High-pressure Torsion Samples Research Articles

An approach for fabrication of Al-Cu composite by high pressure torsion

The method utilizing high-pressure torsion (HPT) processing followed by annealing to fabricate Cu-Al in situ composite with enhanced mechanical properties is presented. The initial sample consists of three metal discs Cu/Al/Cu was processed by HPT (5 GPa, 5 turns). The analyses of the microstructure after HPT carried out by transmission electron microscopy and scanning electron microscopy equipped with energy dispersive spectrometry (EDS). After HPT, the development of nanostructure with the mean crystallite size of about 100 nm in the produced sample was revealed with the small presence of the intermetallic phases. Investigation of the kinetics during subsequent annealing at 450 °C/30 min showed the in situ formation of intermetallic Al2Cu, AlCu, Al3Cu4, Al2Cu3 and Al4Cu9 phases in the copper matrix which was detected by EDS. The formation of intermetallic phases after annealing was also confirmed by X-ray diffraction. The results showed that microhardness of the composite considerably increases from an initial value of ∼380 for HPT sample to a saturation value of ∼900 after annealing.

Structural and mechanical characterization of heterogeneities in a CuZr-based bulk metallic glass processed by high pressure torsion

Cu45Zr45Al5Ag5 bulk metallic glass samples, processed by high pressure torsion (HPT) under various conditions, were characterized using synchrotron X-ray diffraction, nanoindentation, differential scanning calorimetry, atomic force and transmission electron microscopy. The experimental results clearly show that HPT modifies the amorphous structure by increasing the mean atomic volume. The level of rejuvenation, correlated with the excess mean atomic volume, is enhanced at higher shear strains as inferred from relaxation enthalpies. By mapping of structural and mechanical quantities, the strain-induced rejuvenated state is characterized on cross-sectional HPT samples on a local scale. A clear correlation both between elastic and plastic softening and between softening and excess mean atomic volume is obtained. But also the heterogeneity of the HPT induced rejuvenation is revealed, resulting in the formation of highly strain-softened regions next to less-deformed ones. A hardness drop of up to 20% is associated with an estimated increase of the mean atomic volume of up to 0.75%. Based on synchrotron X-ray diffraction and nanoindentation measurements it is concluded that elastic fluctuations are enhanced in the rejuvenated material on different length scales down to atomic scale. Furthermore, the calculated flexibility volume and the corresponding average mean square atomic displacement is increased. The plastic response during nanoindentation indicates that HPT processing promotes a more homogeneous-like deformation.

A novel method for severe plastic deformation at high strain rate

Severe plastic deformation (SPD) processing is defined as any method of forming under an extensive hydrostatic pressure that may be used to impart a very high strain to a bulk solid without any significant change in dimensions of the sample, producing exceptional grain refinement. Most of the SPD techniques employ very low processing speeds, however increased deformation rates are known to have a significant effect on the final microstructure. Most of the SPD processes operating at high rates do not impose hydrostatic pressures to the material and can therefore only be used for very ductile materials, while in others, the microstructural changes are limited to the surface layers of the material. To circumvent these restrictions a novel facility has been designed and developed where high hydrostatic pressures are maintained while a high shear deformation is imposed at high strain rates. The device combines the features of a high pressure torsion (HPT) unit with the principle of a torsional split Hopkinson bar (SHB) setup. A small ring-like sample, placed between two molds, is first subjected to a high, static pressure and subsequently to a high speed shear deformation upon release of torsional energy stored in a long bar. Although, the principle is rather straightforward, the design of the setup was extremely critical because of the high forces and energies involved. Tests have been performed on commercially pure aluminum. The material hardness increased in accordance with the microstructure and processing conditions; viz. annealed, only compressed and applied shear strain. Deformed grains departed from equiaxed shape and showed morphological texture in the direction of the shear even at very low strains indicating the presence of shear strains in the material. Further the material, or more specifically its mechanical properties and microstructure evolution is compared with conventional, statically deformed HPT samples.

