Search

The unique nonuniform deformation characteristic of high-pressure torsion was used to produce nanostructures with systematically varying grain sizes in a copper disk, which allows us to study the grain-size effect on the deformation mechanisms in nanostructured copper using a single sample. The as-processed copper disk has 100–200 nm grains near its center and 10–20 nm grains at its periphery. High densities of full dislocations (2×1016/m2) were distributed nonuniformly in large grains, implying that dislocation slip is the dominant deformation mechanism. With increasing dislocation density, the dislocations accumulated and rearranged, forming elongated nanodomains. The originally formed nanodomains remain almost the same crystalline orientation as their parent large grains. Further deformation occurred mainly through partial dislocation emissions from nanodomain boundaries, resulting in high density of nanotwins and stacking faults in the nanodomains. The elongated nanodomains finally transformed into equiaxed nanocrystalline grains with large-angle grain boundaries. The results suggest that grain boundary rotation and grain boundary sliding might play a significant role in the formation of large-angle grain boundaries in nanocrystalline grains. These experimental results show that different deformation mechanisms operate at different length scales and confirm unambiguously the deformation mechanisms of nanocrystalline grains predicted by molecular dynamic simulations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultrafine-grained Al alloys produced by high-pressure torsion are found to exhibit a very high strength, considerably exceeding the Hall–Petch predictions for ultrafine grains. This phenomenon can be attributed to the unique combination of ultrafine structure and deformation-induced segregations of solute elements along grain boundaries, which may affect the emission and mobility of intragranular dislocations. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure and the spectrum of grain boundary misorientations were studied in Armco iron, following high pressure torsion (HPT) deformation, by means of transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). It was found that HPT deformation results in the formation of an equiaxed grain structure with a mean grain size of 270 and 130nm using a shear strain of γ=210 and 420, respectively. The misorientation spectra in HPT iron have a bimodal character with maxima in low (at 1–2°) as well as in high misorientation angle ranges. A marked increase in the fraction of special boundaries (Σ3–Σ45) was revealed as a result of HPT. The microstructural changes due to HPT are discussed and compared with those obtained during conventional deformation modes. [Copyright &y& Elsevier]

The influence of grain boundary segregation on the strength of a nanostructured austenitic stainless steel was investigated. The steel was nanostructured by severe plastic deformation at two different temperatures to form ultrafine-grained states different by microstructure parameters. It is shown that despite the difference in grain size both nanostructured steels demonstrated the same level of strength. For the first time it is directly observed that severe plastic deformation at elevated temperature leads to formation of Mo–Cr–Si rich grain boundary segregations in the steel. Considering different contributions to the material strengthening, we demonstrate that grain boundary segregations can lead to significant enhancement of the yield stress. [ABSTRACT FROM AUTHOR]

Emission of lattice dislocations from grain boundaries (GBs) specified by deformation-distorted structures with periodic fluctuations of misorientation in deformed ultrafine-grained (UFG) materials is theoretically described. It is theoretically revealed that (i) the dislocation emission from deformation-distorted GBs is significantly enhanced as compared to that from structurally equilibrated GBs; and (ii) the enhancement effect depends on the parameters specifying the deformation-distorted GBs. The influence of deformation-distorted GBs as dislocation sources on the tensile ductility of UFG materials is discussed. [ABSTRACT FROM AUTHOR]

Abstract: The generated results on superplastic flow (SPF) in materials produced by severe plastic deformation (SePD) have been analyzed. Formation of sliding surfaces with subsequent cooperative grain boundary sliding (CGBS) has been demonstrated to be a main deformation mechanism of superplasticity under conditions of microstructural stability. A substantial role of dislocation glide in accommodation processes is shown. When grain growth is observed during deformation, continuous grain boundary migration prevents formation of slide surfaces and CGBS is restrained. In this case, deformation of the material is more likely to occur by dislocation or diffusion creep than true SPF despite relatively large strains being reached. [Copyright &y& Elsevier]

