High-pressure Torsion Samples Research Articles

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.

Wear properties of high pressure torsion processed ultrafine grained Al–7%Si alloy

In this paper, Al–7wt% Si alloy was processed via high pressure torsion (HPT) at an applied pressure 8GPa for 10 revolutions at room temperature. The microstructure and hardness of the HPT samples were investigated and compared with those of the as-cast samples. The wear properties of as-cast and the HPT samples under dry sliding conditions using different sliding distances and loads were investigated by reciprocated sliding wear tests.The HPT process successfully resulted in nanostructure Al–7wt% Si samples with a higher microhardness due to the finer Al matrix grains and Si particles sizes with more homogeneous distribution of the Si particles than those in the as-cast samples.The wear mass loss and coefficient of friction values were decreased after the HPT process. The wear mechanism was observed to be adhesive, delamination, plastic deformation bands and oxidization in the case of the as-cast alloy. Then, the wear mechanism was transformed into a combination of abrasive and adhesive wear after the HPT process. The oxidization cannot be considered as a mechanism that contributes to wear in the case of HPT samples, because O2 was not detected in all conditions.

Microstructure Evolution and Mechanical Properties of Al-1080 Processed by a Combination of Equal Channel Angular Pressing and High Pressure Torsion

Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are the most promising severe plastic deformation (SPD) methods. Both methods impose very high strains, leading to extreme work hardening and microstructural refinement. In this paper, billets of Al-1080 were successfully processed by ECAP conducted for up to 10 passes, HPT at an applied pressure of 8 GPa for 5 revolutions, and a combination of ECAP and HPT (ECAP + HPT) at room temperature. The effects of the different SPD processes (ECAP, HPT, and ECAP + HPT) on the evolution of the microstructure and mechanical properties of Al-1080 were investigated. The HPT and ECAP + HPT processes were observed to produce finer grain sizes with greater fractions of high angle grain boundaries (HAGBs) than the ECAP alone. Although the grain sizes after HPT and ECAP + HPT were similar, the ECAP + HPT sample had more dislocations than the HPT sample. HPT after ECAP enhanced the mechanical properties (hardness, tensile strength, and ductility) of the ECAP-processed Al-1080, showing larger dimple size in the tensile fracture surfaces.

Hydrogen-induced microstructure, texture and mechanical property evolutions in a high-pressure torsion processed zirconium alloy

The gaseous hydriding-induced evolutions of the microstructure, texture and mechanical properties of Zircaloy-4 processed by high-pressure torsion (HPT) were assessed. Much δ-ZrH1.66 precipitation at 15 atm (21%) incurred significant hardening of vacuum-annealed HPT samples, and pure e-ZrH2 obtained at 20 atm showed a superior microhardness of 470 HV0.3 and a low fracture toughness of 0.63 MPa m1/2. The δ-hydrides presented strong (1 1 1) texture and followed the (0 0 0 1)α-Zr//{1 1 1}δ-ZrH1.66 orientation relationship with the α-Zr matrix. During hydriding, α-Zr recrystallization texture was developed from the initial deformation texture.

Microstructural evolution during gaseous hydrogen charging of Zircaloy-4 processed by high-pressure torsion: A comparative study

The original and high-pressure torsion (HPT) processed Zircaloy-4 materials were hydrided using gaseous hydrogen charging at different hydrogen pressures (10, 15 and 20atm). The phase and microstructural evolutions of the samples during hydriding were characterized. It showed that when hydriding at the identical conditions, more hydrides tended to form in the HPT samples compared to that of the original ones. At a hydrogen pressure of 20atm, the HPT sample was completely converted to ε-ZrH2 while some δ-ZrH1.66 hydrides (volume fraction ~5.73%) were present in the material without HPT preprocessing. The HPT samples exhibited high potential for the hydride precipitation, and the large concentration of lattice defects induced by HPT was considered to be responsible for this enhanced susceptibility.

Extreme structural inhomogeneities in high-pressure torsion samples along the axial direction

Structural inhomogeneities along the axial, and not only the radial, direction are shown to occur in Zr 3Al deformed by high-pressure torsion (HPT) at room temperature. Scanning and transmission electron microscopy reveal a structure of nanocrystalline and coarse crystalline banded regions parallel to the shear plane. This contradicts the accepted equation describing microstructural evolution that is derived from the geometry of HPT. Surface treatment of one set of samples prior to deformation allows a systematic study as it leads to the localization of a refined nanocrystalline structure at the sample surface. Once a ∼50 μm wide nanocrystalline banded layer is formed (10 turns), the residual structure remains coarse grained up to the highest deformations studied (80 turns). In the narrow interface region between coarse structure and nanocrystalline structure, the grain size decreases by three orders of magnitude within less than 10 μm. This is accompanied by a change in texture, indicating the transition from a dislocation-mediated deformation mechanism to a grain-boundary-mediated one. The results of the structural study are explained in terms of the occurrence of work softening, as confirmed by microhardness measurements. Once a nanocrystalline layer—softer than the coarse structure—has formed, the vast majority of strain is accommodated by that layer: this leads to extreme inhomogeneities, a large strain gradient along the axial direction and huge deviations of the actual local shear strain from the calculated strain.

