Search

This overview article discusses the Equal Channel Angular Pressing (ECAP) processing of different metallic materials. Particular emphasis is given to the microstructural evolution from the coarse grain (CG) to the ultrafine-grained (UFG) states throughout the electron backscattering diffraction (EBSD) technique. Iron-based alloys, such as duplex and 1020 low-carbon steels reached higher hardening with a lower deformation and lower non-ultrafine average grain sizes than the ultrafine pure iron condition due to fast grain fragmentation, i.e., more geometrically necessary dislocation (GND) grouping. Moreover, due to the magnesium adhesion, copper alloys reached superior mechanical properties compared to pure copper even when the initial grain size for as-cast alloys was over 1000 µm. On the other hand, low melting temperature (TMP) materials processed at 250°C, like the ZK60 magnesium and AA6082 aluminum alloys (i.e., homologous temperatures (TH) of 0.38TMP and 0.37TMP, respectively), showed grain refinement without reaching the ultrafine regime and mechanical softening due to the static and dynamic recrystallization phenomena. CP titanium also displayed heterogeneous grain sizes with average values of above 1 µm after four ECAP passes for temperatures ranging between 150°C and 400°C (TH between 0.09TMP = 0.24TMP). The evolution of the GNDs suggested that the initial deformation stages of severe plastic deformation (SPD) by ECAP produced the most notorious density increments from 1012m-2 to 1014m-2, which level up at high deformations (more than four ECAP passes) around 10141015m-2, explaining the fast and slow grain size reduction rates, respectively. The ECAP processing on different metallic material systems showed a larger grain fragmentation capacity in high melting points and alloyed materials, giving rise to steep yield strength increases and low ductility. The low ductility and grain size saturation correspond to a low capacity to create new grain boundaries manifested by the GNDs saturation in the UFG range. [ABSTRACT FROM AUTHOR]

This report is aimed at giving an overview of the significance of the novel and innovative microstructural and microscopic characterization techniques for bulk nanostructured metals processed by severe plastic deformation, specifically high-pressure torsion (HPT). In practice, the microstructural relaxation behavior upon heating of nanostructured 316L stainless steel and CoCrFeNi high-entropy alloy was characterized by in-situ heating neutron diffraction measurements; the heterogeneous phase distribution of an HPT-bonded hetero-nanostructured AlMg alloy was examined using synchrotron high-energy X-ray diffraction; and the microstructural evolution upon heating of a nanostructured CoCrFeNiMn high-entropy alloy was examined by laser-scanning confocal microscopy. These novel techniques are complementary to each other and any other in- or ex-situ testing methods, especially when nanocrystalline metals are transforming microstructurally and compositionally with temperature and time in a hierarchical manner. The outcomes of the studies emphasize the importance of the methodologies and the development of characterization techniques for further in-depth exploration in the research field of severe plastic deformation. [ABSTRACT FROM AUTHOR]

Hot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a β-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc β-phase recrystallizes dynamically, much faster than the hcp α-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the β-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the excellent deformation behavior of the material, while the presence of two phases effectively suppresses grain growth. [ABSTRACT FROM AUTHOR]

Single-crystalline cast aluminium microwires with a diameter near 15 are characterised by Laue microdiffraction. A microwire in the as-cast condition exhibits a misorientation below over a length of 500. The measured density of geometrically necessary dislocations is low, m−2, though local maxima up to one order of magnitude higher are found. After tensile deformation to failure, the dislocation density is significantly increased in microwires that have mostly deformed in single slip (m−2), and yet higher when deformation has occurred by multiple slip (m−2). In deformed single slip oriented microwires, the streaking directions of Laue spots show that dislocations are stored (though not exclusively) on the primary slip system. Results are consistent with a deformation mechanism governed by rotating, likely single-arm, sources. [ABSTRACT FROM AUTHOR]

Low‐carbon steel sheets are severely plastic deformed to strains of up to ≈3.48 and subsequently heat treated by conventional annealing followed by water‐quenching. Four temperatures are chosen for the annealing below and over the Ac1 and Ac3 transformation lines. The effects of post‐deformation heat treatment are investigated by evaluating the microstructure and mechanical properties, including strength, ductility, work hardening capability, and hardness. A maximum increase of 86% in the strength is obtained through intercritical annealing and quenching of the samples subjected to strain of 1.16. It is interesting that both the elongation and ultimate tensile strength values are higher after heat treatment at a variety of temperatures in comparison with those of the non‐treated samples; up to 197% for the former and up to 33% for the latter. These would be achieved in combination with yield ratios even lower than that of the as‐received material, demonstrating the higher deformation capability of the specimens. However, these beneficial outcomes are deteriorated toward higher amounts of strain. Despite the heat treated undeformed low‐carbon steels, the stress − strain curves for heat treated deformed samples show little or no discontinuous yielding correlated to variations of mobile dislocations density as a result of microstructural evolution. [ABSTRACT FROM AUTHOR]

