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Microstructural changes, such as the density of grain boundary (GB) and dislocation density, due to isochronal annealing in severely plastic-deformed commercial purity aluminium up to 523 K was evaluated using electrical resistivity measurements and scanning transmission electron microscopy. Eventually, the GB density decreases from about 7.2  ×  106to about 2  ×  106 m−1, whilst the dislocation density decreases from an initial value of around 1.3  ×  1014 m−2down to around 4  ×  1013 m−2. [ABSTRACT FROM PUBLISHER]

High-pressure torsion was used for the deformation processing of high-purity aluminum (4N-Al), while high-resolution electron-backscatter diffraction was used for the analysis of evolution of qualitative and quantitative microstructural characteristics. This study reveals a rather full picture of microstructure evolution in the high stacking fault energy fcc material and makes a continuous link between deformation microstructures at low, high and very high strains. Three stages of the microstructure evolution in 4N-Al at ambient temperature have been found: (i) the first stage in the range εeq  ≤ 1; (ii) a transition stage in the range 1 < εeq  ≤ 8; and (iii) a saturation stage in the range εeq  ≥ 8. In stages (i) and (ii), grain subdivision and typical features of deformation microstructures are found. Starting from stage (ii), formation of small equiaxed (sub)grains surrounded by high-angle boundaries (HABs) is found together with minor increase in the average subgrain size. At stage (iii), recrystallized-like microstructure mostly consisting of the dynamically stable equiaxed subgrains surrounded by HABs dominates the microstructure. [ABSTRACT FROM PUBLISHER]

Commercial purity aluminium sheets were severely plastic deformed by accumulative roll bonding (ARB). Changes in electrical resistivity at 77 K and microstructure during the ARB process were traced up to 12 cycles, which corresponded to an equivalent strain of 10. The resistivity at 77 K increased with increasing number of ARB cycles, then saturated after about the sixth ARB cycle with a maximum increment of resistivity from starting material of about 1.1 nΩ m. Since lattice defects affect the resistivity of metals, the internal dislocation density and the density of grain boundaries were evaluated from scanning transmission electron microscopy images using Ham's method and grain boundary maps obtained from electron back-scattering diffraction, respectively. The relationship between the change in resistivity and the lattice defects is discussed. [ABSTRACT FROM AUTHOR]

Ultra-fine grained (UFG) metals fabricated by severe plastic deformation (SPD) sometimes exhibit peculiar mechanical properties. For example, the “hardening by annealing and softening by deformation” was reported in UFG aluminum, which was totally opposite to the behaviors of conventionally coarse-grained materials. In this study, the effect of SPD strain on the peculiar phenomena was investigated. The UFG aluminum was fabricated by various cycles of the accumulative roll-bonding (ARB) process with lubrication at ambient temperature. The specimen ARB-processed by ten cycles certainly showed the peculiar phenomena. On the other hand, the 6-cycle specimen did not show the phenomena but was softened by annealing and hardened by deformation normally. From the results of microstructural characterization, it was suggested that the difference in the change of the mechanical property during annealing and deformation between 6-cycle and 10-cycle specimens was caused by the difference in the grain size and/or the texture components, which depended on the SPD strain. [ABSTRACT FROM AUTHOR]

Abstract: High-purity aluminum (99.99% purity) deformed by accumulative roll bonding to an equivalent strain of 4.8 has been characterized by transmission electron microscopy and electron backscatter diffraction techniques in the deformed state and after various annealing treatments. These two techniques are compared and discussed. The majority of the structure in the as-deformed state is composed of equiaxed crystallites surrounded mostly by high-angle boundaries with misorientation angle above 15°. However, regions that contain a high concentration of low-angle boundaries with misorientation angle below 15° in a lamellar structure elongated along the rolling direction have also been observed. The microstructure is correlated with the local texture as the equiaxed structure contains a mixture of texture components whereas the lamellar structure has orientations corresponding to rolling or shear texture components. Annealing of deformed specimens causes structural coarsening without removing the regions with a high concentration of low-angle boundaries. Heat treatment for a short period of time (0.5h) leads to the start of recrystallization at 200°C, whereas a heat treatment at 175°C for an extended period (6h) allows heat treatments to be carried out at temperatures above 200°C without initiation of recrystallization. The two-step process is a promising route for producing almost uniform equiaxed structures by annealing of samples that have been deformed to large strains. [Copyright &y& Elsevier]

