High Pressure Torsion Technique Research Articles

Mechanisms of Strengthening Aluminum Foils Consolidated by High-Pressure Torsion Technique

The contributions of grain boundary and dislocation mechanisms to the experimentally established strengthening of pure Al samples obtained by consolidation of thin foils by high pressure torsion technique were estimated within the framework of well-known theoretical models using structural parameters (sizes of coherently scattering domains and lattice microstrains) determined by X-ray diffraction. Good agreement between the calculated values and the hardness of deformed and aged samples was found, and possible reasons for their differences for the initial foils were discussed. The influence of deformation and aging on the relative contributions of the analyzed mechanisms to the strengthening of samples consolidated from both Al foils and the rapidly quenched Al95.8Mn3.8Fe0.4 ribbons was determined. The structural features of samples processed by high pressure torsion and the relationship between structural parameters and mechanical properties were discussed.

The effect of severe plastic deformation on the mechanical properties of aluminum composite obtained by additive printing

Abstract The composite material for the study was obtained by wire-arc additive manufacturing on the basis of AA1050 (commercially pure aluminum) and AA5056 (commercial alloy of the Al-Mg system) aluminum alloys. The influence of the degree of deformation by high-pressure torsion technique on the mechanical properties and microstructure of the composite material was investigated. The dependences of the static strength and plasticity of the material on the degree of deformation were obtained. The microstructure of the material was investigated by scanning electron microscopy and X-ray diffraction analysis. The optimal deformation route providing the best combination of mechanical properties is revealed, the key microstructure parameters providing optimal properties are determined.

Nanostructuring of Additively Manufactured 316L Stainless Steel Using High-Pressure Torsion Technique: An X-ray Line Profile Analysis Study.

Experiments were conducted to reveal the nanostructure evolution in additively manufactured (AMed) 316L stainless steel due to severe plastic deformation (SPD). SPD-processing was carried out using the high-pressure torsion (HPT) technique. HPT was performed on four different states of 316L: the as-built material and specimens heat-treated at 400, 800 and 1100 °C after AM-processing. The motivation for the extension of this research to the annealed states is that heat treatment is a usual step after 3D printing in order to reduce the internal stresses formed during AM-processing. The nanostructure was studied by X-ray line profile analysis (XLPA), which was completed by crystallographic texture measurements. It was found that the as-built 316L sample contained a considerable density of dislocations (1015 m-2), which decreased to about half the original density due to the heat treatments at 800 and 1100 °C. The hardness varied accordingly during annealing. Despite this difference caused by annealing, HPT processing led to a similar evolution of the microstructure by increasing the strain for the samples with and without annealing. The saturation values of the crystallite size, dislocation density and twin fault probability were about 20 nm, 3 × 1016 m-2 and 3%, respectively, while the maximum achievable hardness was ~6000 MPa. The initial <100> and <110> textures for the as-built and the annealed samples were changed to <111> due to HPT processing.

Design and Analysis of High-Pressure Torsion Process Using Finite Element Method

High Pressure Torsion (HPT) is a procedure that is frequently used to change the structure and characteristics of materials by using severe plastic deformation. It is widely believed that astraightforward shear causes deformation during HPT. In order to create nanocrystalline structures in metals and alloys as well as to cold weld powders, HPT is utilized. The flow kinematics and stress condition of a material under deformation dictate the HPT treatment’s outcome. Recent studies have revealed that the actual plastic flow during HPT might deviate greatly from the straightforward concept. Based on a modification of the traditional high pressure torsion technique of severe plastic deformation, a new approach called High Pressure Torsion Extrusion (HPTE) will be proposed. A specimen will be extruded via sectional containers that will be spinning in relation to one another during HPTE. The area where the containers converge on the specimen will be exposed to shear deformation. The findings of the FEM inquiry will be presented in this report

Study of Tribological Properties of Bulk Nanostructured Aluminum and Copper Samples Applicable in Automotive Bearing Application

