High-pressure Torsion Processing Research Articles

Using High Speed High Pressure Torsion for Cu–13Al–4Ni Shape Memory Alloy Processing

Recently, severe plastic deformation (SPD) is recognized as a very efficient processing technique for the fabrication of bulk nanostructured (ns)/ultrafine-grained materials. High-speed high-pressure torsion (HSHPT) process, an SPD method, is used as a novel approach to produce ultrafine grains in memory materials. The influence of HSHPT process variables on some Cu-based shape memory alloys (SMAs) is explored and discussed. This paper also envisages providing a comparative study of significant microstructural evolutions and features that are introduced by HSHPT processing. The plastic deformation mechanisms of the copper-based SMAs are investigated from macrostructural and microstructural points of view using optical and scanning electron microscopic techniques. In addition, significant grain size reduction brought about by the process is attested by X-ray diffraction and transmission electron microscopy. The transition temperatures have been determined by differential scanning calorimetry.

Post-treatment of additively manufactured Fe–Cr–Ni stainless steels by high pressure torsion: TRIP effect

High pressure torsion (HPT) at room temperature was used for post-treatment of additively manufactured Fe–Cr–Ni stainless steel with 12.9 wt % Ni as a very strong austenite stabilizer. The results showed that HPT caused a considerable increase in nanohardness of the additively manufactured samples. In contrast with thermodynamic equilibrium-state modeling, a phase transformation from FCC to HCP structure occurred, leading to the formation of ε-martensite during HPT on high angle boundaries, low angle boundaries, and dislocation cells with no detection of deformation twins. It was demonstrated that the combination of additive manufacturing thanks to the high density of dislocations after solidification and HPT process expands the opportunities of both methods to control deformation mechanisms in stainless steels leading to different phase and microstructural features. Thus, the outcome of this study provides a fundamental basis to design advanced structural materials.

Effect of Dislocation Networks on Precipitate Morphology in Maraging Steels

We introduce high dislocation densities in a maraging steel using High Pressure Torsion (HPT) processing, followed by ageing them. This gives rise to tensile behaviour substantially different from that of conventional maraging steels. Our Atom Probe Tomography (APT) studies on the steel, show that the morphologies of the Fe-Mo precipitates in such samples with a high density of networks of dislocations is disc-like. Finite element simulations of stress distributions and strain partitioning in such microstructures help explain the reduced macroscopic ductility in them. Using phase field modelling, we show that such morphologies are a result of faster diffusion paths provided by the dislocations. Thus, in addition to providing nucleation sites for precipitation, the faster diffusion paths influence the precipitate morphology and hence the macroscopic mechanical behaviour.

Impact of cryogenic cycling on tracer diffusion in plastically deformed Pd40 Ni40 P20 bulk metallic glass

The impact of thermo-mechanical treatment, in particular of room temperature high-pressure torsion (HPT), cold rolling and cryogenic thermo-cycling on shear band diffusion in a model Pd40Ni40P20 bulk metallic glass is investigated. Tracer diffusion measurements reveal the existence of fast short-circuit diffusion in deformed glasses and the diffusion enhancement was found to depend on the processing mode. The HPT processing imposes an extremely large shear deformation under quasi-hydrostatic conditions whereas cold rolling induces individual well-separated shear bands, though shear band diffusion is found to be faster after cold rolling. The cryogenic thermo-cycling enhances the shear band diffusion further and a conceptual explanation for the processes leading to kinetic rejuvenation is proposed.

