Room Temperature High-pressure Torsion Research Articles

Structural Aspects of the Formation of Multilayer Composites from Dissimilar Materials upon High-Pressure Torsion.

A multi-metal composite was consolidated from the Ti50Ni25Cu25 and Fe50Ni33B17 alloys by room-temperature high-pressure torsion (HPT). The structural research methods used in this study were X-ray diffractometry, high-resolution transmission electron microscopy, scanning electron microscopy with an electron microprobe analyzer in the mode of backscattered electrons, and the measurement of indentation hardness and modulus of the composite constituents. The structural aspects of the bonding process have been examined. The method of joining materials using their coupled severe plastic deformation has been established to play a leading role in the consolidation of the dissimilar layers upon HPT.

High Pressure Torsion of Copper; Effect of Processing Temperature on Structural Features, Microhardness and Electric Conductivity.

By optimizing the fabrication method, copper components featuring (typically contradicting) advantageous electric conductivity and favorable mechanical properties can be acquired. In this study, we subjected conventional electroconductive copper to a single revolution of high pressure torsion (HPT) at room temperature (RT), searched for the conditions which would yield comparable structure characteristics (grain size) when deformed at a cryogenic temperature, and finally compared the mechanical and electric behaviors to assess specific differences and correlate them with the (sub)structural development. 180° revolution of cryo-HPT imparted structure refinement comparable to 360° revolution of room temperature HPT, i.e., the average grain size at the periphery of both the specimens was ~7 µm. The 360° RT HPT specimen exhibited preferential (111)||SD (shear direction) texture fiber in all the examined regions, whereas the 180° cryo-HPT specimen exhibited more or less randomly oriented grains of equiaxed shapes featuring substantial substructure development of a relatively homogeneous character and massive occurrence of (nano)twins. These structural features resulted in the increase in microhardness to the average value of 118.2 HV0.2 and the increase in the electric conductivity to 59.66 MS·m-1 (compared to 105 HV0.2 and 59.14 MS·m-1 acquired for the 360° RT HPT specimen). The deformation under the cryogenic conditions also imparted higher homogeneity of microhardness distribution when compared to RT processing.

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.

Effect of high-pressure torsion on the structure and properties of the natural layered amorphous-crystalline Ti2NiCu composite

The room-temperature high-pressure torsion (HPT) behavior of a natural layered amorphous-crystalline composite with the precursor phases of the same chemical composition, but differing in structure has been studied by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and the measurements of indentation hardness (HIT) and indentation modulus (EIT) over the cross section of the samples. The force deformation parameters (torsion moment) realized upon HPT of the Ti2NiCu samples tested in three initial structural states (amorphous, crystalline, and composite) have been traced in situ with a special facility. Three different regions of amorphous, crystalline, and transitional structures are distinguished in the composite. It is shown that two different deformation mechanisms are simultaneously realized in crystalline and amorphous regions of the composite upon HPT. An attempt was made to identify the role of the transition layer upon deformation of the composite.

High pressure torsion induced lowering of Young's modulus in high strength TNZT alloy for bio-implant applications

An exceptional combination of low Young's modulus (E ~68 GPa) and high flow strength (σf ~1 GPa) was achieved for a consolidated β-Ti-based metastable Ti-35Nb-7Zr-5Ta (TNZT) alloy subjected to room temperature high-pressure torsion (HPT). The mechanical properties of the alloy were studied by quasistatic nanoindentation tests at different strain rates, where a reduction in Young's modulus E ~73 GPa (NHPT10) and E ~68 GPa (NHPT40) is observed together with an increase in plastic deformability (or HPT rotations). The microstructure evolution with increasing shear strain has been investigated. The stabilized bcc β-Ti phase with homogeneous nanostructure distribution was observed leading to a low Young's modulus. Severe straining causes a uniform hardness distribution without any noticeable change in the strength of the material. This study may be useful for developing excellent removable implant materials.

Magnetic properties of Mn-Al alloy after HPT deformation

In this study, we reported on the possibility to produce a bulk permanent magnet based on Mn55Al43C2 alloy by high-pressure torsion (HPT) deformation. The results show that using room temperature HPT deformation it is possible to obtain a small volume disk-shaped magnetic sample of 0.5 mm thickness and 8 mm diameter deformed for 1, 3 and 5 turns. The maximum coercive force of Hc = 0.37 T (3.7 kOe) at Ms = 0.49 T (75 emu/g) was obtained for the sample deformed for 5 turns (e = 5.16). The reason for enhance of coercive force is accumulation of crystalline defects acting as pinning centers for domain walls. Subsequent heat-treatment of deformed samples at temperature ranging 500–650 °C reduces the coercive force down to 0.24 T (2.4 kOe) with the simultaneous recovery of saturation Ms almost up to initial value 0.75 T (115 emu/g).