A lath martensite steel containing 0.22 mass pct carbon was analyzed in situ during tensile deformation by high-resolution time-of-flight neutron diffraction to clarify the large work-hardening behavior at the beginning of plastic deformation. The diffraction peaks in plastically deformed states exhibit asymmetries as the reflection of redistributions of the stress and dislocation densities/arrangements in two lath packets: soft packet, where the dislocation glides are favorable, and hard packet, where they are unfavorable. The dislocation density was as high as 10 m in the as-heat-treated state. During tensile straining, the load and dislocation density became different between the two lath packets. The dislocation character and arrangement varied in the hard packet but hardly changed in the soft packet. In the hard packet, dislocations that were mainly screw-type in the as-heat-treated state became primarily edge-type and rearranged towards a dipole character related to constructing cell walls. The hard packet played an important role in the work hardening in martensite, which could be understood by considering the increase in dislocation density along with the change in dislocation arrangement. [ABSTRACT FROM AUTHOR]

High-Energy Diffraction Microscopy (HEDM) is a 3-d X-ray characterization method that is uniquely suited to measuring the evolving micro-mechanical state and microstructure of polycrystalline materials during in situ processing. The near-field and far-field configurations provide complementary information; orientation maps computed from the near-field measurements provide grain morphologies, while the high angular resolution of the far-field measurements provides intergranular strain tensors. The ability to measure these data during deformation in situ makes HEDM an ideal tool for validating micro-mechanical deformation models that make their predictions at the scale of individual grains. Crystal Plasticity Finite Element Models (CPFEM) are one such class of micro-mechanical models. While there have been extensive studies validating homogenized CPFEM response at a macroscopic level, a lack of detailed data measured at the level of the microstructure has hindered more stringent model validation efforts. We utilize an HEDM dataset from an alpha-titanium alloy (Ti-7Al), collected at the Advanced Photon Source, Argonne National Laboratory, under in situ tensile deformation. The initial microstructure of the central slab of the gage section, measured via near-field HEDM, is used to inform a CPFEM model. The predicted intergranular stresses for 39 internal grains are then directly compared to data from 4 far-field measurements taken between ~4 and ~80 pct of the macroscopic yield strength. The evolution of the elastic strain state from the CPFEM model and far-field HEDM measurements up to incipient yield are shown to be in good agreement, while residual stress at the individual grain level is found to influence the intergranular stress state even upon loading. Implications for application of such an integrated computational/experimental approach to phenomena such as fatigue are discussed. [ABSTRACT FROM AUTHOR]

This paper introduces a novel severe plastic deformation method entitled Friction assisted tubular channel pressing (FATCP) for producing ultrafine-grained cylindrical tubes. For this purpose, using the precious potential of friction power increases temperature in deformation area and provides localized material softening, severe combined strains and lower pressing force. At first, presented method was executed on the Cu-35Zn alloy and this process exhibits good applicability for this material. Investigation of microstructure of FATCPed sample showed a significant grain refinement from an initial average grain size of 70 μm to less than 5 μm after a single pass. Also, hardness notably increased to 132 Hv from an initial value of 60 Hv. [ABSTRACT FROM AUTHOR]

Ti-based materials are one of the most important materials used in biomedical engineering. Recently, commercially pure (CP) Ti has attracted significant attention of research community due to its full biocompatibility with human body, and there is an ongoing demand to improve its mechanical properties without sacrificing other beneficial properties. A possible approach to achieving this aim is to use the method of severe plastic deformation (SPD) to obtain an ultra-fine grained (UFG) microstructure. Significant grain refinement by SPD processing leads to an improvement in mechanical and functional properties. This paper gives an overview of a range of the properties that can be achieved in the UFG CP Ti including microstructural features, mechanical properties, corrosion performance and biocompatibility. Attention is paid to the main SPD processing techniques developed for the fabrication of UFG CP Ti. Current and potential applications of UFG CP Ti in biomedical engineering are also considered. [ABSTRACT FROM PUBLISHER]

The microstructure, texture and mechanical properties have been studied in PM2000 compressed via dynamic plastic deformation to a strain of 2.1. It is found that dynamic plastic deformation results in a duplex 〈111〉 + 〈100〉 fibre texture and refines the initial microstructure by nanoscale lamellae, which substantially increases the strength of the material, but decreases its thermal stability. In the as-deformed microstructure, the stored energy density is found to be higher in 〈111〉-oriented regions than in 〈100〉-oriented regions. Recovery during annealing at 715 °C reduces the energy stored in the deformed microstructure. This reduction is more pronounced in the 〈111〉-oriented regions. Orientation-dependent recrystallisation takes place in the recovered microstructure, leading to strengthening of the 〈111〉 fibre texture component at the expense of the 〈100〉 fibre texture component. [ABSTRACT FROM AUTHOR]

