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The tensile and fracture behaviors of AA6061 alloy were investigated in order to provide quantitative data about this alloy at cryogenic temperatures. Specimens of AA6061 alloy were solution heat treated before tensile tests at 298, 173 and 77 K and tested at strain rates in the range from 0.1 to 0.0001 s−1. The results indicate the suppression of the Portevin−Le Chatelier (PLC) effect and dynamic strain aging (DSA) at 77 K. In contrast, at 298 K, a remarkable serrated flow, characteristic of the PLC effect, is observed. Furthermore, the tensile behavior at 77 K, compared with that observed at 173 and 298 K, shows a simultaneous increase in strength, uniform elongation, modulus of toughness, strain-hardening exponent and strain rate sensitivity, which is related to a decrease in the dynamic recovery rate at low temperature. These responses are reflected on the fracture morphology, since the dimple size decreases at 77 K, while the area covered by dimples increases. Comparisons of the Johnson−Cook model show that a good agreement can be obtained for tests at 173 and 77 K, in which DSA is suppressed.

AA1050/AA7050 multilayered composite sheets with a proportion of 1:1 were produced by Accumulative Roll Bonding (ARB) and Asymmetric Accumulative Roll-Bonding (AARB), using up to 8 cycles and intermediate annealing treatments at 500°C. The main purpose was to produce one composite sheet with high strength and moderate ductility, taking advantage of the mechanical properties of these aluminum alloys. Microstructural features were investigated in order to evaluate the potential to achieve a refined microstructure and the development of structural patterns. The strain distributions as a function of friction and asymmetry were simulated by finite element analysis. Texture was evaluated by X-ray diffraction and electron backscatter diffraction. A continuous layer pattern was obtained by ARB, up to 6 cycles but after 8 cycles shear bands fragmented the harder layers. In the early AARB cycles, the bending and necking of the AA7050 layers yielded a wavy-pattern. The shear strain in the AARB process has a strong influence on achieving a wavy-pattern, more than the flow stress differences of the alloys in the composite. Shear texture increased with the degree of the layers’ discontinuity. Different sources of shear contributed to the formation of microstructural patterns: the shear due to asymmetry, the frictional shear at roll-sheet interface and at the central layer interface and the shear at the layers’ interface. In addition, the ARB process achieved a better interfacial adhesion at the middle interface and higher strength and elongation than the AARB process.

Cyclic closed-die forging (CCDF) appears to be an easy to operate deformation process, which imposes high levels of strain, even on difficult-to-deform materials. However, despite said potential advantages, the CCDF at cryogenic temperatures has not yet been investigated. Copper samples with dimensions of 10 mm × 10 mm × 20 mm were processed in up to six passes with interpass rotation, enabling the samples to return approximately to their initial dimensions after each pass. The intensity and homogeneity of plastic deformation was evaluated by mapping the Vickers hardness over the entire surface of the sample, and the resulting maps were compared with the strain and stress distribution estimated by FEM numerical simulation. The deformed microstructures were examined by optical and transmission electron microscopy. Cryogenic CCDF has proved to be effective in suppressing the recovery mechanisms of Cu samples, resulting in finer and more heterogeneous strains distribution than those deformed at room temperature. However, long-term observations by TEM have shown that these microstructures are inherently unstable, so that hardness decreases 50% after two years. Keywords: Copper, Cyclic closed-die forging, Numerical simulation, Cryogenic deformation, Microstructural stability

Asymmetric rolling combines direct compression and shear strain imposed by working rolls with different diameters or different rotations speeds. Processing at cryogenic temperatures may partially suppress dynamic recovery mechanisms, modifying the work-hardening rate and increasing the amount of accumulated strain. In this work, samples of AA6061 Al alloy were processed by asymmetric and conventional rolling, at room and cryogenic temperatures. The effects of asymmetric cryorolling were analyzed in terms of strain distribution, grain refinement, hardness, tensile strength and crystallographic texture after post-deformation heat treatments. Artificial aging after conventional rolling at room temperature proved to be more effective in increasing hardness and tensile strength, while the same heat treatment after asymmetric rolling at cryogenic temperature increased uniform elongation and reduced texture intensity. This behavior was associated with recovery and recrystallization phenomena after aging, which cause small dislocation-free grains to be generated inside deformation bands. This was observed directly by TEM and indirectly by texture analysis.

