Papers by Keyword: High Pressure Torsion (HPT)

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Authors: Ke Jing Yang, Yulia Ivanisenko, J. Markmann, Hans Jorg Fecht
Abstract: In situ tensile tests were conducted on ultra fine grained (UFG) pure Pd and Pd-x% Ag (x=20, 60) alloys of different stacking fault energies (SFEs) with the aim to study the general features of the deformation process of UFG materials as well as the peculiarities brought by the alloying. Grey scale correlation analysis (GSCA) was used to determine the true strain as well as the surface flow within the gauge length. It was shown that the largest values of strength and uniform elongation were obtained in Pd-20% Ag alloy. The GSCA revealed different macroscopic flow processes in this sample as compared with pure Pd and Pd-60% Ag alloy. In particular, pure Pd and Pd-60% Ag alloy demonstrated rapid localization of plastic flow in the neck area, whereas Pd-20% Ag samples showed a large contribution of homogenous deformation even after neck formation. It has been proposed that larger strain hardening capacity of Pd-20% Ag alloy is related to its lower SFE as compared with that of pure Pd: the lower is the SFE, the more difficult is the cross slip and climb of split dislocations, which leads to enhanced dislocation storage and, ultimately, to increased strain hardening. At the same time, further decrease of SFE in Pd-60% Ag sample leads to development of deformation twinning and consequent reduction of strain hardening. The dimpled structure of fracture surfaces in the samples will also be discussed in relationship to these findings.
Authors: Shao Kang Guan, Zhen Wei Ren, Jun Heng Gao, Yu Feng Sun, Shi Jie Zhu, Li Guo Wang
Abstract: In this paper the in vitro degradation of ultrafine grained (UFG) Mg-Zn-Ca alloy produced by HPT was investigated by electrochemical measurements and immersion tests in SBF. It was found that UFG Mg alloy had better degradation properties and also higher microhardness value than as-cast Mg alloy. The corrosion current density of UFG Mg alloy decreased by about two orders of magnitude, compared with that of as-cast alloy. Through electrochemical impedance spectroscopy (EIS) test,UFG Mg alloy showed a higher charge transfer resistance value. In immersion test, UFG Mg alloy in SBF exhibited more uniform corrosion and lower degradation rate (0.0763 mm/yr) than as-cast alloy. The degradation properties were related with the microstructure evolution, namely the grain refinement and redistribution of second phase. Keywords: Mg-Zn-Ca alloy; High-pressure torsion (HPT); Degradation behavior; Simulated body fluid (SBF); Microhardness
Authors: Julia Ivanisenko, Ian MacLaren, Ruslan Valiev, Hans Jorg Fecht
Abstract: Recent studies of nanocrystalline materials have often found that the deformation mechanisms are radically different to those in coarse-grained materials, resulting in quite different mechanical properties for such materials. The use of pearlitic steels for the study of the deformation mechanisms in bcc materials with ultrafine grain sizes is quite convenient, because it is relatively straightforward to obtain a homogenous nanocrystalline structure with a mean grain size as small as 10 nm using various modes of severe plastic deformation (SPD). In this paper we show that highpressure torsion of an initially pearlitic steel results in a nanostructured steel in which austenite has been formed at or close to room temperature. The orientation relationship between neighboring ferrite and austenite grains is the well-known Kurdjumov-Sachs orientation relationship, i.e. the same observed in temperature-induced martensitic transformation of iron and steels. It is shown that this must have resulted from a reverse martensitic transformation promoted by the high shear strains experienced by the material during severe plastic deformation of the nanocrystalline structure. This transformation represents an alternative deformation mechanism that can be activated when conventional deformation mechanisms such as slip of lattice dislocations become exhausted.
Authors: Reinhard Pippan, Stephan Scheriau, Anton Hohenwarter, Martin Hafok
Abstract: The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.