A model is proposed explaining enhanced strength of ultrafine-grained alloys that contain grain boundary (GB) solute segregations. In the framework of the proposed model these segregations are treated as homogeneous ellipsoidal inclusions and act as the sources of elastic stresses affecting the emission of lattice dislocations from GBs. These segregations pin the ends of lattice dislocation segments at the initial stage of dislocation propagation along GBs, and the unpinning requires a load increase, leading to the enhanced yield strength. We calculate the contribution of GB segregations to the yield strength for the ultrafine-grained 1570 Al alloy. We demonstrate that the maximum yield strength of this alloy is achieved in the case of clustered, nearly spherical Mg segregations with a high Mg concentration and a diameter to thickness ratio of 1.0–1.4, depending on the Mg concentration inside segregations. We also briefly discuss the possible role of GB dislocations in the formation of such concentrated solute segregations as well as the influence of GB segregations on the strengthening of alloys containing nanoscale twins. The results of the calculations agree well with experimental data. • Enhanced strength of alloys containing grain boundary segregations is modeled. • Segregations are the sources of elastic stresses increasing the yield strength. • Small segregations with a high solute concentration provide maximum strengthening. [ABSTRACT FROM AUTHOR]

Electron backscatter diffraction was applied to examine the fundamental mechanisms governing the development of ultrafine-grain microstructures during equal-channel angular pressing of commercial-purity titanium. Texture analysis and examination of misorientation distributions revealed the enhanced activity of non-prismatic slip systems at prior grain boundaries, which promoted the preferential development of deformation-induced boundaries in these areas. By contrast, deformation in the interior of grains was dominated by prism slip which resulted in sluggish microstructural evolution. This difference in slip activity gave rise to the preferential nucleation of ultrafine grains at prior grain boundaries and thus the development of a bimodal grain structure. The grain refinement was concluded to result primarily from the incompatibility of deformation in neighboring grains. Image 1 [ABSTRACT FROM AUTHOR]

EK-181 is a low-activation ferritic/martensitic steel that is an attractive candidate for in-core component materials for both fast reactors and fusion reactors. To assess the effect of microstructural engineering on radiation response, two variants of EK-181 were studied: one in an annealed condition and the other subject to severe plastic deformation. These specimens were irradiated with 3.5 MeV Fe self-ions up to 400 peak displacements per atom (dpa) at temperatures ranging from 400 °C to 500 °C. The deformation did not suppress swelling over the whole irradiated region. Instead, deformed samples showed higher swelling in the near-surface region. Void swelling was found to be correlated with grain boundary instability. Significant grain growth occurred when steady-state void growth started. [ABSTRACT FROM AUTHOR]

The influence of microstructure evolution on microhardness and electrical resistivity of ultrafine grained (UFG) commercial purity Al under annealing at different temperatures within a range of 363–673 K was studied. The initially coarse grained Al was processed by high pressure torsion (HPT) technique for the formation of UFG structure. The microstructure was characterized by electron backscattering diffraction and X-Ray diffraction. It was shown that annealing of UFG Al at temperatures within a range of 363–473 K leads to simultaneous increase of microhardness (by 6–13%) and electrical conductivity (by 4–8% at 300 K). The correlation between microstructural features and the resulting properties were analyzed. The average width s of potential barriers at grain boundaries (GBs) in HPT-processed Al was estimated in the frame of a tunnel model. The obtained large value of s compared with the GB crystallographic width is associated with elastically distorted lattice near GBs. The obtained results suggest a new way to increase simultaneously strength and electrical conductivity of UFG Al alloys by an appropriate annealing. [ABSTRACT FROM AUTHOR]

