High Pressure Torsion Deformation Research Articles

Magnetostrictive Behavior of Severe Plastically Deformed Nanocrystalline Fe-Cu Materials

Reducing the saturation magnetostriction is an effective way to improve the performance of soft magnetic materials and reduce core losses in present and future applications. The magnetostrictive properties of binary Fe-based alloys are investigated for a broad variety of alloying elements. Although several studies on the influence of Cu-alloying on the magnetic properties of Fe are reported, few studies have focused on the effect on its magnetostrictive behavior. High pressure torsion deformation is a promising fabrication route to produce metastable, single-phase Fe-Cu alloys. In this study, the influence of Cu-content and the chosen deformation parameters on the microstructural and phase evolution in the Fe-Cu system is investigated by scanning electron microscopy and synchrotron X-ray diffraction. Magnetic properties and magnetostrictive behavior are measured as well. While a reduction in the saturation magnetostriction λs is present for all Cu-contents, two trends are noticeable. λs decreases linearly with decreasing Fe-content in Fe-Cu nanocomposites, which is accompanied by an increasing coercivity. In contrast, both the saturation magnetostriction as well as the coercivity strongly decrease in metastable, single-phase Fe-Cu alloys after HPT-deformation.

Microstructure evolution and mechanical properties of a lamellar AlCoCrFeNi2.1 eutectic high-entropy alloy processed by high-pressure torsion

High-pressure torsion (HPT) was applied to a lamellar eutectic high-entropy alloy (EHEA) to study the effect of severe plastic deformation (SPD) on the composite structure and mechanical properties. We found that the existence of multiple phases affects defect distribution and the fragmentation process during HPT. Structural evolution shows orientation dependence with respect to the shear plane, which finally leads to a refined multiphase structure with nanograins and vortex clusters after a shear strain γ of 24. In nanograins, dislocation-mediated deformation prevails. The high density of grain boundaries, forest dislocations, and the generation of deformation twins restrict dislocation movement. As a result, an EHEA with a yield strength of 1.75 GPa, an excellent tensile strength of 2.20 GPa, and an appreciable failure strain of 5 % is realized. Our results demonstrate that the HPT deformation process of the lamellar EHEA is significantly affected by the structure. SPD on the multiphase structure is a suitable route for designing high-strength yet ductile alloys.

Microstructural and material property changes in severely deformed Eurofer-97

Severe plastic deformation changes the microstructure and properties of steels, which may be favourable for their use in structural components of nuclear reactors. In this study, high-pressure torsion (HPT) was used to refine the grain structure of Eurofer-97, a ferritic/martensitic steel. Electron microscopy and X-ray diffraction were used to characterise the microstructural changes. Following HPT at room temperature to a maximum shear strain of 230, the average grain size reduced by a factor of ∼30, with a marked increase in high-angle grain boundaries. Dislocation density also increased by more than one order of magnitude. The thermal stability of the deformed material was investigated via in-situ annealing during synchrotron X-ray diffraction. This revealed substantial recovery between 450 K – 800 K. Irradiation with 20 MeV Fe-ions to ∼0.1 dpa caused a 20% reduction in dislocation density compared to the as-deformed material. However, HPT deformation prior to irradiation only had a minor effect in mitigating the irradiation-induced reductions in thermal diffusivity and surface acoustic wave velocity of the material. Microstructural and material property changes are dominated by deformation compared to irradiation. In light of this, the benefits of using HPT to improve the irradiation resistance of Eurofer-97 are limited. These results provide a multi-faceted view of the changes in ferritic/martensitic steels due to severe plastic deformation, and how these changes can be used to alter material properties.

The formation mechanism of the nano-equiaxed (α+β) duplex phases structure during aging process after high pressure torsion deformation

The aim of this study was to investigate the effects of the nano equiaxed (α+β) duplex phases (NEDP) structure on the mechanical properties of metastable β type titanium alloy. To achieve this, the Ti-3.5Al-5Mo-4 V (Ti-B20) alloy was subjected to high pressure torsion (HPT) deformation and aging treatment. The results demonstrated that the NEDP (α+β) structure exhibited high thermal stability even after prolonged aging. Through systematic transmission electron microscopy (TEM) characterization, it was confirmed that the formation of the NEDP structure was attributed to the generation of crystal defects such as dislocation cells and dislocation pile-ups during HPT deformation. In addition, it was found that the orientation relationship between the α and β phases was [21̅1̅0]α//[111]β after prolonged aging, and the interlocked of coherent interface between the α and β phases allowed the NEDP (α+β) structure to maintain its thermal stability during long-term aging. Therefore, this paper provides valuable guidance for the fabrication of nano-equiaxed dual-phase structures.