Using lightweight systems and friction reduction approaches are two main contributors towards modern and efficient powertrains in the automotive industry. New materials and processes are required to achieve the demanding and ever-increasing performance requirement of automotive systems. Nanostructure induced by severe plastic deformation methods involves bimodal microstructures and hence, shows exceptional mechanical characteristics which can be exploited for automotive application. Through this study, samples were prepared using pure copper, aluminum alloy (series 7000) and were processed to attain bulk nanostructured samples using a Single Step High Pressure Torsion technique with appropriate dimensions applicable as the rolling elements of automotive bearings. The induced nanostructures resulted micro hardness and frictional characteristics of the bulk samples were assessed using transmission electron (TEM) and atomic force (AFM) microscopies as well as microhardness evaluations. The results revealed that a fully refined nanostructured samples were achieved with 90% increase in the hardness at the outer diameter of the sample. The AFM measurements indicated that the friction coefficient of nanostructured copper and aluminum samples were ~ 25 and ~ 45% less than that of both the unprocessed samples, respectively. Characteristics of treated samples suggest that these processes can be potentially used in demanding conditions of rolling element bearings with reduced weight and frictional losses.

Cold consolidation and coercivity enhancement in Sm2Fe17N3 magnets by high-pressure torsion

Sm2Fe17N3 coarse powder was directly consolidated into a disc magnet by the high-pressure torsion technique. The huge superimposed hydrostatic pressure (10 GPa) made it possible to apply a large amount of plastic deformation to the disc as well as the powder particles of the brittle Sm2Fe17N3 compound at room temperature. This large deformation brought about grain subdivision inside powder particles and enabled to produce a bulk magnet having the coercivity of 5.7 kOe from the coarse powder having the coercivity of 1 kOe without reducing the particle size. It was clarified that those grain subdivision events are accompanied by the motion of dislocations confined in non-basal planes. Synchrotron X-ray diffractometry was also performed and revealed that too large strain, however, results in the disappearance of the hard magnetic Sm2Fe17N3 phase and the eventual loss of the coercivity.

Mechanical properties of Al-Nb in situ metal-matrix composites fabricated by constrained high pressure torsion at 10 GPa and subsequent annealing

Mechanical alloying of dissimilar metals by means of severe plastic deformation techniques is known to be a perspective method of obtaining in situ metal matrix composites with advanced properties. Recent investigations found out that besides severe mixing of elements, intensive formation of non-equilibrium intermetallic phases takes place. Presence of intermetallic particles in the composite affects both its physical and mechanical properties. In this work we have realized the fabrication of Al-Nb composites in one step by means of high-pressure torsion technique. A non-trivial phenomenon of growth of tensile strength with increasing number of revolutions during HPT was revealed. It was explained by more homogeneous mixing of components and precipitation of intermetallic phase in the vicinity of Al-Nb interphases. The microstructural observations and X-ray diffraction analysis allowed to reveal Al3Nb intermetallic phase in the as deformed composites, while the equilibrium temperature of formation of this phase is above 600°C. Varying the number of revolutions and post deformation annealing allows obtaining composites with different fractions of intermetallic particles. This approach can be considered as a way for the development of composites with a tailorable complex of physical and mechanical properties.

Effect of ultrafine-grained microstructure on creep behaviour in 304L austenitic steel

The austenitic stainless steel 304 L was subjected to severe plastic deformation using high-pressure torsion technique at room temperature. The severe plastic deformation led not only to the grain size reduction but also to the transformation of austenite into deformation-induced martensite. At high imposed strains the martensitic boundaries disappeared and misorientation distribution exhibited nearly random misorientation distribution. The thermal exposure and tensile creep tests under constant load performed at 923 K and stresses ranging from 50 to 150 MPa revealed that austenitic microstructure contains σ phase. The ultrafine-grained microstructure coarsens during creep testing but the mean grain size is significantly finer than the predicted stationary subgrain. The results demonstrate that during creep testing new grains at precipitates of σ phase are formed and texture changes. The formation of new grains during creep keeps the microstructure of being ultrafine-grained and thus grain boundary mediated processes are enhanced in this steel.

