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The Al–Cu–Li alloy has poor plasticity and hot workability, making it difficult to form by single-pass working, so the multi-stage thermomechanical processing is usually required. However, there is a general distinctly heterogeneous fiber structure in deformed alloys, which has non-negligible effects on the stability and formability of subsequent deformation. Therefore, the deformation mechanism of heterogeneous fibers and intergranular deformation compatibility mechanism are investigated for as-extruded Al–Cu–Li alloys with and fibers in this work. The results show that more slip systems are activated in the fiber, forming LAGBs through dislocations cross-slip and entanglement, thereby stimulating the CDRX. The number of slip systems that be activated in the fiber is relatively smaller, and the dislocations are dominated by single-slip and are easily pinned by the coarsening precipitate, thus activating the DDRX mechanism to force the grains to transform into the fiber. To coordinate the fiber deformation, the DDRX driven by dislocations from precipitate pinning is activated to relieve stress concentration. This study can provide theoretical guidance for rationally formulating the multi-stage thermomechanical processing regime of Al–Cu–Li alloys.

In this study, the T85-treated Al-3.78Cu-1.17Li-0.7 Mg-0.32Ag-0.25Mn-0.33Zn-0.12Zr alloy thermally exposed at a series of temperatures (100–300 °C) for 100 h, 500 h, and 1000 h was systematically investigated. The results show that the thermal stability of the alloy is more closely correlated with thermal exposure temperature than with exposure duration. Excellent thermal stability in strength is obtained below 150 °C, which can be linked to the diameter and number density of T1 (Al2CuLi) precipitates continually increasing with a thickness less than 2 nm. However, the widened precipitation-free zones (PFZs) associated with continuous grain boundary precipitates (GBPs) benefit intergranular fracture, contributing to a decrease in toughness and elongation. In the window of 175–200 °C, T1 precipitates tend to thicken and are gradually substituted by coarse θ′-Al2Cu and S-Al2CuMg with elevating temperature. Simultaneously, the continuous GBPs transverse to discrete circle-like shapes, and thus intergranular ductile fracture becomes dominant. Therefore, the related hardness and strength decrease, while the electrical conductivity and toughness are the opposite. At temperatures over 250 °C, thermal exposure can reduce the dislocation density and dissolve T1 phases to form T2-Al6Cu(Li, Mg)3 and cubic C phases, leading to a dramatic decrease in strength and toughness.

Dynamic recrystallization (DRX) is one of the more convenient and effective means of refining grains during hot deformation, which is closely related to the precipitation behavior. Three alloys with varying pre-precipitations are designed by carrying out different heat treatment processes to explore the effects of precipitation on the recrystallization behavior of an Al–Cu–Li alloy during compression. It is indicated that the recrystallization behavior of the as-extruded alloy is quite sensitive to the precipitation. Precipitation does not change the recrystallization mechanism, but it could significantly affect the recrystallization volume fraction (VDRX). Meanwhile, the diverse extruded texture fibers coordinate deformation through different softening mechanisms during compression, which stimulates the nucleation of the lamellar grain in R–cube textures. Furthermore, the influence mechanism of microstructure on the material flow behavior is also discussed for the three heat-treated alloys. Therefore, this work is expected to provide new insights for Al–Cu–Li alloy to design high-performance products through grain refinement and texture modification in the subsequent processing.

Anisotropy of mechanical properties has limited the further application of Al–Cu–Li alloys. Previous research efforts were mostly focused on the anisotropic tensile properties. In the present study, significant hardness anisotropy evolution, measured during artificial aging, was also established when loading in different directions. It was found that different amounts of inclined/face-on would be encountered when loading along different directions, resulting from the fiber texture and inhomogeneous distribution of T1 (Al2CuLi) variants along different directions for the samples without pre-stretching. The resistance to hardness loading is different between the face-on/inclined T1 and edge-on T1 due to its high aspect ratio, so the measured anisotropic hardness is obtained. Moreover, the characteristics of precipitates also play an important role in the anisotropic hardness evolution, the strong anisotropy of hardness only appears in the alloys that contained large diameters and a low density of T1 phase or θ' (Al2Cu) phase. After applying pre-stretching, the predominant T1 phase precipitates with small diameters and a high density, which helps to facilitate the isotropic hardness evolution during artificial aging. These findings provide a necessary supplement for the hardness evaluation where a high aspect ratio of precipitate is involved.