Phase transformation and mechanical properties of nanocrystalline Ti–2Fe-0.1B alloy processed by high pressure torsion

In this paper, nano-Ti-2Fe-0.1B alloys with different grain sizes were processed by high pressure torsion (HPT) deformation. The study examined the microstructural evolution of the Ti–2Fe-0.1B alloy during HPT and evaluated its mechanical properties through microhardness and tensile tests. The results of the microstructural observations revealed that the grain size of the Ti–2Fe-0.1B alloy progressively decreased from 3.14 μm in the as-annealed state to approximately 20 nm after 10 turns. In the initial state, the alloy consists of α-phase and β-phases, with a volume ratio of approximately 5:1. During the HPT process, transformations of α-phase and β-phase to ω-phase were observed. The primary reason for this is that the shear forces applied during the HPT process facilitated the phase transformation of the α-phase. Additionally, the presence of Fe element in the alloy altered the lattice compatibility between the grains of β-phase and ω-phase, thereby promoting the phase transformation of the β-phase, with the accumulated turns of HPT process, the fraction of ω-phase increased in the beginning gradually, then decreased, which reached to the maximum 80.8% in the 5 turns. This is higher than that of pure titanium and Ti–Fe alloys under the same conditions, because the proportion of the ω-phase increases with the increase in Fe content. Studies of mechanical properties demonstrated that HPT can substantially enhance the hardness of Ti–2Fe-0.1B alloys to as high as 483 HV, and the tensile strength of the Ti–2Fe-0.1B alloy was observed to increase from 725 MPa to 1568 MPa after 5 turns, which is much higher than Ti–6Al–4V subjected to the identical processing conditions. It can be ascribed to the exist of mass ω-phase as well as finer grain size in nanocrystalline Ti–2Fe-0.1B alloy.

Influence of Severe Plastic Deformation on the Magnetic Properties of Sm–Co Permanent Magnets

High pressure torsion (HPT) is presented as a new fabrication route to produce bulk Sm–Co magnets with a strongly refined microstructure down to the nanometer regime. The initial powders, based on the compositions SmCo5, Sm2Co7 and Sm2Co17, are compacted and subsequently deformed by HPT. The microstructural evolution in dependence on the applied deformation parameters is characterized by electron microscopy and the effect of HPT on the phase stability is monitored by synchrotron X‐ray diffraction. An increasing amount of applied strain leads to a stronger reduction in grain size while strain localization counteracts a homogeneous microstructural refinement. The positive effect of elevated deformation temperatures is demonstrated for Sm2Co17, which promotes homogeneous grain refinement, but causes strain‐induced phase transformations at the same time, strongly affecting the magnetic behavior. Superconducting quantum interference device magnetometry is used to characterize the magnetic properties after HPT deformation, which indicates the formation of a magnetic texture depending on the respective phase.

Influence of the Temperature of High Pressure Torsion Deformation on the Recrystallization Kinetics of Iron with a Submicrocrystalline Structure

Influence of the Temperature of High Pressure Torsion Deformation on the Recrystallization Kinetics of Iron with a Submicrocrystalline Structure

Magnetic characterization of HPT-deformed Fe–Si soft magnetic alloys before and after heat treatment and internal stress calculations

Fe, Fe-3wt%Si, and Fe-6.5 wt%Si alloys were subjected to high-pressure torsion (HPT) to achieve a nanostructure (<100 nm grain size). HPT processing was done at 77 and 293 K and heat treatments were performed on the deformed samples to reduce internal stresses that are known to deteriorate soft magnetic properties by impeding the movement of domain walls. X-ray line profile analysis (XLPA) was used to find out the crystallite/subgrain/domain size and the dislocation density in the deformed samples. The coercivity Hc was characterized on ring-shaped samples using a hysteresisgraph. Internal stresses in the deformed and heat-treated samples were determined using magnetic data and the dislocation density data and a comparison is made between the two approaches. HPT deformation increases the coercivity of the samples while heat treatment after deformation reduces the coercivity to a certain extent in Fe samples but has mixed results in Fe–Si alloys. In comparison with Herzer's theory, zero-frequency Hc was plotted against crystallite (or subgrain) size and it was found that coercivity follows a D1.5 relation, where D is crystallite size.