A more defective substrate leads to a less defective passive layer: Enhancing the mechanical strength, corrosion resistance and anti-inflammatory response of the low-modulus Ti-45Nb alloy by grain refinement

Orthopedic and dental implants made of β-type Ti alloys have low elastic modulus which can better relieve the stress shielding effects after surgical implantation. Nevertheless, clinical application of β-type Ti alloys is hampered by the insufficient mechanical strength and gradual release of pro-inflammatory metallic ions under physiological conditions. In this study, the β-type Ti-45Nb alloy is subjected to high-pressure torsion (HPT) processing to refine the grain size. After HPT processing, the tensile strength increases from 370 MPa to 658 MPa due to grain boundary strengthening and at the same time, the favorable elastic modulus is maintained at a low level of 61-72 GPa because the single β-phase is preserved during grain refinement. More grain boundaries decrease the work function and facilitate the formation of thicker and less defective passive films leading to better corrosion resistance. In addition, more rapid repair of the passive layer mitigates release of metallic ions from the alloy and consequently, the inflammatory response is suppressed. The results reveal a strategy to simultaneously improve the mechanical and biological properties of metallic implant materials for orthopedics and dentistry. Statement of significanceThe low modulus Ti-45Nb alloy is promising in addressing the complication of stress shielding induced by biomedical Ti-based materials with too-high elastic modulus. However, its insufficient strength hampers its clinical application, and traditional strengthening via heat treatments will compromise the low elastic modulus. In the current study, we enhanced the ultimate tensile strength of Ti-45Nb from 370 MPa to 658 MPa through grain-refinement strengthening, while the elastic modulus was maintained at a low value (61-72 GPa). Moreover, substrate grain-refinement has been proved to improve the corrosion resistance of Ti-45Nb with reduced inflammatory response both in vitro and in vivo. A relationship between the substrate microstructure and the surface passive layer has been established to explain the beneficial effects of substrate grain-refinement.

Influence of carbon on the mechanical behavior and microstructure evolution of CoCrFeMnNi processed by high pressure torsion

In this study, a Cantor type high entropy alloys with the addition of C interstitials (, 0, 0.5 and 2 at.%) were processed via high pressure torsion (HPT) under 6.5 GPa by 0.5, 1 and 3 turns at room temperature. The microstructures and mechanical properties of samples before and following HPT were investigated. In all compositions studied, HPT deformation led to a dramatic grain size refinement down to a nanoscale range and also resulted in a considerable increase in dislocation density. Atom Probe Tomography studies reveal that in the alloy with 2 at.% C, C atoms segregated at the boundaries of the nano-grains. The hardness of specimens with 0, 0.5 and 2% at. C approached a maximum value at a plateau at 490 HV, 550 HV and 640 HV, respectively. This plateau was reached at an HPT shear strain corresponding to more than ~20. The yield and ultimate tensile strength values of the as-cast alloys increased with increasing C content. The uniform elongation of all three alloys studied exceeded 30%. The yield strength values of the HPT processed samples increased significantly and reached 1.7 GPa, 1.9 GPa and 2.4 GPa, respectively, with 0, 0.5 and 2 at.% C; however, a dramatic decrease of ductility was observed. Analysis of the factors contributing to the strengthening of HPT processed alloys revealed that the conventional approach based on dislocation motion resulted in significantly overestimated yield stress values in comparison with the experimentally obtained ones. It was proposed that the discrepancy between the theoretical estimate and experimental results is related to the emergence of grain boundary sliding. The present results provide new insights on the leverage of strength versus ductility by combining HPT processing with alloying with an interstitial-type element.

Mechanical mixing of Mg and Zn using high-pressure torsion

High-pressure torsion (HPT) processing has proved to be a powerful tool to consolidate metallic particles and fabricate nanostructured metal-matrix composites with a wide range of compositions. In this study, HPT was used to fabricate different Mg-Zn composites at room temperature, and the evolution of microstructure and mechanical properties were analyzed by x-ray diffraction, scanning and transmission electron microscopy, dynamic hardness and Vickers microhardness tests. The results show that ultrafine-grained microstructures were achieved in all composites compositions. Smaller grain sizes are observed in Mg-rich phases near areas with significant segregations of Zn. The Mg-rich phase appears to retain less deformation than the Zn-rich phase. Bending and vortex phenomena, that are usually reported in microscale for materials mixed by HPT, are observed in nanoscale in this work. There is evidence of the MgZn2 intermetallic phase with a morphology that varies with composition. Higher levels of deformation imposed by HPT leads to an increase in hardening and a decrease in strain-rate sensitivity which are attributed to the tendency to form intermetallics and Zn segregations that prevent grain boundary sliding. Moreover, a model is proposed to explain the mixing of phases in microscale and its relation to the evolution of mechanical properties.