Microstructure and Mechanical Properties of High-Entropy Alloy Co20Cr26Fe20Mn20Ni14 Processed by High-Pressure Torsion at 77\u2009K and 300\u2009K

In this work, the mechanical characteristics of high-entropy alloy Co20Cr26Fe20Mn20Ni14 with low-stacking fault energy processed by cryogenic and room temperature high-pressure torsion (HPT) were studied. X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses were performed to identify the phase and microstructure variation and the mechanical properties characterized by Vickers hardness measurements and tensile testing. Cryogenic HPT was found to result in a lower mechanical strength of alloy Co20Cr26Fe20Mn20Ni14 than room temperature HPT. Microstructure analysis by SEM and TEM was conducted to shed light on the microstructural changes in the alloy Co20Cr26Fe20Mn20Ni14 caused by HPT processing. Electron microscopy data provided evidence of a deformation-induced phase transformation in the alloy processed by cryogenic HPT. Unusual softening phenomena induced by cryogenic HPT were characterized by analyzing the dislocation density as determined from X-Ray diffraction peak broadening.

Microstructural and texture evolution of copper-(chromium, molybdenum, tungsten) composites deformed by high-pressure-torsion

Cu-refractory metal composites containing Cr, Mo or W were subjected to severe plastic deformation using room temperature high-pressure torsion (HPT). A lamellar microstructure developed in each of the composites at equivalent strains of ∼75. The refractory metals developed {hkl}⟨111⟩ fibre textures with a slight tilt to the tangential direction. This texture was stronger and more clearly defined in Mo and W than in Cr.By applying additional HPT deformation to these samples, perpendicular to the original shear strain, it was found that the lamellar structure of Cu30Mo70 and Cu20W80 (wt%) composites could be retained at high equivalent strains and the refractory layer thickness could be reduced to 20–50 nm in Cu20W80 and 10–20 nm in Cu30Mo70. Although neighbouring regions of the microstructure were aligned and there was evidence of local texture in both composites, the bulk texture of Cu30Mo70 became weaker during this second step of HPT deformation. This was attributed to the refractory metal lamellae being discontinuous and imperfectly aligned.This work shows that it is possible to form ultrafine composites of Cu-group VI refractory metals via high-pressure torsion, with namolamellar structures being possible where there is a sufficient volume fraction of Mo or W.

Evolution of the Structure of Cu–1% Sn Bronze under High Pressure Torsion and Subsequent Annealing

The evolution of the structure of tin bronze under the room-temperature high-pressure torsion with different degrees of deformation and the subsequent annealing has been investigated. The thermal stability of the structure formed, namely, its behavior upon annealing in the temperature range of 150–400°C has been studied. The possibility of alloying copper with tin has been analyzed with the purpose of obtaining a thermally stable nanostructure with high strength characteristics.

Evolution of Microstructure and Mechanical Properties of a CoCrFeMnNi High-Entropy Alloy during High-Pressure Torsion at Room and Cryogenic Temperatures

High-pressure torsion (HPT) is applied to a face-centered cubic CoCrFeMnNi high-entropy alloy at 293 and 77 K. Processing by HPT at 293 K produced a nanostructure consisted of (sub)grains of ~50 nm after a rotation for 180°. The microstructure evolution is associated with intensive deformation-induced twinning, and substructure development resulted in a gradual microstructure refinement. Deformation at 77 K produces non-uniform structure composed of twinned and fragmented areas with higher dislocation density then after deformation at room temperature. The yield strength of the alloy increases with the angle of rotation at HPT at room temperature at the cost of reduced ductility. Cryogenic deformation results in higher strength in comparison with the room temperature HPT. The contribution of Hall–Petch hardening and substructure hardening in the strength of the alloy in different conditions is discussed.