Исследование усталостных свойств и трибологические испытания стали AISI 316 после кручения под высоким давлением

The effect of high-pressure torsion process in a new die on the fatigue properties of corrosion-resistant steel AISI 316 has been studied. The deformation was carried out at cryogenic and room temperatures. The number of deformation cycles was 8. The initial blank had a ring shape with a diameter of 76 mm, a width of 3.5 mm and a thickness of 3 mm. It is revealed that the fatigue strength of both samples increases due to structure refinement and twinning in austenite during high pressure torsion deformation, partial martensitic transformation and increase of the share of more angular boundaries during cyclic deformation. The main factor of higher fatigue limit of AISI 316 steel after cryogenic deformation in comparison with room temperature is more intensive grinding of microstructure and presence of increased share of large-angle boundaries and more complete martensitic transformation.

Magnetic properties of NdFeB-based alloy under high-pressure torsion

Abstract When a multicomponent NdFeB-based magnetic alloy is deformed using high-pressure torsion (HPT), a quasi-stationary state is reached after 2.5 anvil revolutions, which corresponds to an equivalent strain of ∼40 at the sample mid-radius. In this state, torque self-oscillations are observed with a period of about 1.5 s and an amplitude of ∼10 % around the average value of 550 N m−1. Such self-oscillations are accompanied by strong acoustic emission. Before HPT, the alloy under study has an almost rectangular hysteresis loop with saturation magnetization J s = 135 emu g−1 and coercivity H c = 34.8 kOe. HPT deformation at initial stages transforms this alloy to the class of soft magnets: H c drops to 1.35 × 10−4 kOe, while J s practically does not change. An increase in strain leads to a gradual increase in H c to 9.61 kOe and a decrease in J s to ∼100 emu g−1 at the number of anvil revolutions n = 7. This is explained by HPT modification of the regular grain-boundary network of neodymium-rich paramagnetic phase layers. These layers provide magnetic isolation between grains of the Nd2Fe14B ferromagnetic phase. Periodic changes in torque and J s with increasing torsion angle are caused by transitions from the amorphous phase to the crystalline one and vice versa.

Structure and cryogenic mechanical properties of severely deformed nonequiatomic alloys of Fe–Mn–Co–Cr system

The work is devoted to a study of the structure and mechanical properties of two nonequiatomic medium-entropy nanocrystalline alloys, in which in a coarse state additional mechanisms act during plastic deformation — twinning (TWIP) in the Fe40Mn40Co10Cr10 alloy and phase transformations (TRIP) in the Fe50Mn30Co10Cr10 alloy. The nanocrystalline state in these alloys is achieved by high-pressure torsion (HPT) at 300 K and 77 K after different numbers of revolutions n = 0.25 and 5. In the nanostructural state in the TWIP Fe40Mn40Co10Cr10 and the TRIP Fe50Mn30Co10Cr10 alloys, a basically complete phase transition from the fcc lattice to hcp is observed, the content of which does not depend very strongly on the HPT temperature and deformation. For both alloys in the nanostructured state, there is a significant decrease in differences in the phase composition and microhardness Hv by comparison with the coarse-grained state. A decrease in the HPT temperature and an increase in HPT deformation for all the cases studied lead to an increase in the value of Hv. The Fe40Mn40Co10Cr10 TWIP alloy remains ductile under active compression deformation at 300 and 77 K, while there is no macroscopic plasticity in the Fe50Mn30Co10Cr10 TRIP alloy under similar conditions. For the Fe40Mn40Co10Cr10 TWIP the thermally-activated character of plastic deformation is retained during the transition from the coarse-grained to the nanostructured state.

X-ray line profile analysis study on the evolution of the microstructure in additively manufactured 316L steel during severe plastic deformation

316L stainless steel was manufactured by additive manufacturing (AM), and then, the samples were severely deformed by the high-pressure torsion (HPT) technique. The evolution of the microstructure was monitored by X-ray line profile analysis. This method gives the crystallite size and the density of lattice defects, such as dislocations and twin faults. The AM-processing of the HPT disks was performed in two different modes: the laser beam was parallel or orthogonal to the normal direction of the disks. The subsequent HPT deformation was carried out for ½, 1, 5 and 10 turns. The microstructure and hardness evolution during HPT were similar regardless of the laser beam direction. For both sample series, the minimum achievable crystallite size was about 30 nm, while the dislocation density and the twin fault probability got saturated at the values of 300–350 × 1014 m−2 and 3.5–4%, respectively. The microstructure evolution during HPT of the AM-prepared 316L steel was compared with the HPT-induced changes in an as-cast counterpart. It was found that while the AM-prepared 316L steel remained a single-phase face-centered cubic γ-structure during HPT, in the as-cast samples a body-centered cubic (bcc) martensitic α-phase became the main phase with increasing the imposed strain of HPT due to the lower Ni content. In the saturation state achieved by HPT the initially as-cast 316L steel had a considerably higher hardness (about 6000 MPa) than that for the AM-prepared samples (~ 5000 MPa) due to the large fraction of the hard bcc phase formed during HPT.