The effects of re-solution (RST) and re-aging (RA) treatment on the mechanical property, microstructure, corrosion resistance and electrochemical behavior of 2196 Al-Cu-Li alloy were investigated by tensile testing, conductivity testing, intergranular corrosion and electrochemical corrosion testing. The results show that RST and RA treatment can improve the corrosion resistance without sacrificing the strength compared to T6 temper. The aging precipitation process is reversible, and the number and distribution of δ’, T1 and θ’ precipitates can be effectively regulated by the RA treatment, so that the strength of the alloy approximately reaches the level of T6 temper and the ductility is improved. The corrosion mode of the alloy changed from pitting into intergranular corrosion during RST and RA treatment. The coarsening and discontinuous distribution of grain boundary precipitates as well as the increase of precipitate-free zone (PFZ) width inhibit the generation and propagation of stress corrosion cracking and thereby improve the corrosion resistance of the alloy. The increase of Cu content in grain boundary reduces the electrochemical corrosion sensitivity of the alloy. The optimal process parameters of RST and RA treatment for 2196 Al-Cu-Li alloy are finally determined to be 515 °C for 90 min and 170 °C for 24 h.

In this study, the effects of heat treatment on the microstructure and mechanical properties of extruded 2196 Al-Cu-Li alloy were examined and the precipitation behavior and strengthening mechanism were investigated. It was found that the solution treatment and artificial aging had a significant strengthening effect on the mechanical properties of the extruded alloy, which was related to the type, size, number density, and distribution of the precipitates. Most of the second phases dissolved back into the matrix after solution treatment. The precipitation sequence of 2196 alloy during aging treatment was identified as: supersaturated solid solution→ GP zone + δ′/β′ (Al3(Li,Zr)) → δ′ + θ′ (Al2Cu) + T1 (Al2CuLi) → θ′ + T1, where T1 phase played a leading role in the precipitation strengthening. The fracture mechanism changed from ductile fracture to intergranular fracture. The pre-stretching deformation can introduce a large number of vacancies and dislocations, which provide conditions for atomic diffusion and precipitation of the phases, and further improve the mechanical properties of the alloy. Besides, pre-stretching can effectively shorten the time required for peak strengthening, which is only 20% of the time needed in single-stage peak-aging (170 °C/36 h). Finally, the optimal heat treatment process of 2196 Al-Cu-Li alloy was determined as: 515 °C/90 min solution treatment+3% pre-stretching+170 °C/16 h artificial aging.

[Display omitted] • The microstructure evolution and deformation mechanism in different fibers was revealed by gradient compression tests. • The nucleation mechanism of lamellar grains inside 〈111〉 fiber was elucidated. • A coordinated deformation mechanism based on DDRX and precipitates coarsening mechanism was found in the grain boundaries of 〈111〉 and 〈100〉 fibers. The Al–Cu–Li alloy has poor plasticity and hot workability, making it difficult to form by single-pass working, so the multi-stage thermomechanical processing is usually required. However, there is a general distinctly heterogeneous fiber structure in deformed alloys, which has non-negligible effects on the stability and formability of subsequent deformation. Therefore, the deformation mechanism of heterogeneous fibers and intergranular deformation compatibility mechanism are investigated for as-extruded Al–Cu–Li alloys with <111> and <100> fibers in this work. The results show that more slip systems are activated in the <100> fiber, forming LAGBs through dislocations cross-slip and entanglement, thereby stimulating the CDRX. The number of slip systems that be activated in the <111> fiber is relatively smaller, and the dislocations are dominated by single-slip and are easily pinned by the coarsening precipitate, thus activating the DDRX mechanism to force the grains to transform into the <110> fiber. To coordinate the fiber deformation, the DDRX driven by dislocations from precipitate pinning is activated to relieve stress concentration. This study can provide theoretical guidance for rationally formulating the multi-stage thermomechanical processing regime of Al–Cu–Li alloys. [ABSTRACT FROM AUTHOR]

