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This report summarizes a recent study demonstrating simple and rapid synthesis of a new Al–Mg alloy system and ultimately synthesizing a metal matrix nanocomposite, which was achieved by processing stacked disks of the two dissimilar metals by conventional high-pressure torsion (HPT) processing. The synthesized Al–Mg alloy system exhibits exceptionally high hardness through rapid diffusion bonding and simultaneous nucleation of intermetallic phases with increased numbers of HPT turns through 20, and improved plasticity was demonstrated by increasing strain rate sensitivity in the alloy system after post-deformation annealing. An additional experiment demonstrated that the alternate stacking of high numbers of dissimilar metal disks may produce a faster metal mixture during HPT. Metal combinations of Al–Cu, Al–Fe, and Al–Ti were processed by the same HPT procedure from separate pure metals to examine the feasibility of the processing technique. The microstructural analysis confirmed the capability of HPT for the formation of heterostructures across the disk diameters in these processed alloy systems. The HPT processing demonstrates a considerable potential for the joining and bonding of dissimilar metals at room temperature and the expeditious fabrication of a wide range of new metal systems.
The Al–Mg–Sc alloys have become important materials in research conducted on superplasticity in aluminum-based alloys. Many results are now available and this provides an opportunity to examine the consistency of these data and also to make a direct comparison with the predicted rate of flow in conventional superplasticity. Accordingly, all available data were tabulated with divisions according to whether the samples were prepared without processing using severe plastic deformation (SPD) techniques or they were processed using the SPD procedures of equal-channel angular pressing or high-pressure torsion or they were obtained from friction stir processing. It is shown that all results are mutually consistent, the measured superplastic strain rates have no clear dependence on the precise chemical compositions of the alloys, and there is a general agreement, to within less than one order of magnitude of strain rate, with the theoretical prediction for superplastic flow in conventional materials.
High-pressure torsion (HPT) is a processing technique in which samples are subjected to a high pressure and torsional straining. Anvil alignment and anvil roughness are two important factors related to the successful application of the HPT processing technique. Using a two-phase duplex stainless steel as a model material, experiments were conducted by placing the anvils in different amounts of initial misalignment. Experiments show that the flow patterns (the development of double-swirl patterns) in HPT are dependent upon the alignment of the anvils within the HPT facility. Through carefully designed experiments, it is shown that the presence of a double-swirl is a feature of HPT processing when the initial positions of the anvils have a small lateral misalignment. The effect of the double-swirl patterns on the hardness evolution was also evaluated quantitatively. By comparing the flow patterns developed on the disc upper surface using both rough and smooth anvils with a fixed anvil misalignment, it was demonstrated that there are some differences in the flow patterns which are dependent upon the anvil surface roughness.
Disks of commercial Al-1050 and ZK60A alloys were stacked together and then processed by conventional high-pressure torsion (HPT) through 1 and 5 turns at room temperature to investigate the synthesis of an Al–Mg alloy system. Measurements of microhardness and observations of the microstructures and local compositions after processing through 5 turns revealed the formation of an ultrafine multi-layered structure in the central region of the disk but with an intermetallic β-Al3Mg2 phase in the form of nano-layers in the nanostructured Al matrix near the edge of the disk. The activation energy for diffusion bonding of the Al and Mg phases was estimated and it is shown that this value is low and consistent with surface diffusion due to the very high density of vacancy-type defects introduced by HPT processing. The results demonstrate a significant potential for making use of HPT processing in the preparation of new alloy systems.
A CoCrFeNiMn high-entropy alloy (HEA), in the form of a face-centered cubic (fcc) solid solution, was processed by high-pressure torsion (HPT) to produce a nanocrystalline (nc) HEA. Significant grain refinement was achieved from the very early stage of HPT through 1/4 turn and an nc structure with an average grain size of ∼40 nm was successfully attained after 2 turns. The feasibility of significant microstructural changes was attributed to the occurrence of accelerated atomic diffusivity under the torsional stress during HPT. Nanoindentation experiments showed that the hardness increased significantly in the nc HEA during HPT processing and this was associated with additional grain refinement. The estimated values of the strain-rate sensitivity were maintained reasonably constant from the as-cast condition to the nc alloy after HPT through 2 turns, thereby demonstrating a preservation of plasticity in the HEA. In addition, a calculation of the activation volume suggested that the grain boundaries play an important role in the plastic deformation of the nc HEA where the flow mechanism is consistent with other nc metals. Transmission electron microscopy showed that, unlike conventional fcc nc metals, the nc HEA exhibits excellent microstructural stability under severe stress conditions.
The paradox of strength and ductility is now well established and denotes the difficulty of simultaneously achieving both high strength and high ductility. This paradox was critically examined using a cast Al–7%Si alloy processed by high-pressure torsion (HPT) for up to 10 turns at a temperature of either 298 or 445 K. This processing reduces the grain size to a minimum of ∼0.4 μm and also decreases the average size of the Si particles. The results show that samples processed to high numbers of HPT turns exhibit both high strength and high ductility when tested at relatively low strain rates and the strain rate sensitivity under these conditions is ∼0.14 which suggests that flow occurs by some limited grain boundary sliding and crystallographic slip. The results are also displayed on the traditional diagram for strength and ductility and they demonstrate the potential for achieving high strength and high ductility by increasing the number of turns in HPT.
Deformation mechanism maps are well established in the field of high temperature creep for materials having conventional coarse grain sizes but they are almost unknown within the field of nanostructured materials. This paper summarizes the background to deformation mechanism mapping, presents simplified examples that may be used to easily construct appropriate maps for any selected condition, demonstrates the potential extension of this approach to other areas such as creep fracture, and then considers the potential limitations associated with using the same approach to predict the deformation mechanisms in true nanostructured materials. Two representative deformation mechanism maps are shown for an ultrafine-grained alloy processed either by equal-channel angular pressing or by high-pressure torsion.
Impurity effects were investigated in (Ba,Sr)TiO3 (BST) systems in order to suppress leakage currents under relatively low oxygen pressure conditions by Pulsed Laser Deposition (PLD). We tried to dope transition metals, such as Mo, Mn, Cr, W and Fe into the BST target and used the targets to fabricate the films. By measuring electrical properties, we found Fe-doping had a significant effect on suppressing leakage current. Subsequently, we changed the amount of Fe doping from 0.1mol% to 6%. As a result, with post annealing, the sample with Fe:4% showed the lowest leakage current among those analyzed. Even without post annealing, the sample with Fe:6% showed the lowest leakage current. As for the dielectric constants, they decreased as the doping increased. At most, a 30% reduction was observed, compared with non-doped BST. XANES (X-ray Absorption Near Edge Structures) results indicated that the valency of the Fe ion was 3+ and located at the B-site of BST.
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