A fully annealed AA8011 aluminum alloy sheet containing a number of large particles (∼5 µm) was severely deformed up to an equivalent strain of 12 by an accumulative roll-bonding (ARB) process. The texture evolution during the ARB process was clarified, along with the microstructure. The ARB-processed aluminum alloy sheets had a different texture distribution through the sheet thickness, due to the high friction between the roll and the material during the ARB process. The shear textures composed of {001}<110> and {111} <110> orientations developed at the sheet surface, while the rolling textures, including Cu {112}<111> and Dillamore {4,4,11}<11,11,8> orientations, developed at the sheet center. The textural change from a shear texture to a rolling texture at the sheet center during the ARB process contributed to an increase in the fraction of high-angle boundaries. Also. a large number of second-phase particles in the AA8011 alloy sheets weakened the texture. Up to the medium strain range (below ε = 6.4), relatively weak textures developed, due to the inhomogeneous deformation around the second-phase particles; after the strain of 6.4, strong rolling-texture components, such as the Dillamore and Cu orientations, developed. This remarkable textural change can be explained by the reprecipitation of fine particles in grain interiors. [ABSTRACT FROM AUTHOR]

The surface of the various kinds of metallic materials sheets were severely deformed by wire-brushing at ambient temperature to achieve nanocrystalline surface layer. The surface layers of the metallic materials developed by the near surface severe plastic deformation (NS-SPD) were characterized by means of TEM. Nearly equiaxed nanocrystals with grain sizes ranging from 30 to 200 nm were observed in the near surface regions of all the severely scratched metallic materials, which are Ti-added ultra-low carbon interstitial free steel, austenitic stainless steel (SUS304), 99.99 wt.%Al, commercial purity aluminum (A1050 and A1100), Al–Mg alloy (A5083), Al-4 wt.%Cu alloy, OFHC-Cu (C1020), Cu–Zn alloy (C2600) and Pb-1.5%Sn alloy. In case of the 1050-H24 aluminum, the depth of the surface nanocrystalline layer was about 15 μm. It was clarified that wire-brushing is an effective way of NS-SPD, and surface nanocrystallization can be easily achieved in most of metallic materials. [Copyright &y& Elsevier]

Aging behavior of an ultrafine grained Al–Cu alloy was studied. Al–2 wt%Cu alloy sheets were solution-treated at 550 °C for 1.8 ks and then severely deformed up to an equivalent strain of 4.8 by the accumulative roll bonding (ARB) process at ambient temperature. The ARB processed material showed the ultrafine lamellar boundary structure with mean boundary interval of 67 nm. The ARB processed sheets and the starting sheets having conventionally coarse grains were aged at 190 °C. The coarse grained specimen for comparison showed typical aging behavior showing in-grain precipitation of the thin-plate shaped θ′ phase coherent to {001} crystallographic plane of the matrix aluminum. Hardness of the coarse grained specimen increased as precipitation proceeds, reached the maximum around HV60, and decreased owing to over-aging. On the other hand, the ARB processed sample with the ultrafine-grained structure showed very high hardness of HV115 in the as-deformed state, and the hardness decreased monotonously with increasing the aging time. The detailed microstructural observations clarified that recovery and grain growth of the matrix proceeds during 190 °C aging and the precipitation only at grain boundaries occurs in the ultrafine grained material. Instead of the θ′ phase, the equilibrium θ phase preferentially appeared in the ultrafine grained specimen. The coarsening of the matrix was mainly responsible for monotonous decrease of hardness. The kinetics of precipitation in the ARB processed sample was much faster than that of the coarse grained materials. It was concluded that the aging behaviors in severely deformed material having the ultrafine grained structure are completely different from the conventional ones in the coarse-grained materials. [Copyright &y& Elsevier]