Continuous high-pressure torsion of pure Al and Al-2 wt% Fe alloy using multi-wires

The process of high-pressure torsion (HPT) is applied for grain refinement in a wire form of pure Al and an Al-2% Fe alloy. An earlier version of the continuous HPT (CHPT) was further developed in this study, so that two wires were processed simultaneously. The two wires were placed symmetrically along a circumferential direction so that the pressure was equally applied on the wires to allow a stable processing. Guiding holes were made at both the inlet and outlet sides of the grooves on the upper anvils so that no interference of the two wires occurs. As in the earlier version, the surface roughness of the upper grooves and lower grooves were made different. Thus, this difference causes a continuous material flow when the lower anvil is rotated with respect to the upper anvil due to the occurrence of more slippage with the upper anvil. The modified CHPT process was then performed using the upsized anvils having two grooves with a half circular cross section on the upper and lower anvils with 100-mm-outer and 94-mm-inner diameters. The grain size was reduced to ~ 1 μm for pure Al and ~ 400 nm for the Al-2% Fe alloy after rotation by a quarter turn. The tensile strength of 280 MPa was achieved in the Al-2% Fe alloy, and an equivalent strain of ~ 25 was introduced through this CHPT process. It is suggested that CHPT process with multi numbers of wires is feasible when the anvil diameter is enlarged.

Effect of severe plastic deformation by high-pressure torsion at different temperatures and subsequent annealing on structural and phase transformations in Zr-2.5% Nb alloy

The effect of the temperature of the high-pressure torsion (HPT) deformation and subsequent annealing in a wide temperature range on the structural and phase transformations in the Zr-2.5% Nb alloy and its microhardness has been studied. In the course of the HPT process, the grain structure in the alloy is refined to the nano- and submicron level, the density of crystal defects increases and the α-Zr → ω-Zr phase transformation occurs, which are accompanied by an increase in the microhardness values. The microstructure of the alloy after HPT is characterized by significant inhomogeneity, and the crystallite size increases slightly with increasing the HPT temperature. Annealing of the HPT-processed alloy samples leads to their softening due to the reverse ω-Zr → α-Zr phase transformation and due to the grain growth. With an increase in the HPT-deformation temperature from room temperature to 325 °C, the amount of the ω-Zr phase in the alloy structure decreases, while its thermal stability increases. The HPT-processed Zr-2.5% Nb alloy has an advantage for medical applications compared to other commercial zirconium alloys.

Microstructural evolution and properties of a Ti-Nb-Ta-Zr-O prepared by high-pressure torsion

In this work, the Ti-29Nb-13Ta-4.6Zr-0.07O (TNTZO) alloy was processed by high-pressure torsion (HPT), and its microstructure, phase stability, elastic behavior, mechanical properties and deformation mechanisms were determined for samples prepared under different conditions. An increase in β-phase stability relative to martensite and a significant hardening was observed after severe plastic deformation. The increase in β-phase stability is attributed to the sharp increase in the system’s Gibbs free energy that occurs when the α"-laths reach a critical nanometer-size during the HPT processing. This system’s Gibbs free energy increase triggers the occurrence of the α"→ β reverse martensitic transformation. The greater hardness can be related to three factors: microstructural refinement; high dislocation density and stress-induced formation of the ω-phase. Plastic deformation is based on conventional slip, detwinning, stress-induced martensite formation, as well as the reverse martensitic transformation. The reverse martensitic transformation observed during HPT corresponds to a recently discovered mechanism that has not been well addressed in the literature until now. In this paper, this new mechanism is clearly detected and detailed discussed based on a systematic Rietveld refinement, in-depth microscopy, and calorimetry.