The Influence of the Thermomechanical Processing Regime on the Structural Evolution of Mo-Nb-Ti-V Microalloyed Steel Subjected to High-Pressure Torsion

In the present study the effect of the thermal-mechanical processing regime—cold high-pressure torsion (HPT) at room temperature, cold HPT followed by annealing at 773 K (500 °C), and warm HPT at 723 K (450 °C)—on the peculiarities of the microstructure and microhardness of a Mo-Nb-Ti-V-0.08C microalloyed steel was analyzed. HPT processing resulted in high deformation and refinement of the initial structure and produced an ultrafine-grained microstructure featuring different morphologies with fine (<5 nm) and coarse (15 to 20 nm) carbides and with a mean size of the ferrite grain-subgrain structural elements of ≈100 nm. After room-temperature HPT, a mixed grain-subgrain structure with a high microhardness of 608 HV was obtained. Warm HPT caused formation of an ultrafine-grained structure with a microhardness of 553 HV. After annealing of cold HPT-processed specimens at 773 K (500 °C), a partial recovery of the structure occurred, but the average size of the structural elements and their distribution varied only slightly whereas the microhardness increased to 642 HV. The variations in the microhardness with the processing regime were correlated to changes in microstructural parameters (grain size, dislocation density and precipitation strengthening).

Effect of the initial state on nanostructuring and strengthening of middle- and high-strength age-hardenable aluminum alloys under severe plastic deformation (Review)

General requirements to the initial structure/phase state of the commercial middle- and high-strength age-hardenable aluminum alloys, especially to precipitates of aluminides of transition metals and main strengthening phases, under thermomechanical processing, involving severe plastic deformation (SPD), preliminary aging and re-aging, and aiming at nanostructuring the alloy matrix, are reviewed. The data on the alloy structure, hardness and tensile strength, realized in disc-shape samples severely deformed via room temperature high pressure torsion (HPT), have been taken into consideration. Influence of the nature, morphology and densities of secondary phases on efficiency of the alloys nanostructuring and strengthening are analyzed. The role of main strengthening phases in the alloys (sub)grain refinement to nano sizes, as well as in complete suppression of the matrix nanostructuring, are discussed. Nature of the alloys structure transformations and structure-strength relations owing to evolution of the grain and second phase structures due to SPD, preliminary and post-SPD heat treatment are considered. It was concluded that the «direct» impact of the formation of nanosized net of grain boundaries on the alloys strength is not exceeding the impact of its dispersion hardening under conventional aging. The main alloy hardening effect under HPT is due to an increasing the crystal defects densities, predominantly of dislocations.

In situ observation of the “crystalline⇒amorphous state” phase transformation in Ti2NiCu upon high-pressure torsion

The evolution of the structure and mechanical behavior upon deformation has been studied experimentally in situ by room-temperature high-pressure torsion (HPT) of the initially crystalline Ti2NiCu alloy. An abrupt increase in the shear stress upon HPT has been revealed. It was found that the observed effect is due to the deformation-induced “crystalline⇒amorphous state” phase transformation and the corresponding change in the mechanism of severe plastic deformation. It is shown that the amorphization of the material begins at the boundaries of grains and fragments of the crystalline phase as a result of grain boundary sliding processes. It was established that the amorphous boundaries form a “grain-boundary framework”, which upon deformation is expanded and transformed into bulk amorphous phase.

Intermetallics formed at interface of ultrafine grained Al/Mg bi-layered disks processed by high pressure torsion at room temperature

In this work, Mg/Al bi-layered disks were fabricated by high pressure torsion (HPT) at room temperature. A 0.26µm layer of γ-Mg17Al12 intermetallic formed at the interface of Mg/Al disks. This is very different to the interface of Mg/Al produced by other techniques such as diffusion bonding and friction stir welding where both γ-Mg17Al12 and β-Al3Mg2 form and the β-Al3Mg2 layer is much thicker than the γ-Mg17Al12 layer. This unusual phenomenon is attributed to the interfacial energies of intermetallics and the effect of room temperature HPT on diffusion of Al in Mg and the γ-Mg17Al12 layer.

Structural and phase transitions in the amorphous and nanocrystalline Ti50Ni25Cu25 alloys upon high-pressure torsion

The evolution of the structure and phase composition upon room temperature high-pressure torsion of the amorphous and nanocrystalline Ti50Ni25Cu25 alloy has been studied. It is shown that the accumulation of deformation regardless of the initial state of the alloy leads to the occurrence of cyclic «amorphous state⇒crystal» and «crystal⇒amorphous state» phase transformations. After the application of only hydrostatic pressure, without torsion, to the initially amorphous Ti50Ni25Cu25 alloy, a small amount of crystalline phase is present in the structure. The observed features of the structural transformations are explained in terms of the dissipation of mechanical energy, which is continuously introduced into the solids upon high-pressure torsion.