The thermal stability of the lightweight, T83 heat treated 2099 Al-Cu-Li alloy was assessed in the temperature range 200–305 °C, through both hardness and tensile tests after overaging. After prolonged thermal exposure, the alloy exhibited a better performance compared to aluminium alloys specifically developed for high temperature applications, with the advantage of a considerable lower density. The tensile behaviour was modelled through Hollomon's equation as a function of residual hardness. The changes in the alloy performance were explained through both SEM and STEM investigations. Microstructural analyses gave evidence of Ostwald ripening, while fractographic analyses revealed a transition from an intergranular to a ductile fracture mechanism in the overaged alloy. STEM investigations highlighted the superior thermal stability of the T 1 phase compared to ϑ and S strengthening phases, which dissolved during overaging at 245 °C. The study underlines the need to enhance the formation of T 1 precipitates when high temperature strength is required. The results of the present study suggest that the 2099 alloy is a very promising candidate for automotive engine components, which are extremely demanding in terms of both thermal resistance and lightweight. [ABSTRACT FROM AUTHOR]

[Display omitted] • The T85 treated Al-Cu-Li alloy was systematically investigated under thermal exposure at 100–300 °C for up to 1000 h. • The evolution of precipitation characteristics during thermal exposure was revealed. • Fracture toughness was greatly affected by the morphology and distribution of grain boundary precipitates. • The qualitative relationship between mechanical properties and multiscale microstructure was established. In this study, the T85-treated Al-3.78Cu-1.17Li-0.7 Mg-0.32Ag-0.25Mn-0.33Zn-0.12Zr alloy thermally exposed at a series of temperatures (100–300 °C) for 100 h, 500 h, and 1000 h was systematically investigated. The results show that the thermal stability of the alloy is more closely correlated with thermal exposure temperature than with exposure duration. Excellent thermal stability in strength is obtained below 150 °C, which can be linked to the diameter and number density of T1 (Al 2 CuLi) precipitates continually increasing with a thickness less than 2 nm. However, the widened precipitation-free zones (PFZs) associated with continuous grain boundary precipitates (GBPs) benefit intergranular fracture, contributing to a decrease in toughness and elongation. In the window of 175–200 °C, T1 precipitates tend to thicken and are gradually substituted by coarse θ′-Al 2 Cu and S-Al 2 CuMg with elevating temperature. Simultaneously, the continuous GBPs transverse to discrete circle-like shapes, and thus intergranular ductile fracture becomes dominant. Therefore, the related hardness and strength decrease, while the electrical conductivity and toughness are the opposite. At temperatures over 250 °C, thermal exposure can reduce the dislocation density and dissolve T1 phases to form T2-Al 6 Cu(Li, Mg) 3 and cubic C phases, leading to a dramatic decrease in strength and toughness. [ABSTRACT FROM AUTHOR]

[Display omitted] • The dominant recrystallization mechanism gradually changed from DDRX to CDRX/GDRX with increasing temperature, decreasing strain rate and solid solubility of the alloy. • A lamellar grain nucleated was discovered in the 〈1 1 0〉 fiber during the R-cube transformation to copper, and its growth induced the GDRX. • Precipitation only affected the volume fraction of recrystallization and did not alter the recrystallization mechanism. • The coupling effect of DPN and solution strengthening in as-soluted alloys exhibited strong recrystallization resistance. Dynamic recrystallization (DRX) is one of the more convenient and effective means of refining grains during hot deformation, which is closely related to the precipitation behavior. Three alloys with varying pre-precipitations are designed by carrying out different heat treatment processes to explore the effects of precipitation on the recrystallization behavior of an Al–Cu–Li alloy during compression. It is indicated that the recrystallization behavior of the as-extruded alloy is quite sensitive to the precipitation. Precipitation does not change the recrystallization mechanism, but it could significantly affect the recrystallization volume fraction (V DRX). Meanwhile, the diverse extruded texture fibers coordinate deformation through different softening mechanisms during compression, which stimulates the nucleation of the lamellar grain in R–cube textures. Furthermore, the influence mechanism of microstructure on the material flow behavior is also discussed for the three heat-treated alloys. Therefore, this work is expected to provide new insights for Al–Cu–Li alloy to design high-performance products through grain refinement and texture modification in the subsequent processing. [ABSTRACT FROM AUTHOR]