Grain Size Effects on Static and Dynamic Strength of Ultrafine-Grained Al-Mg-Mn Alloy Produced by High-Pressure Torsion

In the paper, the structure and static and dynamic mechanical properties of ultrafine-grained A5083 alloy (Al-Mg-Mn) produced by high-pressure torsion (HPT) are reported. The static yield stress and tensile strength were determined in tensile tests at a strain rate of ~ 10−3 s−1, and the dynamic yield stress and spall strength were calculated from free-surface velocity histories obtained during shock-wave loading at a strain rate of 105 s−1. The HPT technique provides strong grain refinement. The average grain size of the alloy after HPT is 100-180 nm and depends on the accumulated true strain. HPT significantly improves the static strength properties of the alloy. The static yield stress is increased by 360-390% and the static ultimate tensile strength by 166-182%. It is shown that the dynamic yield stress improved by 168-181%, while the dynamic spall strength was not improved by HPT. Moreover, the nanostructured alloy with a grain size of ~ 100 nm demonstrates the lowest spall strength.

Improving the corrosion resistance of ZEK100 magnesium alloy by combining high-pressure torsion technology with hydroxyapatite coating

Magnesium alloys have better biocompatibility and biodegradability than conventional biomedical metal materials, but the corrosion resistance is so poor that the implant loses its mechanical integrity before the damaged tissue completely recovers. In this study, the ZEK100 magnesium alloy was pre-deformed with a high-pressure torsion (HPT) process and then fabricated hydroxyapatite (HA) coatings with different contents of Mg(OH)2 nanopowder via hydrothermal method. The specimens were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). Meanwhile, the corrosion behavior of the specimens was evaluated by electrochemical impedance spectroscopy (EIS) and hydrogen evolution tests. Results showed that HPT processing refined the grain size and introduced a great number of twins, resulting in the enhancement of microhardness and Young's modulus of ZEK100. The abundant fine grains, twins and grain boundaries in HPT-ZEK100 can provide more nucleation sites for the HA crystal, resulting in a denser, thicker HA coating with smaller crystal sizes. In terms of the amount of Mg(OH)2 nanopowder, a 0.3 mg/mL content was most appropriate for the deposition of HA. In addition, HPT technique and surface modification by HA coating simultaneously reduced the corrosion rate of ZEK100 magnesium alloy, which virtually expands the biological application of magnesium alloys.

The effect of bismuth on microstructure evolution of ultrafine grained copper

The effect of bismuth on the microstructure evolution of ultrafine grained copper at elevated temperatures has been studied. Ultrafine grained copper polycrystals were produced employing the High-Pressure Torsion technique. Bismuth had little effect on kinetics of recrystallization and grain growth of as-processed copper. Bi-rich intergranular films at the grain boundaries were observed in the samples annealed between 700 and 900°C. Complete wetting of the grain boundary triple junctions by the Bi-rich liquid phase was observed at the temperatures below the onset of complete grain boundary wetting. We attributed the fact that Bi does not affect the kinetics of grain growth to the abundance of twin boundaries not affected by Bi segregation.

Dehydrogenation-hydrogenation characteristics of nanocrystalline Mg2Ni powders compacted by high-pressure torsion

High-pressure torsion technique was applied on nanocrystalline Mg2Ni powders to produce bulk disks by simultaneous uniaxial compression and severe shear deformation. The hydrogen absorption and desorption behavior of the disks has been characterized by high-pressure calorimetry. During desorption, the decomposition of the Mg2NiH4 phase takes place, which is followed by the dehydrogenation of Mg2NiH0.3 solid solution. In order to monitor the sorption properties in details, partially dehydrogenated states of the fully absorbed disk have been performed by interrupting the desorption process at 75%, 50% and 25% hydrogen contents in a Sieverts' type apparatus. Microstructural evolution during dehydrogenation has been investigated by X-ray diffraction. The variation of average crystallite size, lattice parameters and unit cell of the competing phases has been determined by the Rietveld refinement method of X-ray diffractograms. The unit cell volume of the Mg2NiH0.3 hydride solid solution decreases with decreasing hydrogen content. Coupled differential scanning calorimetry and thermogravimetry measurements were also taken on the partially desorbed states in order to determine the activation energy of hydrogen release.