A Ti-25Nb-25Ta β-type alloy was subjected to severe plastic deformation via accumulative roll bonding; the method was applied on two different precursor samples, in order to establish the influence of the precursor structural state on the final alloy properties processed by ARB. $$\overline{Bo}$$ and $$\overline{Md}$$ electronic parameters have been calculated in order to establish, by plotting the alloy on $$\overline{Bo}$$ - $$\overline{Md}$$ diagram, the alloy β stabilizing grade, and the dominant deformation mechanisms. Structural modifications during ARB process (changes in phase structure, phase quantities, lattice parameters, and coherent crystalline domain size) were described and analyzed using XRD investigations. Texture data expressed by pole figures, inverse pole figures, and orientation distribution functions for the (110), (200), and (211) β-Ti peaks were obtained through XRD investigations. The results showed that it is possible to obtain high intensity share texture modes ({001}〈110〉) and well-developed α- and γ-fibers, the most important fiber being the α-fiber. [ABSTRACT FROM AUTHOR]

Novel forming processes for light metal alloys utilize recovery and recrystallization to extend their total elongation and enhance formability. To attain optimum efficiency in such processes, it is necessary to understand and quantify the kinetics of recovery and recrystallization in work-hardened metal alloys. An electron backscatter diffraction based method, using local average orientation spread, is shown to identify the end of recovery as well as the onset of recrystallization. Local average orientation spread results from dislocation flux and storage during plastic deformation and hence, captures the evolution of static recovery process. The method has been demonstrated using pre-strained Al–Mg alloys. The recovery kinetics is shown to be consistent with results from dislocation density based recovery models. In addition, a direct observation of the coexistence of static recrystallization and recovery illustrates competing processes for energy minimization. [ABSTRACT FROM AUTHOR]

The existing concepts of the mechanisms of forming fragmented structures under the conditions of severe plastic deformation of crystalline solids are analytically reviewed. The translational and rotational plasticity modes that develop at micro- and mesoscopic structural levels, respectively, are sequentially taken into account. This allows us to correctly describe the morphological features of the evolution of fragmented structures, to predict misorientation spectra for various fragmentation mechanisms, and to determine the partial contribution of each mechanism in the cases where several deformation grain refinement mechanisms are involved in fragmentation. The computer simulation of deformation-induced misorientation spectra that was developed using these concepts is a new method for studying the physical nature of structure formation processes, and this method can be applied for various materials, temperature-rate deformation conditions, and technological loading schemes. As an example, we comprehensively consider the formation of fragmented structures under the extreme conditions of explosion welding of commercial-purity copper plates. A comparison of the model misorientation spectra calculated for a reference structure and the fragmented structure in the near-contact zone of the welded joint with the existing experimental data demonstrates the efficiency and reliability of the proposed method. [ABSTRACT FROM AUTHOR]

The results obtained from an experimental study on the structure of deformed polycrystalline materials of the meso- and microscale grain size, using transmission electron microscopy, are presented. The slip line pattern, accumulation of the scalar dislocation density and dislocation density components, lattice curvature-torsion, and internal stresses are investigated. The difference between the characteristics of the defect polycrystalline structure at the micro- and mesoscale levels is established. [ABSTRACT FROM AUTHOR]

Pure copper processed by a twist extrusion process with a reduced twist-line slope is investigated. Twist extrusion is a severe plastic deformation technique which is promising for scale-up because it allows for processing of relatively large metallic bars. On the way to commercialization, decreasing costs associated with processing is critical for twist extrusion. As one of the measures, reducing the twist-line slope-an important geometrical feature of twist extrusion-can be advantageous in terms of processing costs. The current study seeks to elucidate effects of reducing the twist-line slope on the microstructure, mechanical properties, and their heterogeneity in the processed metal, which were unclear because most of the previous research has focused on twist extrusion with high twist-line slopes ( e.g., 60 deg). Results of the present study suggest that reducing the twist-line slope moderates grain refinement and limits improvement of mechanical properties of the twist-extruded metal. These findings are discussed in terms of the shear strain reversal which is inherent in the process. [ABSTRACT FROM AUTHOR]

The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ µLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ µLaue experiments are useable. A critical size is evidenced below which, depending on Young's modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample. [ABSTRACT FROM AUTHOR]

Severe plastic deformation (SPD) techniques have attracted considerable attention due to their capability to refine the grain size of metals and alloys down to the ultrafine grained (UFG) regime. A characteristic feature of SPD processes is that they generate semi‐finished parts such as rods or sheets, which require further shaping in order to become marketable products. One approach for the shaping of SPD processed materials is the use of metal forming processes, which can be a challenge with regard to the low work hardening rate and tendency to strain localizations of most UFG metals. This article summarizes the current state of knowledge on the formability and forming behavior of UFG metals under different loading scenarios, discussing both limitations and application potential. Furthermore, the interaction between microstructural aspects and the formability, such as the occurrence of strain localizations during and the role of heat treatments are also addressed. Given that the formability of UFG metals has received far less attention than other aspects so far, this work also aims at identifying apparent contradictions as well as scientific and technological questions that need to be addressed in future to expand the current knowledge of the field. This article provides an overview of the formability and forming behavior of ultrafine grained metals under various loading scenarios of sheet and bulk forming. It also addresses the interaction between microstructure and forming behavior as well as strategies to improve the formability. [ABSTRACT FROM AUTHOR]