Vacancy release upon heating of an ultrafine grain Al-Zr alloy: In-situ observations and theoretical modeling

The present study demonstrates the direct observation of pore formation by rapid release of vacancies in ultrafine grain (UFG) Al-0.4at%Zr alloy during in-situ annealing in a scanning transmission electron microscope. The ultrafine grain structure was preliminary obtained by high pressure torsion processing at a hydrostatic pressure of 6 GPa up to 10 revolutions. In-situ annealing reveals the rapid formation of pores in triple junctions (TJs) of grain boundaries (GBs) during the first few minutes followed by their subsequent slow resorption with the complete disappearance of some of them. During annealing, no noticeable displacement of GBs is observed. By considering the evolution of non-equilibrium GBs inherited by the severe plastic deformation, a theoretical description is suggested which describes: (i) the pore formation at disclinated TJs as a thermodynamically driven process of free volume dissolution through generation of vacancies, which then migrate to the TJs and coagulate at them with growth of the pores diminishing the strain energy of the TJ disclinations, and (ii) further decrease of the TJ disclination strain energy through the climb of extrinsic GB dislocations towards the disclinated TJs, accompanied with dissolution of the TJ pores by emission of vacancies which provide the dislocation climb. The rapid release of excess vacancies during the early stage of heat treatment is consequently identified as a phenomenon responsible for accelerated atomic mobility. This work hence provides a new perspective for understanding the accelerated precipitation kinetics observed in severely deformed alloys.

Severe Plastic Deformation of Copper, Binary Cu-Zn Solid-Solution Alloys, and High-Strength Brass by High-Pressure Torsion

Severely-deformed high-strength brasses were investigated by leveraging high-pressure torsion (HPT) processing in order to obtain more enhanced mechanical properties of copper alloys. Pure copper, binary Cu-Zn solid-solution alloys and high-strength brasses including aluminum and manganese additions were selected for experiments. For comparison of these materials, zinc equivalent parameter was used. These materials were subjected to the HPT processing, followed by hardness tests and tensile tests. The maximal hardness value of the nanostructured high-strength brass with β phase matrix was reached 420 HV. The HPT processed high-strength brass with β phase matrix showed significant increase in the yield stress and tensile strength with sacrificing ductility. The tensile specimen of the high-strength brass with β phase matrix was fractured before initiation of necking. It was found that utilizing β phase matrix is also beneficial for controlling enhanced strength of high-strength copper alloys for not only casting process but also severe plastic deformation.

Hardness Development of Mechanically-Bonded Hybrid Nanostructured Alloys through High-Pressure Torsion

Processing through the application of high-pressure torsion (HPT) provides significant grain refinement in bulk metals at room temperature. These ultrafine-grained (UFG) materials after HPT generally demonstrate exceptional mechanical properties. Recent reports demonstrated the bulk-state reactions for mechanical bonding of dissimilar lightweight metal disks to synthesize hybrid alloy systems by utilizing conventional HPT processing. Accordingly, the present report provides a comprehensive summary of the recent work on processing of several UFG hybrid alloy systems including Al-Mg and Al-Cu by HPT under 6.0 GPa at room temperature and a special emphasis was placed on understanding the evolution of hardness. This study demonstrates a significant opportunity for the application of HPT for a possible contribution to current enhancements in diffusion bonding, welding and mechanical joining technologies as well as to an introduction of hybrid engineering nanomaterials.

Microstructure and magnetic properties of Mn-Al-C permanent magnets produced by various techniques

Bulk Mn52Al46C2 in τ-phase was prepared by vacuum induction melting and used as precursor for the production bulk permanent magnets by suction casting and hot-extrusion. Part of the precursor alloy was mechanically milled into a τ-phase powder and used as precursor for production of samples by electron beam melting, hot-compaction and high pressure torsion processes. The microstructure and magnetic properties of all samples were investigated and correlated. It was found that the mechanical deformation enhances coercivity, up to 0.58 T, while the absence of this strain is beneficial for magnetization. Among the observed techniques, hot extrusion and high pressure torsion have shown promising possibilities to further develop Mn-Al-C as permanent magnets. However, it should be taken into account the challenges related to design a proper processing window for hot extrusion and the limitation of HPT regarding the absence of texture.