鉱物由来の鉄硫化物に着目した熱電材料開発

Development of thermoelectric materials based on iron-sulfide minerals is reviewed with focusing on n-type doped CuFeS2. It is found that the thermoelectric properties of Zn-doped CuFeS2 strongly depend on the synthesis method, especially on the cooling rate and heat treatment conditions after spark-plasma sintering (SPS). A high power factor exceeding 1 mW/K2m is achieved around room temperature for samples annealed and slowly cooled after SPS. With aiming at reducing thermal conductivity, several methods are applied including the high-pressure torsion technique and nano-structuring by ball milling. These experiments successfully yield reduced thermal conductivity, though experimental conditions needs to be optimized to reduce electrical resistivity simultaneously.

Microstructure and mechanical properties of ultrafine-grained aluminum consolidated by high-pressure torsion

Coarse-grained aluminum powder with 99.5wt% purity was consolidated by high-pressure torsion (HPT) technique at room temperature using a low carbon steel powder holder. In this process, the powder experiences a semi-constrained condition because the internal wall of the powder holder can expand under the load applied during HPT. After 4 turns of HPT a relative density of 99.83% was achieved. The microstructure was characterized by electron backscatter diffraction and X-ray line profile analysis. It was found that the grain size decreased while the dislocation density increased with both increasing the distance from the disk center and the number of HPT turns. The smallest grain size and the maximum dislocation density with the values of 0.41μm and 6.8×1014m−2, respectively, were achieved at the periphery of the disks processed for 4 turns. Tensile tests showed that the consolidation of this Al powder by 4 turns of HPT yielded a high ultimate tensile strength and a good ductility (~373MPa and ~22%, respectively). It turned out that the yield strength versus grain size relationship obeys the Hall-Petch equation. This study demonstrates the capability of HPT technique for processing consolidated Al powder with high strength and good ductility.

High Pressure Torsion Extrusion as a new severe plastic deformation process

A new method named High Pressure Torsion Extrusion (HPTE) is proposed based on a modification of the conventional high pressure torsion technique of severe plastic deformation. During HPTE, a specimen is extruded through sectional containers rotating relative to each other. The specimen is subjected to shear deformation in the area where the containers meet each other. One of the main advantages of the HPTE process is that already after a single extrusion pass a high accumulated strain can be achieved in the specimen. Furthermore, the presence of a strong velocity gradient in the specimen cross-section during HPTE provides the possibility to process hybrid materials or composite parts with helical architecture of functional elements. The HPTE method is evaluated theoretically by using finite element methods (FEM) and experimentally by using HPTE for processing copper specimen and the results are presented and discussed.

X-ray studies of dynamic aging in an aluminum alloy subjected to severe plastic deformation

In this work, X-ray scattering methods were applied for a quantitative characterization of the microstructure of an aluminum alloy of the Al–Mg–Si system during dynamic aging realized through the high pressure torsion technique. A qualitative and quantitative phase analysis of the alloy was performed, together with Al alloy lattice parameter determination. From the reflections broadening the effective size of the coherent scattering domains and the lattice microstrain were determined in the framework of the Halder–Wagner approach. Using the method of small-angle X-ray scattering, the quantitative characteristics of the size, shape and spatial distribution of the secondary phase particles formed in the Al alloy during dynamic aging were established. In order to validate the obtained results, the method of small-angle X-ray scattering was preliminarily tested on similar samples after artificial aging and compared with the results from small-angle neutron diffraction widely known in literature.

