Severe plastic deformation processes involve large grain rotations due to the action of different modes of plastic deformation and other microstructural changes which lead to characteristic texture formation. The present review deals with the evolution of texture during the most important severe plastic deformation processes, namely Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Friction Stir Processing (FSP), Accumulative Roll Bonding (ARB) and Multi-Axial Forging (MAF). First three of the processes are shear based, while the latter two are plane-strain based. The textures formed during ECAP are visually different from simple shear textures due to (i) the inclination of the shear plane, (ii) additional contribution of non-shear based deformation. The relative intensities of texture components are function of deformation micro-mechanisms, amount of straining and configuration of the strain path. The texture evolved during HPT is very similar to simple shear texture, with additional consequences of microstructural changes that occur due to very large deformations. The textures formed in FSP process also resemble shear textures. On the other hand, texture evolution during ARB and MAF can be described using plane strain deformation. The present review deals with texture evolution during severe plastic deformation as a function of nature of processes and type of materials.

Severe Plastic Deformation of Semiconductor Materials Using High-Pressure Torsion

Abstract: Severe plastic deformation (SPD) has been widely studied in order to enhance the strength and ductility of metallic materials. Among various SPD processing techniques, high-pressure torsion (HPT) can be applied to various brittle materials including semiconductors. In this overview, we report on the HPT processing of Si, Ge, and compound semiconductor GaAs. When crystalline Si was subjected to HPT, metastable body-centered-cubic (bcc) Si-III and rhombohedral Si-XII as well as amorphous regions were formed. After annealing, Si-III and SiXII reversely transformed to diamond-cubic Si-I. No appreciable photoluminescence (PL) peak was observed from the as-HPT processed samples while a broad PL peak originating from Si-I nanograins appeared after annealing. The electrical resistivity was increased just after compression without anvil rotation, but it decreased after HPT-processing because of the formation of semimetallic Si-III. In the case of Ge, metastable tetragonal Ge-III was formed by room-temperature HPT processing. A broad PL peak originating from diamond-cubic Ge-I nanograins was observed after annealing. The metastable bcc Ge-IV was observed in the cryogenic-HPT-processed samples. In the case of GaAs, no metastable phase was observed in the HPT-processed samples. A strong PL peak associated with the bandgap disappeared after HPT processing. An additional PL peak in the visible light region appeared after annealing. These results suggested that noble properties such as optical and electrical properties can be obtained by applying HPT processing to semiconductor materials. [doi:10.2320/matertrans.MF201907]

An Overview on the Continuous Severe Plastic Deformation Methods

Abstract: Severe Plastic Deformation (SPD) processes have been extensively investigated in the last 30 years to facilitate the production processes of nanostructured (NS) or ultrafine-grained (UFG) materials with unique properties. However, the majority of the efforts were limited on the laboratory-scale investigations not to be able to overcome the demands for industrial scale applications. Researchers have tried to introduce effective industrial methods for processing large and long UFG/NG materials. In this review, all industrial processes especially continuous SPD methods which are more suitable for mass production are categorized and explained. Furthermore, the factors influencing the process efficiency were presented to give a vision to the researchers who want to step in this scientific field. Finally, the industrial processes were compared regarding final microstructure and properties. [doi:10.2320/matertrans.MF201905]

Severe Plastic Deformation of Amorphous Alloys

Abstract: Bulk metallic glasses having a disordered amorphous structure have been in the focus of recent intensive materials research due to their special mechanical properties, however, these materials exhibit brittleness in conventional unconstrained deformation modes. High-pressure torsion as a special severe plastic deformation method, which applies constraints on the material, can induce significant plasticity in metallic glasses. Apart, the deformation can promote structural changes in the glass, such as anisotropy and nanocrystallization. The inhomogeneity generated by torsional shear deformation in bulk metallic glasses can be detected in internal surfaces. The final structure and morphology of the deformed material depend on the processing parameters (deformation rate, shear strain, temperature and pressure) of the high-pressure torsion apparatus. [doi:10.2320/matertrans.MF201917]

Hydrogen Storage in Mg and Mg-Based Alloys and Composites Processed by Severe Plastic Deformation

Abstract: Mg-based hydrides have been extensively studied in the last 20 years due to its great potential as hydrogen storage materials, especially for stationary applications. Severe plastic deformation (SPD) can be used to produce Mg-based materials for hydrogen storage applications, with good activation (first hydrogenation) and H-absorption/desorption kinetics, combined with enhanced air resistance. Both advanced (e.g. highpressure torsion, equal-channel angular pressing) and more conventional (e.g. cold rolling, cold forging) techniques were investigated as means of production of bulk samples with refined microstructures and controlled textures. Depending on the processing parameters, SPD or SPD-like techniques can produce sub-microcrystalline or even nanocrystalline structures, with a fair level of dispersion of the additives and high level of the desired [0002] fibre type texture. In this review we discuss how the processing of hydrogen storage materials by SPD techniques matches the following desirable aspects: fast kinetics (many interfaces ­ nano-grains and additives), easy activation (clean surfaces, interfaces, and adequate texture) and thermodynamic stability (alloying; synergy between phases in composites). The results suggest new and in most cases simpler and cheaper alternatives to produce hydrogen storage materials with proper hydrogen absorption and desorption kinetics and, in the case of composites, lower hydride stability. [doi:10.2320/matertrans.MF201927]

Severe Plastic Deformation of Polymers

Abstract: 1Donetsk Institute for Physics and Engineering named after A.A. Galkin, National Academy of Sciences of Ukraine, pr. Nauki 46, Kyiv, Ukraine 2Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza Street, 112, Lodz, Poland 3Department of Materials Science and Engineering, Monash University, Clayton 3800, Australia 4Department of Mechanical Engineering, The University of Western Australia, Crawley 6009, Australia 5Institute of Nanotechnology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen 76344, Germany

Development of Metal Oxide High-Pressure Phases for Photocatalytic Properties by Severe Plastic Deformation

Abstract: Photocatalytic activity of most metal oxides is restricted to the ultraviolet (UV) range of solar spectrum due to their large band gap. Since UVaccounts for only 5% of the solar spectrum, designing metal oxide semiconductors with capability of absorbing visible light has been widely attempted. The large band gap of metal oxides can be reduced by various methods like doping with metallic or non-metallic ions, however a better photocatalytic activity can not be achieved necessarily by these methods due to fast recombinations of electron and hole. In recent years, authors have paid attention to the high pressure phases of metal oxides, which theoretically possess narrow band gaps, being able to absorb visible light. In this review, high pressure phases of well-known metal oxides like titania (TiO2), zirconia (ZnO), and yttria (Y2O3) have been stabilized by applying a severe plastic deformation method, and photocatalytic properties of them have been evaluated. [doi:10.2320/matertrans.MF201916]