Processing at cryogenic temperatures is one of the ways of improving mechanical properties of copper alloys. However, such processing routes may result in unstable microstructures, which make it difficult to understand the deformation mechanisms controlling microstructure evolution during cryogenic deformation. In this study, the microstructure evolution of a Cu-0.7Cr-0.07Zr alloy was analyzed as a function of the flow behavior during tensile tests performed at 123 K and 298 K, through in-situ X-ray diffraction (XRD) experiments in a synchrotron source. The tensile behavior was analyzed in terms of the dynamic recovery rate using the Kocks-Mecking model, while the evolution of stacking-fault density, dislocation density and crystallite size during the tests was estimated by the modified Williamson-Hall method. Solution-treated and peak-aged conditions were tested to assess the influence of particles distribution. At 298 K, the strain-hardening rate was controlled by dislocation glide and the flow stress was affected mainly by particle strengthening. At 123 K, a simultaneous increase in strength and ductility was attributed to the twinning induced plasticity (TWIP) effect, promoted by precipitate interfaces acting as nuclei for stacking-faults, thus delaying necking and increasing the plasticity. Based on the evolution of dislocation and stacking-fault densities with increasing plastic strain, it was concluded that, at 123 K, mechanical twinning predominated at strain levels below 0.15, mainly for aged conditions, while dislocation glide between twin boundaries, and its accumulation, started after TWIP mechanism reached its saturation.

The deformation behaviour of metals and alloys is significantly affected by both stacking-fault energy and processing temperature. By lowering the former, deformation twinning is favoured over dislocation slip, whilst cryogenic processing partially suppresses dynamic recovery. Three materials having different stacking-fault energies, that is, Al AA1050 (high), pure Cu (medium) and Cu–15Zn alloy (low) were rolled at room (RTR) and at cryogenic (CR) temperatures up to a true strain equal to 1.5. Annealed coarse-grained samples were tested in tension at room and cryogenic temperatures. The processed samples were characterized by optical and transmission electron microscopy, hardness measurements and room-temperature tensile tests. CR increases the tensile strength with respect to RTR for the three materials; elongation to failure is decreased for AA1050, whilst for Cu and Cu–15Zn the CR effectively increases the ductility. Cu–15Zn sample after CR exhibits a microstructure relatively heterogeneous, suggesting static recrystallization events in the temperature excursion from 77 to 298 K. Finally, it was concluded that cryogenic deformation increases strength, whilst low SFE increases the ability to absorb plastic deformation, an effect strongly increased at cryogenic temperatures. The present investigation gives some basis for the development of cryogenic severe plastic deformation processes.

The present study was designed to determine the effect of shear strain on the macro- and microstructure and texture evolution in the AA1050/AA7050 Al multilayered composite sheets. The sheets were processed by Accumulative Roll-Bonding (ARB) and Asymmetric ARB (AARB) at 450 °C and 500 °C. The shear strain in the AARB process has a remarkable influence in achieving a macrostructural wavy-pattern, more than the flow behavior of the materials. For ARB, a flat-pattern with slight waviness was produced mainly due to flow incompatibility, as estimated by finite element simulation and observed by macrostructural analysis. In addition, friction at the surfaces and at the interfaces, and the strain distribution, together with the coarsening or dissolution of precipitates in the AA7050 layers have also influenced the shear banding and microstructure evolution in the sheets. A reduction in the texture intensity by AARB was observed. The grain refinement in the AA1050 layers was controlled by continuous dynamic recrystallization and static recovery during the thermo-mechanical cycles for both processing routes. The AA7050 layers presented a more complex grain refinement behavior that was influenced by dissolution of the η- and η′-phases, which was observed for both temperatures in the ARB. In contrast, the shear strain accelerated this dissolution kinetics and grain growth in the AA7050 layers of sheets processed by AARB.