Aluminum matrix composites reinforced with multi-walled boron nitride nanotubes fabricated by a high-pressure torsion technique

ABSTRACT Aluminum matrix composites loaded with various fractions of multi-walled, well-structured boron nitride nanotubes (BNNTs), up to 5 wt.% fractions, were fabricated using powder constituents by means of a high pressure torsion technique (HPT) at room temperature under 5 GPa pressurization. Transient ultrathin amorphous-like layers, with a thickness of 2–5 nm, composed of Al(BNO) phases, which formed under severe plastic deformation and developed under further heat treatments of the composites at 350 °C and 450 °C, were detected at the interfacial regions between Al grains and tightly embracing them BN layers. Room temperature hardness and tensile tests on fabricated composites before and after heat treatments were conducted. The highest value of room temperature tensile strength was obtained on Al-5 wt.% BNNT samples annealed at 450 °C, that reached up to ~ 420 MPa, thus exhibiting more than a doubled increase in strength compared to HPT-fabricated pure Al samples under identical compacting conditions.

Effects of carbon nanotube content and annealing temperature on the hardness of CNT reinforced aluminum nanocomposites processed by the high pressure torsion technique

In this paper, the microstructure and hardness of CNT reinforced aluminium (CNT/Al) nanocomposites prepared by the advanced powder metallurgy method and subsequently processed by the high pressure torsion (HPT) technique are studied. The effects of CNT content and annealing temperature on the hardness of the nanocomposites are investigated. The results show that annealing materials at temperatures below 150°C leads to secondary hardening, while annealing at higher temperatures soften the nanocomposites. HPT-processed CNT/Al nanocomposites with 1.5wt.% of CNTs are shown to have the highest hardness in comparison with other composites containing CNTs from 0 up to 2wt.%. Microstructures, CNT distribution and the phase composition of CNT/Al nanocomposites are investigated by transmission and scanning electron microscopy and X-ray diffraction techniques.

Analysis of microstructure and microhardness of Zr-2.5Nb processed by High-Pressure Torsion (HPT)

Nanostructured materials have been widely studied due to the improvement of their mechanical properties comparing to those of coarse grain materials. The present work intended to analyze the microstructure and microhardness of Zr-2.5Nb processed by high-pressure torsion (HPT), one of the severe plastic deformation techniques. The deformations were carried out at room temperature using a pressure of 5 GPa and 5 anvil turns. Vickers indentation was used to evaluate the microhardness of the samples. Transmission electron microscopy and X-ray diffraction were used to analyze the microstructure. The results showed a significant refinement from the initial microstructure achieving nanometric grain size around 50 nm and phase transformation α → ω + βI induced by shear. The Vickers microhardness values of the material submitted to HPT technique were significantly higher than those of non-deformed material. Also, HPT procedure resulted in a huge grain refinement of the material and in phase transformation.

Attenuation of ultrasound in severely plastically deformed nickel

Ultrasound attenuation was measured in nickel specimens of about 30mm diameter prepared using the high pressure torsion technique. The cold working process produced an equivalent shear strain increasing from zero at the center up to 1000% at the edge of the specimen. The fragmentation of the grains due to multiple dislocations led to an ultrafine microstructure with large angle grain boundaries. The mean value of the grain size distribution gradually decreased from ∼50μm at the center to 0.2μm at the edge. Laser pulses of 5ns were employed for the excitation of broadband ultrasound pulses covering the spectral range of 0.1–150MHz. The ultrasound pulses were measured from the opposite side of the specimen by means of an optical interferometer and a piezoelectric foil transducer in two experimental setups. The features of the detected signal forms are discussed. The absolute value of the attenuation decreases from the center to the edge of the specimen showing nearly linear frequency dependence. The variation of the phase velocity was measured in a 6mm-thick high pressure torsion nickel sample, revealing a velocity increase from the center to the edge.