The microstructure and superior mechanical properties of bulk nanocrystalline Fe–Al–C alloys made by mechanically alloying (MA) with subsequent spark plasma sintering (SPS) were investigated. Three kinds of nanocrystalline Fe–24 at% Al–X at%C (X=1,2,4) alloy powder were produced by MA from iron and aluminum powder with addition of methanol, and were subsequently consolidated at 1073–1273 K under 64 MPa by SPS. These compacts have the relative densities of 99.97% (1 at%C) to 99.5% (4 at%C). The structure of compacts with 1at%C is composed of grains of Fe3Al of 1.5 μm in diameter and nano κ-carbides (Fe3AlC0.5) precipitates, while those of compacts with 2 and 4 at%C are composed of nanocrystalline Fe3Al of about 80 nm in diameter, nano κ-carbides and small amount of large α-grains of about 1 μm in diameter. These structures maintain the nanostructure even at 973 K, that is, they have the good thermal stability. The mechanical properties of these compacts were measured by compression tests at room temperature (RT) to 973 K in vacuum. The compacts with 1 and 2 at%C of this work perform the superior mechanical properties (e.g. yield strength of 2150 MPa and rupture strain of 0.14 for compact with 2 at%C at R.T.) when compared with the ordinary Fe3Al casting (e.g. the yield strength of 380 MPa and rupture strain of 0.12). [Copyright &y& Elsevier]

Abstract Solid solution strengthening effect was investigated in Al x CoCrNi medium-entropy alloy (MEA) with different Al content (x = 0–7 at.%). The result of X-ray diffraction indicates that the fully recrystallized phase was a single face-centered cubic (FCC) structure for all compositions. The average grain size regarding a twin boundary as a grain boundary was approximately 16 μm. Tensile testing shows that the yield strength increases with the addition of Al. Here, Labusch's approach to random solid solution alloys is applied to confirm the strengthening mechanism in Al x CoCrNi MEA by using lattice constants and elastic moduli. The experimental results were found to be in good agreement with quantitative predictions of the solid solution contribution in Al x CoCrNi MEA. Graphical abstract Image 1 Highlights • The effect of dilute addition of Aluminum in equiatomic CoCrNi is investigated. • Single phase Al x CoCrNi have similar grain size but have different yield strength. • Strengthening mechanism in Al x CoCrNi is quantified by extended Labusch's approach. • Elastic constant and lattice constant highly involve in the strength prediction. [ABSTRACT FROM AUTHOR]

Accumulative rolling bonding (ARB) is a kind of severe plastic deformation process which can produce high strength metals with ultrafine (sub-micron) grained microstructure. Metals produced by this process are also expected to show excellent damping capacity since they contain significant amounts of lattice defects which give rise to internal friction. We examined the damping capacity of high purity (99.99% Al) and commercial purity (99% Al) aluminum sheets ARB processed to an equivalent strain of 4. Internal friction values (Q−1) of the 99.99% Al and the 99% Al were as high as 12×10−3 and 6×10−3, respectively. Microstructures were observed to clarify the damping mechanism. The mean grain thickness of the pancake-like shaped ultrafine grains were 640 and 250 nm in 99.99% Al and 99% Al, respectively. A significant amount of dislocations was observed within the ultrafine grains in both of the ARB-processed aluminum sheets. The high damping capacity was attributed to vibration of the dislocations whose ends were tightly bound with the grain boundaries. [Copyright &y& Elsevier]

The effect of 7.5 at.% Al addition in Al 7.5 CoCrNi medium-entropy alloy (MEA) was investigated. The recrystallized Al 7.5 CoCrNi exhibited the microstructure of fine-grain face-centered cubic (FCC) phase altogether with the formation of NiAl-rich B2-phase precipitate around the grain boundary and grain interior. The formation of NiAl-rich precipitate led to the significant grain size refinement compared to the Al-free CoCrNi MEA. The small addition of Al was also discovered to be able to significantly improve the mechanical properties of CoCrNi matrix, with a substantial yield strength increment from 400 MPa to 680 MPa was obtained. This increment occurs owing to the combination of several strengthening mechanisms by the addition of Al via substitutional solid solution, grain refinement, and the B2-phase precipitation strengthening that has a good agreement with the theoretical results. The presence of NiAl-rich B2 phase also changed the plastic deformation mechanism that the dislocation slip without ε martensitic transformation was found to replace the deformation twinning mechanism which generally observed in a CoCrNi matrix during the plastic deformation. Image 1 • Small Aluminum addition increase the lattice constant of CoCrNi. • Al 7.5 CoCrNi produce a fine-grained microstructure with B2 phase precipitate. • Combination of grain refinement effect and B2 precipitate increase the yield strength. • Dislocation slip is more favorable than deformation twinning in Al 7.5 CoCrNi. [ABSTRACT FROM AUTHOR]