Deformation lagging characteristics of IF steel disks in the plastic deformation process of high pressure torsion

Deformation lagging characteristics of IF steel disks in the early torsion stage in the process of High-Pressure Torsion (HPT) are investigated with DEFORM-3D finite analysis software through the effective strain distribution of HPT sample by computer simulation and experimental verification. The results indicate that in the early torsion stage (not more than 2 turns) , deformation lagging characteristics will exhibit remarkably in the severe plastic deformation (SPD) process. As to the Equivalent Von Mises strain, values in the sample edge is significantly higher than that in center, gradually decreasing from the edge to the center along the radius direction. Compared with the upper surface of sample, the effective strain of the bottom surface is obviously increasing. The SPD area in upper surface of sample is smaller than bottom surface of sample. Following the increasing rotation angel and the distance from the center, the deformation lagging characteristics of upper surface will appear more obviously. The mechanism of characteristics analysis indicate that the active rotation for the bottom die and friction plays an important role in the deformation lagging process. The theoretical analysis, the micro-structure and hardness investigation all agree well with the simulation results.

Hardness, Electrical Conductivity and Thermal Stability of Externally Oxidized Cu-Al2O3 Composite Processed by SPD

Cu-Al2O3 composites prepared by external oxidation method were further enhanced by severe plastic deformation (SPD) processing, including equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) methods. The HV hardness and electrical conductivity of the samples before and after SPD processing were tested. Results revealed that ECAP samples (with an equivalent strain of about 5.34) showed a relative small increase in hardness, whereas a significant decrease in electrical conductivity. The HPT samples (with an equivalent strain of about 6.94) showed not only a much improved hardness but also a higher electrical conductivity. Thermal stability of the SPD-processed Cu-Al2O3 composites was tested, and the HPT samples maintained good HV hardness together with high electrical conductivity even at 600 °C. The combination of external oxidation method and HPT processing resulted in enhanced mechanical properties, good electrical conductivity, acceptable thermal stability, and much simplified oxidation process.

Magnetic properties and structure of Fe50Pd50-xNix alloys (x = 4 and 8) in the as-deformed and annealed state

The structural and hysteresis properties of ternary Fe50Pd50-xNix (x = 4 and 8) alloys have been studied. In order to accelerate the formation of the hard magnetic L10 phase in these alloys, they were deformed by rolling with a reduction of 96% and by the method of high-pressure torsion (HPT). The annealing treatment for ordering for up to 100 h was performed at 400–500°С. It is shown that the structure transformation of the initial fcc phase into the ordered tetragonal phase at 400°С proceeds very slowly. After annealing for 100 h at this temperature, the Fe50Pd46Ni4 and Fe50Pd42Ni8 alloys are multiphase and contain residues of the cubic phase (Fm3¯m), cubic phase ordered by the L12 type (Pm3¯m), and tetragonal phase L10* appropriately described by the symmetry space group C4/mmm. The isothermal annealing at 400–500 °C for up to 100 h does not result in the maximum of Hc for the as-rolled Fe50Pd46Ni4 and Fe50Pd42Ni8 alloys. Implementation of the annealing routine with a temperature decreasing stepwise from 500 to 400 °C allowed us to increase Hc to 851 and 527 Oe for as-rolled samples and to 1192 and 980 Oe for the HPT samples of the Fe50Pd46Ni4 and Fe50Pd42Ni8 alloys, respectively.

Behavior of the grade 5 titanium alloy in different structural states in conditions of high-speed erosion

Behavior of a Grade 5 titanium alloy after high pressure torsion (HPT) and equal channel angular pressing (ECAP) under conditions of high-speed erosion was studied. Samples of the alloy in the ultrafine-grained state after the HPT and ECAP treatment were tested in an abrasive flow with various velocities using a technique similar to those presented in article Petrov et al. (2017). Two abrasive powders were used: with 109 and 230 μm average particle size. The tests were carried out at room temperature. Fracture surface parameters were studied for all tested specimens and fracture type (ratio of viscous and brittle fracture) was evaluated in each case. Additionally, surface roughness mass loss dependencies on particle flow velocity was investigated for both HPT and ECAP samples.