The relatively low strength and poor ductility of conventional AZ80 Mg alloys have been attributed to the limited number of independent slip systems, in combination with the formation of fragile eutectic β-Mg 17 Al 12 networks at grain boundaries. In an effort to overcome these limitations, spray forming followed by equal channel angular pressing (ECAP) was employed to obtain a unique bi-modal microstructure: coarse grains were separated and surrounded by deformation networks consisting of ultrafine-grained Mg with an average grain size of 0.6 µm and ellipsoidal shaped β-Mg 17 Al 12 particles with sizes of 200–300 nm. Tensile tests revealed the advantage of this structure: a yield strength of 235 MPa combined with an elongation to failure of 14%; the values are significantly higher than those of their conventional counterparts (100 MPa-12%, and 140 MPa-5%). The underlying strengthening and deformation mechanisms of this particular microstructure are discussed and analyzed. [ABSTRACT FROM AUTHOR]

High-resolution electron backscatter diffraction (EBSD) was used to examine the grain structure in long rods of ultrafine-grain (UFG) commercial-purity titanium produced for medical implants. The UFG material was obtained by equal channel angular pressing (ECAP) using a Conform scheme followed by rod drawing. The microstructure was found to be bimodal consisting of relatively coarse elongated grains containing well-developed substructure and ultrafine grains. The mean grain size was ~0.3 μm, and the fraction of high-angle boundaries was ~45%. The material had a strong < 10 1 ¯ 0 > -fiber texture. An analysis of the microstructure–properties relationship showed that the increase in material strength developed during severe plastic deformation resulted from grain refinement and substructure evolution in comparable measure. [ABSTRACT FROM AUTHOR]

Exploring an approach to grain boundary (GB) mesoscopic structure in 3D polycrystals, we develop a model description for equilibrium and non-equilibrium states of GBs in graphene. Non-equilibrium states of GBs in graphene are analyzed for the first time. On the base of this model, dependence of total energy for symmetric GBs on the misorientation angle, E ( θ ), in graphene was calculated. It was found that for the same average misorientation angle the energy in a non-equilibrium state of a GB in graphene can significantly exceed the energy in the equilibrium state. In graphene, the GB energy depends to a greater extent on the deviation from the equilibrium state than on the misorientation angle. It is proposed to account for the presence of non-equilibrium GBs in the explanations of unusual physical properties of graphene. [ABSTRACT FROM AUTHOR]

Abstract: The effects of high-pressure torsion (HPT) processing on an Al–Mg–Si alloy (AA6060) have been investigated comprehensively. We show that the processing temperature has complex effects on the strength, grain refinement and solute nanostructures of the alloy. Ten-revolution HPT processing at room temperature produced the highest yield strength of 475MPa, which is similar to a high-strength Al alloy. However, processing at 100°C produced the finest grains due to the strong solute segregation to grain boundaries and the formation of high-density precipitates that pin grain boundaries. Processing at 180°C led to significant decomposition of the alloy and the formation of coarse precipitates. This research demonstrates that solute nanostructures provide key information for unravelling the origins of HPT-induced strengthening and grain refinement, and reveals the important opportunities for “engineering” solute nanostructures to enhance grain refinement in HPT processing. [Copyright &y& Elsevier]

In ultra-fine grained Al–Zn alloys after high pressure torsion Al/Al grain boundaries (GBs) completely, partially or pseudopartially wetted by Zn in the solid state have been observed using analytical and high resolution electron microscopy. In the latter case the solid Zn particles in the triple joints form a non-zero contact angle θ=80–160° with Al/Al GBs. Simultaneously, a 2–10nm thick uniform Zn-rich layer is present in the Al/Al GBs. [ABSTRACT FROM AUTHOR]

Abstract: Ultrafine-grained structures were obtained in a solution-treated Al–Mg–Si (6060) alloy processed by high-pressure torsion at ambient temperature, 100°C and 180°C. Processing at 100°C provided the highest hardness and the smallest average grain size because of the enhanced segregation of solutes at grain boundaries which hindered grain growth during the processing. This research demonstrated the importance of optimising processing temperature to enhance grain refinement in high-pressure torsion processing. [Copyright &y& Elsevier]

Abstract: Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called “non-equilibrium” grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed. [Copyright &y& Elsevier]