Dynamic Interactions between Precipitation and Plastic Deformation in Aluminium Alloys

Dynamic interactions between plastic deformation and precipitation processes in Aluminium alloys can lead to large changes in precipitation kinetics as compared to conventional heat treatments. This can be due to a number of different mechanisms such as changes in vacancy concentrations or movement of structural defects. Understanding the underlying mechanisms requires a quantitative measurement of the precipitate evolution as a function of deformation conditions, temperature and time. The present contribution will review experimental results obtained by Small-Angle X-ray Scattering in three different configurations, all concerning Al-Zn-Mg-Cu alloys: first, dynamic precipitate evolution during plastic deformation at ageing temperature; second, dynamic precipitation during low cycle fatigue at room temperature; and third, precipitation during ageing of a material processed by room temperature high pressure torsion. The experimental evidence will be used to discuss the role of the different mechanisms of interaction with plasticity on the precipitate microstructure evolution.

Thermal stability of nanocrystalline structure in niobium processed by high pressure torsion at cryogenic temperatures

Evolution of structure of Nb subjected to high-pressure torsion (HPT) in liquid nitrogen and further annealing in the temperature range of 100–600°C has been studied by transmission electron microscopy (TEM). HPT at the cryogenic temperature of 80K results in the formation of nanocrystalline structure in Nb, with crystallite sizes of about 75nm and the record-breaking microhardness of 4800MPa. The structure obtained is stable at room temperature but possesses relatively low thermal stability, namely, it undergoes recrystallization at lower temperatures than the structure after the conventional deformation or room-temperature HPT.

Atomic-scale structural evolution in amorphous Nd9Fe85B6 subjected to severe plastic deformation at room temperature

Understanding the deformation-induced nanocrystallization in amorphous alloys at room temperature is proved to be a great challenge. In the present study, the formation of vacancy-type defects richly surrounded with Fe atoms in amorphous Nd9Fe85B6 subjected to room-temperature high-pressure torsion deformation is directly evidenced by positron lifetime measurements combined with a coincident Doppler broadening measurement of the positron-electron annihilation photons. This demonstrates a direct experimental evidence for shear inducing the enrichment of Fe atoms in the deformed amorphous alloy. These results presented here are of importance for understanding the deformation-induced nanocrystallization and for the technique development of nanocrystalline materials.

Volume changes in Vitreloy bulk metallic glass during room temperature high-pressure torsion

Commercial Zr44Ti11Cu10Ni10Be25bulk metallic glass (Vitreloy 1b) disk was subjected to extreme plastic deformation by high-pressure torsion at room temperature. Two-dimensional mapping by high-intensity synchrotron x-ray diffraction in the plane of the shear deformation reveals no evidence of nanocrystallization; however, average effective volume changes as a function of the deformation can be evaluated.

Stress- and Strain Induced Phase Transformations in Pearlitic Steels

An overview of the mechanically driven phase transformations taking place in nanocrystalline pearlitic steels in conditions of the severe plastic deformation (SPD), i.e. combination of high pressure and strong shear strains will be given. Conditions of the discussed experiments (room temperature and moderate strain rates) exclude any thermal origin of the observed transformations. One of them is strain induced cementite decomposition, which is a well-documented phenomenon taking place at cold plastic deformation of pearlitic steels. We explain this process taking into account friction forces at the interface between the hard cementite and ferrite. Under the high pressures and stresses higher than the ferrite matrix yield stress, the later one behaves like a viscoelastic fluid. The friction at the precipitate/matrix interface leads to two effects. One is to induce high strains on the precipitates. This leads to shift of thermodynamic equilibrium towards dissolution of the cementite. The second is wear of the cementite phase due to friction at the ferrite/cementite interface and mechanically induced drag of carbon atoms by the ferrite. This had been recently confirmed in 3D AP experiments, which demonstrated that the process of cementite decomposition starts with depleting of carbides with carbon and formation of non-stoichiometric cementite. The existing theories of atom drag by moving dislocations (ballistic models) can be regarded as one of the many possible mechanism of wear discussed by the wear theory. In that respect the process can be called athermal, as temperature indirectly influences wear processes but is not their main cause. We observed also another strain driven transformation in nanocrystalline pearlitic steel during room temperature high pressure torsion. This is a stress induced α→γ transformation, which has never been observed at conventional deformation of coarse grained iron and carbon steels. This was concluded to have occurred due to a reverse martensitic transformation.