Deformation and failure behavior of nanocrystalline WCu

The technical potential of WCu alloys is limited by the modest fracture characteristics of the material system in its coarse-grained condition. To provide a nanocrystalline microstructure and improve mechanical properties, a W-50 at.% Cu composite was processed using high-pressure torsion deformation at a temperature of 200 °C. Therefore, two specimens were subjected to 100% and 1000% shear strains, respectively. Scanning electron and scanning transmission electron microscopy, including nanoscale energy dispersive X-ray spectroscopy mappings, were used to quantify the resulting microstructures. The average grain sizes for the 100% and 1000% deformed specimens were determined to be 14.7 ± 6.6 nm and 10.5 ± 5.6 nm, with the amount of mechanically intermixed W in the Cu grains increasing from 15.4 at.% to 15.9 at.%. X-ray diffraction and selected area electron diffraction studies both revealed strained lattice parameters of the W and Cu phases, respectively. Mechanical properties were investigated using in-situ notched microcantilever tests. The mean conditional fracture toughness and J-integral values were comparable for both conditions, at 3.7 ± 0.4 MPa√m and 245 ± 58 J/m2, respectively. The related behavior could be attributed to the low fault tolerance of the highly deformed states and was substantiated by cleaved globular W grains along the fractured surfaces. In addition, the detailed relationship between the altered grain boundary conditions, the degree of mechanical intermixing and the influence of the different microstructures on the fracture properties were carefully evaluated and discussed to pave the way for future application of these high-strength nanocomposites.

EFFECT OF HIGH PRESSURE TORSION ON STRUCTURE AND MECHANICAL PROPERTIES OF Al–Ca–Cu ALLOY

To improve the balance of strength and ductility of the Al–6% Ca–8% Cu (wt %) alloy, the high-pressure torsion (HPT) deformation followed by annealing was applied. The structure of the as-cast alloy consisted mainly of two eutectics [(Al) + AlCaCu] and [(Al) + (Al, Cu)4Ca + AlCaCu]. HPT through three turns leads to the formation of a predominantly submicrocrystalline structure, refinement of eutectic particles and their more uniform distribution in the sample volume, calcium segregation from AlCuCa and (Al, Cu)4Ca particles, and supersaturation of the (Al) solid solution with copper. Such a structure provides a strengthening of the alloy by a factor of 3.5, but contributes to its embrittlement. Subsequent annealing at 400°C achieves a good balance of strength and ductility of the alloy.

The key role of grain boundary state in deformation-induced softening effect in Al processed by high pressure torsion

The influence of type and amount of additional severe plastic deformation (SPD) on the magnitude of the deformation-induced softening (DIS) effect in ultrafine-grained (UFG) aluminum processed by high-pressure torsion (HPT) has been studied for the first time. Drastic DIS effect (the increase of plasticity from ∼1% to ∼23%), while maintaining high strength (ultimate tensile strength ∼ 180 MPa) is observed after a small additional deformation by HPT to 0.25 turns in UFG aluminum annealed at 150 °C for 1 h. An increase of additional HPT deformation to 0.75 turns does not lead to an increase of the DIS effect. The change of the type of additional deformation from HPT to cold rolling does not lead to any DIS effect, but slight hardening occurs and plasticity remains at low level (1–3%). The analysis of the evolution of the microstructure in correlation with mechanical behavior after annealing and subsequent deformations of both types indicates the key role of the state of the grain boundaries for the DIS effect in pure aluminum processed by HPT.

Exceptional strength-plasticity synergy in β-Ti alloy via HPT and short-period annealing

A new metastable β-type Ti-3.5Al-5Mo-4 V (Ti-B20) alloy with a high strength has been recently developed. However, the plasticity of the alloy after solution or aging treatment is poor due to its large initial β grain size, and conventional deformation-induced grain refinement is quite limited. In the present work, a high-pressure torsion (HPT) method was used to manufacture a Ti-B20 alloy with large deformed grains. The Ti-B20 alloy subjected to HPT was annealed in a β-phase field (850 ℃) for different periods. The deformation mechanism during HPT processing and the recrystallization behavior of the deformed alloy were systematically revealed. The results show that the deformation mode during HPT processing is mainly dislocation slip and shear band splitting of the original grains and martensitic phase (α″). During recrystallization annealing, the shear bands and high-density dislocations generated by the HPT deformation of the Ti-B20 alloy significantly promote nucleation and growth of recrystallized grains, which shortens the time to complete recrystallization. After recrystallization annealing, exceptional strength-ductility synergy with a high tensile strength of 941.9 MPa and a high plasticity of 39.7% is achieved in the fine-grained (14.68 µm) alloy, which is the highest elongation reported in the Ti-B20 alloy at present. The fine grains with a low dislocation density provide more space for dislocation slip and accumulation while maintaining high strength. This work provides a new possibility for preparing fine-grained β-titanium alloys with synergistic strength and plasticity.