Abstract: Fully dense, nanocrystalline tantalum (average grain size as small as ∼40nm) has been processed for the first time by high-pressure torsion. High-resolution transmission electron microscopy reveals non-equilibrium grain boundaries and grains decorated with high-density dislocations. Microhardness measurements and instrumented nanoindentation experiments indicate that the mechanical property is quite uniform except for the central area of the disks. Nanoindentation experiments at different strain rates suggest that the strain rate sensitivity of nanocrystalline tantalum is increased compared to the coarse- and ultrafine-grained counterparts and is accompanied by an activation energy of the order of a few ∼b 3 (b is the magnitude of the dislocation Burgers vector), implying a shift in the plastic deformation mechanism from the screw dislocation dominated regime. We thus infer the plastic deformation mechanisms of nanocrystalline body-centered cubic (bcc) and face-centered cubic metals converge. To examine the stress–strain behavior, we have used microcompression to measure the compressive stress–strain curves on microscale pillars fabricated by focused ion beam technique. Yield strength as high as 1.6GPa has been observed. High-strain rate behavior has been investigated using a miniature Kolsky bar system. We have found that at high-strain rates the nanocrystalline tantalum specimens exhibit adiabatic shear banding, a dynamic plastic deformation mode common to many ultrafine-grained and all nanocrystalline bcc metals. [Copyright &y& Elsevier]

Abstract: The microstructure and the mechanical properties of ultrafine grained Pd and Pd–x% Ag (x =10, 20, 40) alloys processed by high pressure torsion (HPT) were thoroughly investigated using transmission and high resolution electron microscopy, XRD analysis and tensile tests. The increased Ag content resulted in decrease of the stacking fault energy (γ SF), which made it possible to produce by means of HPT different ultrafine-grained structures in the alloys and to study their superior strength and ductility. The nature of such unique mechanical behaviour is discussed on the basis of analysis of strain hardening mechanisms and effect of stacking fault energy. [Copyright &y& Elsevier]

Abstract: An in situ tensile test with grey scale correlation has been performed to study the deformation process in ultrafine grained (UFG) Pd and Pd–x at.% Ag (x =5 or 20) alloys produced by high-pressure torsion. Shear band nucleation and propagation was found to be an important deformation mechanism after strain localization during the tensile test. The underlying microscopic mechanism is related to cooperative grain boundary sliding. Moreover, the additional influence of stacking fault energy was found to change the nature of the deformation mechanism from localized strain in Pd to more homogeneous deformation in Pd–20% Ag. In situ analysis and the findings are new and give innovative insight into the basics of deformation in UFG face-centred cubic metals. [Copyright &y& Elsevier]

Abstract: Fully dense nanocrystalline tungsten (nc-W) with extremely high strength (∼3.0GPa under quasi-static compression and ∼4.0GPa under dynamic compression) has been obtained by high-pressure torsion (HPT) at low temperature (500°C). The nanocrystalline microstructure is revealed by transmission electron microscopy (TEM). The grain boundaries (GBs) are mostly of the large-angle type. High-resolution TEM (lattice images) suggests that the GBs are clean and well defined (atomically sharp). GBs are non-equilibrium and of a high-energy nature. Edge dislocations are present within the grains. The authors hypothesize that these edge dislocations, combined with a depleted impurity concentrations along pre-existing GBs, contribute to enhance the ductility of nc-W. Under dynamic compression, the specimens exhibit localized shearing followed by cracking and subsequent failure, similar to their ultrafine-grain (UFG) counterparts processed by equal-channel angular pressing plus cold rolling, and to many other body-centered cubic metals with UFG/nanocrystalline microstructures. The shear band width in the HPT-processed nc-W is much smaller (shear band width <5μm) than that observed in the UFG counterparts (shear band width ∼40μm). [Copyright &y& Elsevier]

We investigated the corrosion behavior of commercially pure (CP) Ti with both ultrafine-grained (UFG) and coarse-grained (CG) microstructures. It was found that the UFG Ti is more resistant to corrosion than its CG counterpart. The superior corrosion resistance of UFG Ti is believed to result from rapid passivation of UFG Ti and the impurity segregation to grain boundaries in CG Ti. [Copyright &y& Elsevier]