The effect of an electric field on the microstructure and mechanical properties of an Al-Cu-Li alloy was investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), microhardness tests, electric conductivity tests, and tensile tests. The results show that the field-aged sample exhibited a shorter stable hardening plateau period and an earlier peak-aged stage than the field-free-aged sample. Also, the hardness and conductivity of the field-aged sample during the hardening plateau interval were generally greater than that of the field-free-aged sample. This might be due to the fact that the electric field promoted the precipitation of the β'/δ' phase and accelerated the growth of the θ' phase. The improvement in mechanical properties of the field-aged sample was caused by the segregation of the T 1 phase at the subgrain boundaries and the grain boundaries. A vacancy-atom complex diffusion model successfully explains the microstructure evolution and the change in mechanical properties of the alloy during the ageing process accompanied by an electric field. [ABSTRACT FROM AUTHOR]

In this paper, hot gas forming with integrated heat treatment was carried out to investigate the microstructure and strengthening behavior of an Al–Cu–Li alloy sheet. The cup-shaped specimen was free bulged by compressed air within 3 s after the sheet was heated at 520 °C for 30 min. The corresponding strengthening behavior was reflected by Vickers hardness measurement. Microstructure observation was conducted to reveal the deformation and strengthening mechanisms by using electron backscatter diffraction and transmission electron microscope techniques. It was found that the hardness in the undeformed area could arrive at 181 HV, which increased as deformation increased. Dynamic recovery occurred while the effective strain was 0.31. Dynamic recrystallization occurred with further increase in deformation, accompanying with grain fragmentation. The grains in {110} <112> orientation were produced gradually with increasing deformation. Precipitation strengthening was the main strengthening mechanism in the undeformed area, which could be enhanced by the deformation during hot gas forming. A high density of dislocations and low angle grain boundaries were produced in the deformed area, which accelerated more fine precipitates precipitating. Enhanced formability and hardness were obtained during rapid hot gas forming by combining with heat treatment. [ABSTRACT FROM AUTHOR]

The effects of different ageing treatments on microstructure evolution, properties and fracture are investigated in the present study. 2198 alloy exhibits strong ageing response during ageing. It is found that tensile properties, hardness and conductivity of 2198 alloy are very sensitive to ageing temperatures, which corresponds to different microstructures. In the naturally-aged condition (T3), only &#948' (Al3Li) was detected. After artificial ageing (T8), large amounts of precipitates emerged and major precipitates that were detected turned to be &#948', θ' (Al2Cu) and T1 (Al2CuLi) phase. Exposure to higher temperature caused greater amounts of the precipitation. The constitution and morphology of precipitates varies with different ageing temperature; the major precipitates are &#948', θ' when ageing below 160 °C, while above 160 °C, T1 phase comes out in large numbers, becoming dominate strengthening phases gradually. Fracture transforms from a typical dimple type to a dimple-intergranular mixed type with the rise of ageing temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • Anisotropic hardness evolution appears during artificial aging when changing the loading axis of samples. • Due to fiber texture and inhomogeneous distribution of T 1 (Al 2 CuLi) variants, different amounts of face-on/inclined T 1 will be encountered in different loading directions. • The face-on/inclined T 1 has a higher resistance ability to hardness loading compared to edge-on T 1. • The pre-stretching prior to aging can effectively facilitate the isotropy of hardness evolution. Anisotropy of mechanical properties has limited the further application of Al–Cu–Li alloys. Previous research efforts were mostly focused on the anisotropic tensile properties. In the present study, significant hardness anisotropy evolution, measured during artificial aging, was also established when loading in different directions. It was found that different amounts of inclined/face-on would be encountered when loading along different directions, resulting from the fiber texture and inhomogeneous distribution of T 1 (Al 2 CuLi) variants along different directions for the samples without pre-stretching. The resistance to hardness loading is different between the face-on/inclined T 1 and edge-on T 1 due to its high aspect ratio, so the measured anisotropic hardness is obtained. Moreover, the characteristics of precipitates also play an important role in the anisotropic hardness evolution, the strong anisotropy of hardness only appears in the alloys that contained large diameters and a low density of T 1 phase or θ' (Al 2 Cu) phase. After applying pre-stretching, the predominant T 1 phase precipitates with small diameters and a high density, which helps to facilitate the isotropic hardness evolution during artificial aging. These findings provide a necessary supplement for the hardness evaluation where a high aspect ratio of precipitate is involved. [ABSTRACT FROM AUTHOR]