Authors: Seung Won Lee, Daichi Akama, Z. Horita, Tetsuya Masuda, Shoichi Hirosawa, Kenji Matsuda
Abstract: This study presents an application of high-pressure torsion (HPT) to an Al-Li-Cu-Mg alloy (2091). The alloy was subjected to solid solution treatment at 505oC for 30 minutes and was processed by HPT under 6 GPa for 5 revolutions at room temperature. The hardness increased with straining and saturated to a constant level at 225 Hv. Aging was undertaken on the HPT-processed alloy at 100, 150 and 190oC for the total periods up to 9.3 days. The aging treatment led to a further increase in the hardness to ~275 Hv. It is shown that the simultaneous strengthening of the alloy due to grain refinement and age hardening was successfully achieved by application of HPT and subsequent aging treatment. The enhancement of the strength is prominent when compared with the application of a conventional rolling process.
Authors: Hirotaka Matsunaga, Z. Horita, Kazutaka Imamura, Takanobu Kiss, Xavier Sauvage
Abstract: An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~150 nm and the Vickers microhardness was significantly increased through the HPT process. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis revealed that fine precipitates with sizes of ~20 nm or smaller were formed in the Cu matrix and some particles consist of Ni and Si with no appreciable amount of Cu.
Authors: Aicha Loucif, Thierry Baudin, François Brisset, Roberto B. Figueiredo, Rafik Chemam, Terence G. Langdon
Abstract: This investigation uses electron backscatter diffraction (EBSD) to study the development of microtexture with increasing deformation in an AlMgSi alloy having an initial grain size of about 150 µm subjected to high pressure torsion (HPT) up to a total of 5 turns. An homogeneous microstructure was achieved throughout the disc sample at high strains with the formation of ultra-fine grains. Observations based on orientation distribution function (ODF) calculation reveals the presence of the torsion texture components often reported in the literature for f.c.c. materials. In particular, the C {001}<110> component was found to be dominant. Furthermore, no significant change in the texture sharpness was observed by increasing the strain.
Authors: Anton Hohenwarter, Reinhard Pippan
Abstract: Motivated by the large variety of enhanced properties of ultrafine and nanocrystalline materials such materials are under extensive investigation. Besides focusing on classical material parameters, like strength and ductility, the fracture toughness of these materials is also of great importance, especially when the damage tolerance is required. In this contribution an overview of the fracture behavior of different metals covering ultrafine-grained iron and nickel as well as a nanocrystalline steel processed via high pressure torsion (HPT) will be given. It will be shown that the specimen orientation can have a tremendous influence on the fracture behavior and toughness. Due to this toughness anisotropy an unexpectedly good combination of high strength and high fracture toughness can be achieved very often in these materials.
Authors: Radik R. Mulyukov, Ayrat A. Nazarov, Renat M. Imayev
Abstract: Deformation methods of nanostructuring (DMNs) of materials are proposed to classify into severe plastic deformation (SPD) and mild plastic deformation (MPD) methods according to fundamentally different low- and high-temperature grain refinement mechanisms they exploit. A general analysis of the fundamentals and nanostructuring efficiency of three most developed DMNs, high pressure torsion (HPT), equal-channel angular pressing (ECAP), and multiple isothermal forging (MIF) is done with a particular attention to ECAP and MIF. It is demonstrated that MIF is the most efficient method of DMNs allowing one to obtain the bulkiest nanostructured samples with enhanced mechanical properties.
Authors: Seung Won Lee, Z. Horita
Abstract: An Fe-50at %Ni alloy was processed by high-pressure torsion (HPT) and annealed at lower homologous temperatures. Vickers microhardness and microstructural evolutions were examined with respect to the annealing time. Disks with 10 mm diameters having 0.85 mm thicknesses were subjected to HPT under a pressure of 6 GPa for 1-10 revolutions at a rotation speed of 1 rpm. The annealing after the HPT processing was conducted at homologous temperatures of ~0.3 for up to 40 days. The hardness increases with straining and saturates to a constant level of ~400 Hv at large equivalent strain. Microstructure analysis using XRD revealed that there was a peak corresponding to an α (Fe, Ni) phase.
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