Microstructural evolution and superplasticity in an Mg–Gd–Y–Zr alloy after processing by different SPD techniques

Mg–Gd–Y–Zr alloys have attracted much attention recently due to their ability to exhibit stable fine-grained microstructures and their potential superplastic behavior. However, the microstructure and superplasticity of these alloys processed by severe plastic deformation (SPD) methods remain less understood. In this work, the microstructure and superplastic behavior of an Mg–5Gd–4Y–0.4Zr (GW54) alloy were investigated after processing using extrusion and the SPD processes of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). Microstructural characterization by transmission electron microscopy and electron backscattered diffraction showed that nano-sized grains of ~72±5nm were obtained after 8 HPT turns whereas the grain sizes were about ~4.6±0.2 and ~2.2±0.2µm after extrusion and 4 ECAP passes, respectively. Shear punch tests revealed that the optimum temperature for superplasticity is 623K for the HPT samples and 723K for the ECAP and extrusion samples, at which the strain rate sensitivities were measured as about 0.42±0.05, 0.46±0.05 and 0.50±0.05 for the extrusion, ECAP and HPT samples, respectively, and the corresponding activation energies were about 117, 101 and 110kJ/mol for these three processing conditions. These results suggest that grain boundary sliding controlled by grain boundary diffusion is the dominant mechanism of deformation at the optimum temperatures for superplastic flow.

Microstructural evolution, strengthening and thermal stability of an ultrafine-grained Al–Cu–Mg alloy

To gain insight into the origin of the ultra-high strength of ultrafine-grained (UFG) alloys, the solute clustering, precipitation phenomena, and microstructural evolutions were studied in an UFG Al-4.63Cu-1.51 Mg (wt.%) alloy (AA2024) processed by high-pressure torsion (HPT). The thermal analysis was performed using differential scanning calorimetry. The microstructures, internal microstrains and hardness following heating at a constant rate were characterised at room temperature using X-ray diffraction (XRD), transmission electron microscopy (TEM) and atom probe tomography (APT). The microhardness of the HPT processed sample initially increases following heating to 140 °C, and then remains unchanged on further heating to 210 °C. As the temperature increases up to 210 °C, the crystallite size calculated from XRD line broadening remains about 60–70 nm, while the dislocation densities remain in excess of 2 × 1014 m−2. A multimechanistic model is established to describe the strengthening due to grain refinement, dislocation accumulation, solid solution, precipitation, solute clusters and their segregation. The analysis reveals that solute clusters and lattice defects are key factors in HPT-induced strengthening of alloys, and illustrates the interactions between alloying elements, dislocations and grain boundaries enhance strength and stabilize ultrafine microstructures. Furthermore, for an HPT sample heated beyond 210 °C, the formation of nano-precipitates also contributes to hardness increment. The multimechanistic model for hardness contribution indicates the short-range order strengthening due to cluster-defect complexes is the dominant mechanism, which accounts for more than 40% of overall hardness.

Microstructure and recrystallization behavior of pure W powder processed by high-pressure torsion

High-pressure torsion (HPT) was conducted on pure W powder (99.9wt.%) at the temperature of 713K and the Vickers microhardness along the disk radius was measured. The microstructure and thermal stability after HPT were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The results show that the increasing equivalent strain resulted in grain refinement, microhardness improvement and thermal stability enhancement. The nanostructured W samples with crystallite size of 32.5nm, dislocation density of 9.19×1014m−2 and average microhardness of 1020Hv were obtained through 4GPa and 10 revolutions of HPT. For HPT samples at 2GPa and 5 turns, uniformly distributed nucleation sites around grain boundaries led to the recrystallization temperature increasing. A complex recrystallization mechanism occurred in the sample after 4GPa and 10 turns, including continuous dynamic recrystallization (cDRX) during HPT and continuous static recrystallization (cSRX) during DSC heating.

Role of interfaces on the trapping of He in 2D and 3D Cu–Nb nanocomposites

The role of interface structure on the trapping of He in Cu–Nb nanocomposites was investigated by comparing He bubble formation in nano-multilayers grown by PVD, nanolaminates fabricated by accumulative roll bonding (ARB), and 3D nanocomposites obtained by high pressure torsion (HPT). All samples were implanted with 1 MeV He ions at room temperature and characterized by cross section transmission electron microscopy (TEM). The critical He concentration leading to bubble formation was determined by correlating the He bubble depth distribution detected by TEM with the implanted He depth profile obtained by SRIM. The critical He dose per unit interfacial area for bubble formation was largest for the PVD multilayers, lower by a factor of ∼1.4 in the HPT nanocomposites annealed at 500 °C, and lower by a factor of ∼4.6 in the ARB nanolaminates relative to the PVD multilayers. The results indicate that the (111)FCC||(110)BCC Kurdjumov-Sachs (KS) interfaces predominant in PVD and annealed HPT samples provide more effective traps than the (112)KS interfaces predominant in ARB nanolaminates; however, the good trapping efficiency and high interface area of 3D HPT structures make them most attractive for applications.