Bauschinger test in uniaxial tension-compression mode was carried out for the first time on the pure Al specimens having homogeneous ultra-fine grained (UFG) microstructures fabricated by equal-channel angular pressing (ECAP) and subsequent annealing processes. Significant Bauschinger stress (transient softening), Bauschinger energy parameter and their strong dependences on the tensile plastic pre-strain at the very early stage of the tensile deformation were measured in the UFG specimens, in sharp contrast to their coarse-grained (CG) counterpart. The grain size dependence of the Bauschinger effect in pure Al was qualitatively discussed in terms of the back stress arising from the formation of dislocation pile-up against the grain boundary during plastic deformation. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: A pure aluminum single crystal with (112) [1 1 −1] orientation was deformed by nine cycles of conventional rolling and accumulative roll bonding (ARB) which corresponded to a rolling strain (ε) of 7.18. (4 4 11) [11 11 −8] was the main component of the deformation texture. Between the third and seventh cycles (ε=2.4–5.6), (4 4 11) [11 11 −8] oriented areas existed stably in the center layer, and this stability could be rationalized by the crystal rotation mechanism reported by Kamikawa et al. and Heason et al. The deformation texture after the ninth cycle included (4 4 11) [11 11 −8] and (−4 −4 11) [11 11 8] components. The formation of the (−4 −4 11) [11 11 8] orientation was attributed to crystal rotation from (001) [110], which was formed in the lower surface layer after the seven cycle under lubrication conditions. [Copyright &y& Elsevier]

Bimodal grain structure was fabricated in a 99.99% purity aluminium by high pressure torsion at ambient temperature in reversal deformation mode at a strain amplitude Δε =1, strain rate and hydrostatic pressure P =5GPa. Enhanced boundary mobility due to the high purity of the aluminium and homologous temperature coupled with reversal simple shear deformation are found to be responsible for bimodal microstructure formation. [ABSTRACT FROM AUTHOR]

Abstract: Dislocation density within ultrafine grains in severely deformed aluminium was evaluated by the scanning transmission electron microscopy. The values were around 1014 m−2 in commercial pure aluminium samples with equivalent strain up to 10. The obtained dislocation densities values correspond to strength between 55MPa and 70MPa according to the Bailey–Hirsch relationship. [Copyright &y& Elsevier]

Abstract: In this study, 2mm-thick commercial 1050-Al plates with an ultrafine grained (UFG) structure were obtained by the accumulative roll bonding (ARB) technique after a 5 cycle process and were subsequently joined by friction stir welding (FSW) at various revolution pitches (welding speed/rotation speed) of 1mm/r, 1.67mm/r and 2.5mm/r. To understand the effect of the initial grain size on the welding properties, ARB processed samples followed by annealing under H24 conditions as well as the as-received samples in the fully annealed state were also applied to the FSW process. The microstructure evolution and Vickers hardness in the stir zone of all the samples were investigated. It was revealed that the annealed samples with an intermediate grain size finally obtained the most refined grain size and highest value of Vickers hardness in the stir zone. However, for the UFG samples, significant grain growth and corresponding decrease in hardness can be found in the stir zone. [Copyright &y& Elsevier]

The evolution of texture in pure aluminum subjected to monotonous and reversal deformation in high-pressure torsion was first clarified. In a low-strain regime the texture development is similar for both deformation modes, while significantly different behavior is observed at high strain levels. The monotonously deformed materials showed typical torsion textures. In contrast, strain reversal results in development of texture components not observed for simple shear deformation. [Copyright &y& Elsevier]

Abstract: Most of severe plastic deformation processes involve strain reversal. Till now quite big number of researches has been done on indirect study of its role, which discusses the effect of loading path in ultra-fine grained structure formation. This work is aimed to study directly the role of the strain reversal in grain refinement by severe plastic deformation. A 99.99% purity aluminum was processed by high pressure torsion up to 96° rotation (maximal equivalent strain ɛ ≈8) with two deformation modes: monotonic and reversal deformations with a step of 12° rotation (maximal equivalent strain ɛ ≈1). It was shown that strain reversal retarded the grain refinement in comparison with the monotonic deformation. This explains slower rate of the nanostructure formation of the SPD processes that involve strain reversal. [Copyright &y& Elsevier]