In this investigation, AA1050/AA7050 multilayered composite sheets were produced by Asymmetric Accumulative Roll-Bonding (AARB) in order to evaluate microstructure evolution and the interfacial zone. The asymmetric rolls introduced an extra component of shear strain, which provided conditions for necking and/or rupture in the AA7050 layers during processing at 500 °C yielding a wavy-pattern. On the other hand, at 450 °C the continuity of layers was maintained. Thus, AARB parameters can be controlled to produce wavy- or flat-pattern structures in multilayered composites. The hardening of the AA7050 layers after processing at 450 °C was caused by overaging and grain refinement, whereas at 500 °C it was due to the precipitation of η’ and η-phases. The hardening of the AA1050 layers was attributed to grain refinement achieved from continuous dynamic recrystallization. In addition, diffusion bonding has a significant role in the formation of AA1050/AA7050 interfaces by the AARB process, as observed in the interdiffusion zone. Taken together, these findings provide insights into structural pattern formation and microstructural control in Al-based multilayered composites using the AARB process.

Equal-channel angular pressing (ECAP) is capable of improving the mechanical properties of many aluminum alloys by means of intense grain refinement. Meanwhile, a general relationship between grain size and stress corrosion cracking (SCC) resistance is not yet established, since the introduction of microstructural heterogeneities affects, in many ways, the localized corrosion susceptibility. This work evaluates the simultaneous impact of ECAP processing on microstructure, mechanical strength and SCC resistance of an Al-4wt.%Cu alloy, in comparison with a commercially pure aluminum. Microstructure was observed by optical and electronic microscopy, while mechanical response was evaluated by hardness measurements and tensile tests. SCC resistance was evaluated by slow strain rate tests and by constant load tests performed in an aqueous solution with 3.5 wt.% NaCl. ECAP promoted grain refinement, breakdown/redistribution of coarse secondary phase particles and contributed to reduce the local chemical heterogeneity. Consequently, mechanical properties and SCC resistance were simultaneously improved.

Accumulative Roll-Bonding (ARB) and Asymmetric Rolling (AR) techniques were combined to produce ultrafine-grained aluminum sheets with the mechanical characteristics of a Severe Plastic Deformation (SPD) process. Temperature and number of bonding cycles were varied to promote grain refinement, texture randomization and high-quality sheet bonding. Finite element simulation for a single pass was performed to clarify the strain distribution differences between symmetric and asymmetric roll -bonding. The microstructure and crystallographic texture were measured by Electron Backscatter Diffraction (EBSD) and X-ray diffraction. Hardness and tensile tests characterized strain distribution and bonding efficiency. A fine grain structure with a mean grain size of 1.0 μm was achieved at 350 °C, whereas a coarser grain structure was obtained at 400 °C. The grain size and shape distribution were linked to enhancing the mechanical strength in a transversal direction. During repeated bonding cycles at both temperatures, extra shear in the interfacial region yielded favorable homogeneous strain distribution and a weak shear texture across the sheet. Rotated-cube orientation was the strongest component in both processing temperatures. To increase the interfacial strength, mainly on the last bond interface, an extra 50% reduction step was added. This improved the adhesion in the last bonding interface, and thus enhanced the ductility. These findings helped to provide a basis for determining the processing conditions for aluminum alloys.

The combination of Equal-Channel Angular Pressing (ECAP) with cryogenic processing temperatures can be an efficient method to obtain very refined microstructures. However, there is a lack of research on the combination of these processes and the effects they have on microstructure and mechanical properties, particularly in precipitation hardened aluminum alloys. The present study describes the effects of cryogenic ECAP-processing and subsequent heat treatments on the microstructure and mechanical properties of AA6061 Al alloy. Samples in the solutionized state were processed by ECAP at 173 K (Cryo) and at room temperature (RT) up to an equivalent strain of 4.2. After Cryo-ECAP, the microstructure was very refined, with an average grain size close to 0.5 μm and strain concentrated within microscopic shear bands. EBSD analysis after Cryo-ECAP shows new fine equiaxed grains with a higher proportion of low-angle grain boundaries. The temperature of 173 K was effective in suppressing partially precipitation during ECAP, and made the process possible without extensive cracking. However, recovery mechanisms were not fully hindered, which undermined the strengthening effect of Cryo-ECAP. The yield stress was higher after RT-ECAP, while uniform elongation was improved by Cryo-ECAP plus T6 treatment, a result attributed to the higher fraction of low-angle grain boundaries observed in the microstructure. Finally, it was demonstrated that cryogenic processing is not in itself a guarantee of improvements in strength and ductility, in comparison with RT processing. Both recovery and precipitation processes dictated the feasibility of the process, the final microstructure and the mechanical response of the ECAP-processed AA6061 Al alloy.