Impact of high pressure torsion on the microstructure and physical properties of Pr 0.67Fe 3CoSb 12, Pr 0.71Fe 3.5Ni 0.5Sb 12, and Ba 0.06Co 4Sb 12

Both p- and n-type skutterudites (Pr 0.67Fe 3CoSb 12, Pr 0.71Fe 3.5Ni 0.5Sb 12 and Ba 0.06Co 4Sb 12) have been deformed by high pressure torsion (HPT) with 2 GPa resulting in a lamellar shaped nanograined structure. The crystallite size distribution and dislocation density are evaluated using X-ray powder diffraction data, revealing a crystallite size of 47 nm and a dislocation density of 7 × 10 14 m −2 for Ba 0.06Co 4Sb 12. Whilst at T < 5.6 K the electrical resistivities of HPT processed Pr 0.67Fe 3CoSb 12 and Pr 0.71Fe 3.5Ni 0.5Sb 12 do not indicate long-range magnetic order, the temperature dependent susceptibility elucidates antiferromagnetic ordering after HPT although the anomaly at the phase transition becomes washed out. The effective magnetic moments are 4.18 μ B and 4.07 μ B for Pr 0.67Fe 3CoSb 12 before and after HPT, revealing a non-zero effective moment on the Fe 3CoSb 12 framework. Metamagnetic transitions at μ 0H = 0.9 (before HPT) and 0.8 (after HPT) are clearly seen in isothermal magnetization curves. In comparison with the microstructures of milled and hot pressed samples, those after HPT exhibit markedly smaller although lamellar grains, and also amorphous aggregates. The thermal conductivity of HPT samples is smaller, but the electrical resistivity is markedly higher than that of milled material, which in sum results in a lower figure of merit ZT. The increase of resistivity is caused by the high density of microcracks observed in the HPT samples, which may be avoided by suitable modification of the HPT processing parameters.

Comparison for Thermoelectric Properties of BiTe Based Semiconductor Processed by the Mechanical Alloying and the High Pressure Torsion after Melt Grown by the Vertical Bridgman Method

Bi 2 Te 3 -related thermoelectric semiconductors were prepared by the mechanical alloying (MA) and the vertical Bridgman (VBM) - High Pressure Torsion (HPT) methods. Structures and electric properties of these alloys were then investigated. These samples had a nominal composition of Bi 0.5 Sb 1.5 Te 3.0 with 0.07 mass% excess Te. The MA sample was sintered into a compact after mechanical alloying by hot-pressing while the VBM - HPT sample was subjected by HPT after being cut into a disk from a melt-grown ingot by the vertical Bridgman method (VBM). The orientation factors of the MA and VBM - HPT samples were 0.054 and 0.525, respectively, indicating that the preferred orientation was formed by VBM - HPT. Average grain sizes of both samples were approximately 2 μm. By estimating the slope of carrier mobility versus temperature the carrier scattering factor of the MA and VBM - HPT samples were estimated to be -2.8 and -1.4. These values are indicative of carrier scatterings by acoustic phonons and by the interaction between acoustic and optical phonons, respectively. The maximum power factor obtained for the isotropic MA sample was 4.1 × 10 -3 W m -1 K -2 at 310 K and this was about twice the 2.1 × 10 -3 W m -1 K -2 at 440K obtained for the anisotropic VBM - HPT sample.

Microstructure and properties of cold consolidated amorphous ribbons from (NiCu)ZrTiAlSi alloys

Amorphous ribbons of compositions (Ni 56Cu 2)Zr 18Ti 13Al 6Si 5 and (Ni 36Cu 23)Zr 18Ti 14Al 5Si 4 were consolidated by high pressure torsion (HPT) at room temperature. In the HPT experiments a 6 GPa pressure and two turns were applied. Samples in the form of discs, 6–7 times thicker than the ribbons and about 10 mm in diameter were achieved. The minimal deformation for the homogenous consolidation was estimated to be in the range of 400%. XRD showed that the microstructure was dependent on the composition. The sample with high Cu content remained amorphous while the sample with low Cu content revealed some crystallization. DSC experiments allowed a comparison of the glass transition temperature T g and crystallization process of the amorphous ribbon and HPT sample which were different. The glass transition temperature T g of the amorphous HPT sample of (Ni 36Cu 23)Zr 18Ti 14Al 5Si 4 composition decreased. For both alloys the nanohardness and the elastic modules showed decrease for cold consolidated samples in comparison to the ribbons.