High-Pressure Double Torsion as a Severe Plastic Deformation Process: Experimental Procedure and Finite Element Modeling

In the present study, a severe plastic deformation process of high-pressure torsion (HPT) has been modified. The new process is called high-pressure double torsion (HPDT) as both anvils of the conventional HPT process rotate in opposite directions. We manufactured sets of aluminum and pure copper samples using both the HPT process and the newly developed HPDT process to compare between microstructures and microhardness values. Our investigations showed that the copper samples processed by HPDT exhibited larger gradients in microstructure and higher values of hardness. Subsequently, we carried out a set of finite element simulations in ABAQUS/explicit to better understand the differences between the HPT process and the HPDT process. A comparison of the strain distributions of the HPT and HPDT samples revealed a decreasing trend in strain values as the radius increased at the middle surface of the samples. Analysis of the equivalent stress values revealed that stress values for the HPDT samples were higher than those of the HPT samples. Finally, the comparison of the max principal stress values indicated that in the HPDT sample, the extent of the compressive stresses was larger than those in the HPT sample.

An evaluation of high temperature tensile properties for a magnesium AZ31 alloy processed by high-pressure torsion

A magnesium alloy AZ31 was processed by high-pressure torsion (HPT) at room temperature. Microstructure investigations show the material has a grain size as fine as ~450 nm after 5 turns of HPT processing. X-ray texture measurements show most grains have the {0001} fibre, with their c-axis parallel to the HPT torsion axis. Tensile specimens were cut from HPT disc and pulled to failure over a range of strain rates (4.5×10-5, 1.3×10-4, 1.3×10-3 and 1.3×10-2 s-1) at temperatures of 623 and 673 K. The tensile elongations from HPT specimens are lower than for published results using equal-channel angular processing (ECAP) specimens although AZ31 has a finer grain size after HPT than after ECAP. The reasons for the lower elongations in HPT specimens are related to the thermal stability of the processed microstructure, the texture components and the tensile specimen size. Earlier investigations confirmed there was significant grain growth at 623 and 673 K in HPT samples, which would contribute to the low ductility of AZ31 in tensile testing. The main {0001} fibre in HPT samples means in tensile specimens most grains have their basal plane parallel to the surface of the tensile specimen, leading to the low ductility because the critical resolved shear stress does not operate on the basal plane due to the small Schmid factor that is nearly zero. The tensile specimen thickness in HPT is thinner than in ECAP and it is known that the ductility decreases when reducing the specimen thickness

High-Pressure Torsion of Ti: Synchrotron characterization of phase volume fraction and domain sizes

Rods of grade 2 Ti were processed by Equal-Channel Angular Pressing (ECAP) (ϕ = 120° at 573 K) employing 2, 4 and 6 passes. The same billets were further deformed by High- Pressure Torsion (HPT) at room temperature, varying both the hydrostatic pressure (1 and 6 GPa) and the number of rotations (n = 1 and 5). The ECAP and HPT samples were studied by synchrotron radiation at DESY-Petra III GEMS line. On the ECAP samples, textures were thus determined while for both ECAP and HPT samples the measurements were further analyzed by MAUD. Domain sizes and phase volume fractions were determined as a function of the radial direction of the samples. Alpha and Omega phases were detected in different amounts depending mostly on hydrostatic pressure and shear deformation. These transition phases can be pressure-induced during HPT processing and the results of Vickers microhardness measurements were related to the processing parameters and to the amounts of these phases.

Mechanical property evaluation of an Al-2024 alloy subjected to HPT processing

An aluminum-copper alloy (Al-2024) was successfully subjected to high-pressure torsion (HPT) up to five turns at room temperature under an applied pressure of 6.0 GPa. The Al-2024 alloy is used as a fuselage structural material in the aerospace sector. Mechanical properties of the HPT-processed Al-2024 alloy were evaluated using the automated ball indentation technique. This test is based on multiple cycles of loading and unloading where a spherical indenter is used. After two and five turns of HPT, the Al-2024 alloy exhibited a UTS value of ~1014 MPa and ~1160 MPa respectively, at the edge of the samples. The microhardness was measured from edges to centers for all HPT samples. These results clearly demonstrate that processing by HPT gives a very significant increase in tensile properties and the microhardness values increase symmetrically from the centers to the edges. Following HPT, TEM examination of the five-turn HPT sample revealed the formation of high-angle grain boundaries and a large dislocation density with a reduced average grain size of ~80 nm. These results also demonstrate that high-pressure torsion is a processing tool for developing nanostructures in the Al-2024 alloy with enhanced mechanical properties.