A review paper on magnesium alloy fabricated by severe plastic deformation technology and its effects over microstructural and mechanical properties

Abstract This review paper mainly focused on microstructural and mechanical properties ofultrafine grained nanocrystalline (Nc) Magnesium alloy fabricated by Severe Plastic Deformation(SPD) process. The influence of SPD over mechanical, microstructural, physical, optical, and corrosion properties of Nanocrystalline magnesium alloy are detailed and discussed. It has been observed that nanocrystalline (Nc) magnesium materials has shown superior properties over the bonding of grains while during the Extrusion, Annealing, quenching, Normalizing, Spark Plasma Sintering (SPS) processes. The outcome based by many researchers showed that nanocrystalline Magnesium alloy processed by Severe Plastic Deformation (SPD) showed an improved properties over the Hardness, Dislocation density, Compressive, Tensile and texture, and it’s found to be increased up to certain limit beyond that decrease of microstructural stability, micro hardness and dislocation density were seen to be observed. The Microstructural and Mechanical properties are well improved by Spark Plasma Sintering (SPS), Extrusion, Annealing, Equal channel angular pressing (ECAP), Accumulative Roll Bonding (ARB) and High-Pressure Torsion (HPT) processes. Grain growth Mechanism, Energies Activation during grain growth, Thermal stability, Relative density and stress corrosion rate were observed by previous Researchers during Fractography analysis over several nanocrystalline (Nc) Magnesium alloy specimens.

Microstructural and Hardness Evolution in a Duplex Stainless Steel Processed by High-Pressure Torsion

The duplex stainless steel 2205, designated DSS2205 and having a duplex structure comprising ferrite and austenite phases, was processed by high-pressure torsion (HPT) and the microstructural and hardness evolutions were investigated after various HPT revolutions and at different positions within the specimens. The results show that the grain refinement induced by severe deformation processing is different in the ferrite and austenite phases such that the ferrite grains are refined via dislocation subdivision, whereas grain refinement in the austenite phase depends mainly on the interaction of dislocations and twin boundaries at relatively low strains. When the numbers of revolutions increases, the grain refinement in austenite restricts the occurrence of deformation twinning so that dislocation slip becomes dominant. During HPT processing, the effect of the phase boundaries on the mechanical properties of the alloy is very significant. The results show the average width between two adjacent phases and the hardness of the alloy are generally consistent with the classical Hall–Petch relationship.

High pressure torsion processing of maraging steel 250: Microstructure and mechanical behaviour evolution

Maraging steels are precipitation strengthened martensitic steels with an unusual combination of strength and ductility. High Pressure Torsion (HPT) has been used in this study to produce maraging steel 250 grade (AMS 6512) with finer laths and higher dislocation density, both of which act as nucleation sites for precipitation, and reverted austenite formation. This study focusses on the effect of such a processing on the evolution of the microstructure, including kinetics of precipitation, recrystallisation and austenite reversion as well as the stability of the precipitates thus formed. It was found that the aging kinetics accelerated substantially in the HPT processed samples, by achieving peak aging conditions at considerably shorter temperature/time durations and also a peak hardness higher than the as-received sample by 41%. Detailed microstructural characterisation revealed a change in the precipitate morphology from spherical to plate like form in the overaged conditions. The impact of this on mechanical response of these steels was quantified using tensile tests. A 70% increase in ultimate tensile strength was achieved in HPT processed samples after peak aging. Changes in strength and ductility were correlated to the changes in the microstructure and their impending contributions to different strengthening mechanisms at play to enable better design of maraging steels.