The effects of re-solution (RST) and re-aging (RA) treatment on the mechanical property, microstructure, corrosion resistance and electrochemical behavior of 2196 Al-Cu-Li alloy were investigated by tensile testing, conductivity testing, intergranular corrosion and electrochemical corrosion testing. The results show that RST and RA treatment can improve the corrosion resistance without sacrificing the strength compared to T6 temper. The aging precipitation process is reversible, and the number and distribution of δ', T 1 and θ' precipitates can be effectively regulated by the RA treatment, so that the strength of the alloy approximately reaches the level of T6 temper and the ductility is improved. The corrosion mode of the alloy changed from pitting into intergranular corrosion during RST and RA treatment. The coarsening and discontinuous distribution of grain boundary precipitates as well as the increase of precipitate-free zone (PFZ) width inhibit the generation and propagation of stress corrosion cracking and thereby improve the corrosion resistance of the alloy. The increase of Cu content in grain boundary reduces the electrochemical corrosion sensitivity of the alloy. The optimal process parameters of RST and RA treatment for 2196 Al-Cu-Li alloy are finally determined to be 515 °C for 90 min and 170 °C for 24 h. [Display omitted] • RST and RA can improve corrosion resistance without sacrificing the strength. • Corrosion mode changes from pitting to intergranular corrosion during RST and RA. • Coarse discontinuous GBPs and wide PFZ improve corrosion resistance of alloy. • Increase of Cu content in GBs reduces electrochemical corrosion sensitivity. [ABSTRACT FROM AUTHOR]

In this study, the effects of heat treatment on the microstructure and mechanical properties of extruded 2196 Al-Cu-Li alloy were examined and the precipitation behavior and strengthening mechanism were investigated. It was found that the solution treatment and artificial aging had a significant strengthening effect on the mechanical properties of the extruded alloy, which was related to the type, size, number density, and distribution of the precipitates. Most of the second phases dissolved back into the matrix after solution treatment. The precipitation sequence of 2196 alloy during aging treatment was identified as: supersaturated solid solution→ GP zone + δ′/β′ (Al 3 (Li,Zr)) → δ′ + θ′ (Al 2 Cu) + T 1 (Al 2 CuLi) → θ′ + T 1 , where T 1 phase played a leading role in the precipitation strengthening. The fracture mechanism changed from ductile fracture to intergranular fracture. The pre-stretching deformation can introduce a large number of vacancies and dislocations, which provide conditions for atomic diffusion and precipitation of the phases, and further improve the mechanical properties of the alloy. Besides, pre-stretching can effectively shorten the time required for peak strengthening, which is only 20% of the time needed in single-stage peak-aging (170 °C/36 h). Finally, the optimal heat treatment process of 2196 Al-Cu-Li alloy was determined as: 515 °C/90 min solution treatment+3% pre-stretching+170 °C/16 h artificial aging. Unlabelled Image • The precipitation sequence of this alloy during aging was: SSS → GP zone + δ′/β′ → δ′ + θ′ + T 1 → θ′ + T 1. • The optimum heat treatment process of extruded 2196 Al-Cu-Li alloy was determined, the strength was significantly improved. • The pre-stretching can effectively shorten the time required for peak strengthening, only 20% of the T6 peak-aging state. • The dimple fracture was transformed to intergranular fracture with aging time, accompanied by partial cleavage fracture. [ABSTRACT FROM AUTHOR]