Nanostructures and Microhardness in Al and Al–Mg Alloys Subjected to SPD

Nanostructures and microhardness of a commercial purity Al, three binary Al–Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion (HPT) at room temperature were comparatively investigated using high-resolution transmission electron microscopy, X-ray diffraction (XRD) and high-resolution XRD line profile analysis. The hardness values of HPT samples are twice to three times larger than that of the undeformed counterparts. Grain sizes measured by XRD are in the range 10–200 nm with typical average values ranging from 46 to 120 nm. The hardness values and the dislocation densities increased, whereas, the average grain size decreased significantly with increasing Mg contents. Typical dislocation densities are in the range 1.7 × 1014 m-2 – 2.3 × 1015 m-2. However, local densities in grain boundary and triple junction areas might be as high as 1017 m-2. The strengthening mechanisms contributing to high hardness may primarily be attributed to the cooperative interactions of high dislocation densities, grain boundaries and planar interfaces.

Microstructure and mechanical properties of pure Cu processed by high-pressure torsion

Pure Cu was subjected to severe plastic deformation through high-pressure torsion (HPT) using disc and ring samples. Vickers microhardness was measured across the diameter and it was shown that all hardness values fall well on a unique single curve regardless of the types of the HPT samples when they are plotted against the equivalent strain. The hardness increases with an increase in the equivalent strain at an early stage of straining but levels off and enters into a steady-state where the hardness remains unchanged with further straining. It was confirmed that the tensile strength also follows the same single function of the equivalent strain as the hardness. The elongation to failure as well as the uniform elongation also exhibits a single unique function of the equivalent strain. Transmission electron microscopy showed that a subgrain structure develops at an early stage of straining with individual grains containing dislocations. The subgrain size decreases while the misorientation angle increases and more dislocations are formed within the grains with further straining. In the steady-state range, some grains appear which are free from dislocations, suggesting that recrystallization occurs during or after the HPT process. The mechanism for the grain refinement was discussed in terms of dislocation mobility.

Mechanical Behavior and Stress-Induced Martensitic Transformation in Nanocrystalline Ti&lt;sub&gt;49.4&lt;/sub&gt;Ni&lt;sub&gt;50.6&lt;/sub&gt; Alloy

Amorphous-nanocrystalline Ti49.4Ni50.6 alloy in the shape of a disc 20 mm in diameter has been successfully produced using high pressure torsion (HPT). Application of HPT and annealing at temperatures of 300–550°C resulted in formation of a nanocrystalline (NC) structure with the grain size (D) about 20–300 nm. The HPT samples after annealing at Т = 400°C with the D= 20 nm possess high yield stress and high ultimate tensile strength (more than 2000 MPa). There is an area of strain-induced transformation B2-B19’ on the tensile curve of the samples with the grain size D =20 nm. The stress of martensitic transformation (σm) of samples is 450 MPa, which is three times higher than σm in the initial coarse-grained state (σm ≈ 160 MPa). The HPT samples after annealing at Т = 550°C with the D= 300 nm possess high ductility (δ&gt;60 %) and high ultimate tensile strength (about 1000 MPa).

Microstructural characteristics of nickel processed to ultrahigh strains by high-pressure torsion

The microstructures of highly strained high-pressure torsion (HPT) nickel processed from bulk master metals pre-strained to different levels of strain and from electrodeposited (ED), ball milled (BM) samples and rapidly quenched (RQ) ribbons were studied by transmission electron microscopy and X-ray diffraction. High-resolution X-rays showed that the BM + HPT specimens had the smallest coherent domain size of ∼9 nm and the RQ + HPT samples possessed a high dislocation density of ∼6.7 × 10 15 m −2. Gradually increasing the strain in bulk nickel led to an increasing dislocation density and a decreasing coherent domain size. Microhardness measurements revealed differences in the mechanical behavior in HPT-strained nickel specimens. There is a good correlation between the microhardness measurements and the dislocation density measured with high-resolution X-rays.

Microstructure and mechanical properties of titanium (Grade 4) processed by high-pressure torsion

The influence of high-pressure torsion (HPT) at various temperatures on microstructure and mechanical properties of commercially pure (CP) titanium (Grade 4) has been studied using transmission electron microscopy and tensile tests. The temperature range of 70–450 °C for isothermal HPT processing has been used to fabricate nanocrystalline samples with diameter 20 mm under a pressure of 6 GPa. Using the new installation, we demonstrated that the ultimate tensile strength of CP Ti (Grade 4) after HPT processing achieves 1600 MPa as a result of significant grain refinement. At the same time, ductility of the HPT samples has been affected strongly by processing and annealing temperature.

High-pressure torsion using ring specimens

A facility is developed for high-pressure torsion (HPT) using ring samples and the results are compared with those of conventional disk HPT samples. With pure Al (99.99%), both types of HPT are consistent. When hardness values are plotted against equivalent strain, they fall on a single line having a maximum at a strain of ∼2 followed by a steady-state at a strain of ∼6 or more where hardness remains constant with straining.