The effect of high-pressure torsion on the microstructure and outstanding pseudoelasticity of a ternary Fe–Ni–Mn shape memory alloy

Abstract Experiments were conducted to examine the effect of high-pressure torsion (HPT) processing on the microstructure and pseudoelastic behavior of a ternary Fe–10Ni–7Mn (wt.%) shape memory alloy in both the solution-annealed and intercritically-annealed conditions. X-ray diffraction (XRD) patterns and electron backscatter diffraction (EBSD) analyses showed that the initial microstructure of the alloy in the solution-annealed condition was a fully lath α′-martensite which partially transformed to a strain-induced austenite (α′→γ) by HPT processing. Also, the austenite formed in the dual phase (α′+γ) specimens after intercritical annealing treatment at 600 °C for 7.2 ks underwent a γ→e→α′ transformation during subsequent HPT processing such that a multi-phase microstructure was formed consisting of α′-martensite, austenite and e-martensite. The HPT processing led to a significant increase in the microhardness value to ~690 Hv due to a high density of dislocations and the associated grain refinement of the microstructure. Cyclic loading-unloading tensile tests at room temperature revealed a strain hysteresis and pseudoelastic behavior in the HPT-processed specimens with different initial microstructures. Outstanding pseudoelasticity values of about 67% and 75% were obtained at the fourteenth loading-unloading cycle after 20 HPT turns in the solution-annealed and intercritically-annealed specimens, respectively.

Structural Characteristics of Multilayered Ni-Ti Nanocomposite Fabricated by High Speed High Pressure Torsion (HSHPT)

It is generally accepted that severe plastic deformation (SPD) has the ability to produce ultrafinegrained (UFG) and nanocrystalline materials in bulk. Recent developments in high pressure torsion (HPT) processes have led to the production of bimetallic composites using copper, aluminum or magnesium alloys. This article outlines a new approach to fabricate multilayered Ni-Ti nanocomposites by a patented SPD technique, namely, high speed high pressure torsion (HSHPT). The multilayered composite discs consist of Ni-Ti alloys of different composition: a shape memory alloy (SMA) Ti-rich, whose Mf > RT, and an SMA Ni-rich, whose Af < RT. The composites were designed to have 2 to 32 layers of both alloys. The layers were arranged in different sequences to improve the shape recovery on both heating and cooling of nickel-titanium alloys. The manufacturing process of Ni-Ti multilayers is explained in this work. The evolution of the microstructure was traced using optical, scanning electron and transmission electron microscopes. The effectiveness of the bonding of the multilayered composites was investigated. The shape memory characteristics and the martensitic transition of the nickel-titanium nanocomposites were studied by differential scanning calorimetry (DSC). This method opens up new possibilities for designing various layered metal-matrix composites achieving the best combination of shape memory, deformability and tensile strength.

Optimization of high pressure torsion processing for fabrication of the Al-Nb hybrid system

A hybrid composite material fabricated from the Al-Nb system using high-pressure torsion (HPT) up to 30 turns has been studied in the present work. To fabricate the composite, deformation of a three-layer Al-Nb-Al package was carried out at room temperature on Bridgman anvils with grooves under a pressure of 5 GPa at N=10, 25 and 30 revolutions, at a strain rate of ω=1 and 2 rpm. Initial disc diameters from pure metals and HPT conditions were experimentally optimized to obtain monolithic and defect-free composite samples. The most intensive fragmentation and stirring of niobium in the aluminium matrix was observed if diameter of aluminium discs was 10 mm and deformation conditions N=25 and 30 revolutions and ω=2 rpm were applied. Three microstructural zones were observed after HPT under optimal conditions: the central zone with wide curved layers of niobium in aluminium, the mid-radius zone with finely dispersed layered structure, and periphery with a uniform distribution of niobium in the aluminium matrix. It was shown that HPT led to occurrence of strain-induced ageing resulting in formation of the intermetallic Al3Nb phase. The microhardness measured along the diameter of the obtained composite materials changed nonmonotonically depending on the produced structure (microstructural zone).