Microstructure and mechanical properties of ultrafinegrained Mg-Zn-Ca alloy

In this paper the influence of high pressure torsion (HPT) on the structure of Mg-Zn- Ca alloy is studied. The microstructure before and after HPT and after additional annealing has been studied by the scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results of microhardness measurements and tensile tests of HPT samples are discussed.

Nanostructured Cu-Cr alloy with high strength and electrical conductivity

The influence of nanostructuring by high pressure torsion (HPT) on strength and electrical conductivity in the Cu-Cr alloy has been investigated. Microstructure of HPT samples was studied by transmission electron microscopy with special attention on precipitation of small chromium particles after various treatments. Effect of dynamic precipitation leading to enhancement of strength and electrical conductivity was observed. It is shown that nanostructuring leads to combination of high ultimate tensile strength of 790–840 MPa, enhanced electrical conductivity of 81%–85% IACS and thermal stability up to 500 °C. The contributions of grain refinement and precipitation to enhanced properties of nanostructured alloy are discussed.

Finite element analysis of the effect of friction in high pressure torsion

High pressure torsion (HPT) is one of the most important techniques among various methods that create severe plastic deformation in the production of bulk materials with nano/ultrafine grained microstructures. Since the driving force in deforming the workpiece in HPT is surface friction, understanding of the friction effect is critical for successful application of HPT. In this study, the friction effect in HPT was analyzed using the finite element method. The distribution of effective strain on the contact surface of the HPT samples under different friction conditions was investigated. The friction force influenced the effective strain more in the middle and edge regions than in the central region. The condition for the minimum friction factor that could achieve a sticking condition between the surfaces of the dies, and the samples in the middle and edge regions, was investigated. There was a critical friction coefficient in which the effective strain varies sharply with an increasing friction coefficient.

Powder metallurgy routes toward aluminum boron nitride nanotube composites, their morphologies, structures and mechanical properties

Aluminum/boron nitride nanotube (BNNT) composites with up to 5wt% (i.e., 9.7vol%) nanotube fractions were prepared via spark plasma sintering (SPS) and high-pressure torsion (HPT) methods. Various microscopy techniques, X-ray diffraction, and energy dispersive X-ray analysis confirmed the integration of the two phases into decently dense and compact composites. No other phases, like Al borides or nitrides, formed in the Al–BNNTs macrocomposites of the two series. The BNNTs were found to be preferentially located along Al grain boundaries in SPS samples (grain size was 10–20μm) creating micro-discontinuities and pores which were found to be detrimental for the sample hardness, whereas in HPT samples, the tubes were rather evenly distributed within a fine-grained Al matrix (grain size of several hundred nm). Therefore, the hardness of HPT samples was drastically increased with increasing BNNTs content in Al pellets. The value for Al–BNNT 3.0wt% sample was more than doubled (190MPa) compared to a pure Al–HPT compact (90MPa). And the room temperature ultimate tensile strength of Al–BNNTs HPT samples containing 3.0wt% BNNT (~300MPa) became ~1.5 times larger than that of a BNNT-free HPT–Al compact (~200MPa).

Analysis of stress states in compression stage of high pressure torsion using slab analysis method and finite element method

High pressure torsion (HPT) is useful for achieving substantial grain refinement to ultrafine grained/nanocrystalline states in bulk metallic solids. Most publications that analyzed the HPT process used experimental and numerical simulation approaches, whereas theoretical stress analyses for the HPT process are rare. Because of the key role of compression stage for the deformation of HPT, this paper aims to conduct a theoretical analysis and to establish a practical formula for stress and forming parameters of HPT process using the slab analysis method. Three equations were obtained via equations derivation to describe the normal stress states corresponding to the three zones of plastic deformation for HPT process as stick zone, drag zone and slip zone. As to the compression stage of HPT, the stress distribution results using the finite element method agree well with those using the slab analysis method. There are drag and stick zones on the contact surface of the HPT sample, as verified by the finite element method (FEM) and slab analysis method.