Exchange Bias Demonstrated in Bulk Nanocomposites Processed by High-Pressure Torsion.

Ferromagnetic (Fe or Fe20Ni80) and antiferromagnetic (NiO) phases were deformed by high-pressure torsion, a severe plastic deformation technique, to manufacture bulk-sized nanocomposites and demonstrate an exchange bias, which has been reported predominantly for bilayer thin films. High-pressure torsion deformation at elevated temperatures proved to be the key to obtaining homogeneous bulk nanocomposites. X-ray diffraction investigations detected nanocrystallinity of the ferromagnetic and antiferromagnetic phases. Furthermore, an additional phase was identified by X-ray diffraction, which formed during deformation at elevated temperatures through the reduction of NiO by Fe. Depending on the initial powder composition of Fe50NiO50 or Fe10Ni40NiO50 the new phase was magnetite or maghemite, respectively. Magnetometry measurements demonstrated an exchange bias in high-pressure torsion-processed bulk nanocomposites. Additionally, the tailoring of magnetic parameters was demonstrated by the application of different strains or post-process annealing. A correlation between the amount of applied strain and exchange bias was found. The increase of exchange bias through applied strain was related to the microstructural refinement of the nanocomposite. The nanocrystalline maghemite was considered to have a crucial impact on the observed changes of exchange bias through applied strain.

Gripping Prospective of Non-Shear Flows under High-Pressure Torsion.

The article presents a theoretical study of the regimes of high-pressure torsion (HPT) for which slippage of the deforming material on the interfaces with anvils is possible. The approach taken is a generalisation of the currently accepted view of the HPT process. It enables a rational explanation of its salient features and the effects observed experimentally. These include a lag in the rotation angle of the specimen behind that of the anvils, an outflow of the material from the deformation zone, enhancement in gripping the specimen with anvils with increasing axial pressure, etc. A generalised condition for gripping the specimen with anvils, providing a basis for an analytical investigation of the HPT deformation at a qualitative level, is established. The results of the analytical modelling are supported by finite-element calculations. It is shown that for friction stress below the shear stress of the specimen material (i.e., for the friction factor m < 1), plastic deformation is furnished by non-shear flows, which expands the range of possible process regimes. The potential of these flow modes is impressive, which is reflected in the second meaning of the word “gripping” in the title of the article. Non-shear flows manifest themselves in the spreading of the material over the anvil surfaces whose cessation signifies the end of deformation and the beginning of slippage of the specimen as a whole. The model shows that for m < 1 such a finale is inevitable at any axial pressure. It predicts, however, that the highest achievable strain is increased when the axial pressure is raised in the course of the HPT process. Unlimited deformation of the specimen is only possible for m = 1, when slippage of the deforming material relative to the anvils is suppressed.

Temperature-Rate Conditions of Deformation and Structure-Forming Processes in Nickel during High-Pressure Torsion

Scanning and transmission electron microscopy has been used to investigate the nickel structure formed by high pressure torsion deformation at temperatures of 100–250°C. The relationship between the structure and temperature–rate conditions of deformation, which is characterized by the Zener–Hollomon parameter (Z), has been found. A change in the temperature–rate conditions of deformation has been shown to alter the dominant structure-forming process and to change the resultant structure. We have identified lnZ ranges where these processes occurred, namely, dynamic recovery at lnZ 25, dynamic recrystallization at 21 lnZ 25, and dynamic polygonization at lnZ 21.

Distribution of Order Parameter in Solids under High Pressure Torsion.

Severe plastic deformation (SPD) can lead to various phase transformations. High-pressure torsion (HPT) is one of the most important variants of SPD. In principle, HPT can continue almost indefinitely long, as long as the plungers are not destroyed. However, the number of defects in a material during HPT deformation cannot increase indefinitely. When the rate of defects production becomes equal to the rate of their annihilation, a steady state or dynamic equilibrium is reached. Unexplored is the issue of establishing equilibrium at the initial stage of plunger torsion, when there is an angular acceleration. The parameters of the steady state are described here using the idea of an order parameter in solids in the framework of Landau phenomenological theory and the Landau–Khalatnikov equation.