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The microstructure evolution and composition distribution of the industrially cast Al-Cu-Li alloy during single-step and tow-step homogenization were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that severe dendrite segregation exists in the Al-Cu-Li as-cast alloy. Cu distributes unevenly from the grain boundary to inside. But the changes of Mg, Zn, Mn and Ag are not obvious. At the grain boundary, there are a large amount of coarse nonequilibrium eutectic phases Al 2 Cu, Al 2 Cu containing a trace of Mg and Al 2 CuMg phase. After the optimized two-step homogenization treatment, most of the nonequilibrium eutectic phase and second phase (Al 2 CuMg and Al 2 CuLi) dissolve into α(Al) matrix. A small amount of Fe-rich and Mn-rich phase are still distributed at the grain boundaries. Al 2 CuMg phase melting point is lower than that of Al 2 Cu phase. Al 2 CuMg and Al 2 Cu phase gradually dissolve into the matrix at 495 and 515 °C, respectively. The suitable homogenization treatment for the Al-Cu-Li alloy is 495 °C/24 h + 515 °C/24 h. The results of homogenization can be described by homogenization kinetic analysis, which agrees well with experimental observation. [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]

The 2195 Al-Cu-Li alloy is widely utilized for the fabrication of key aerospace vehicle components; hence, an in-depth understanding of its hot deformation behavior is essential for consistently achieving high-performance products. In this study, isothermal compression tests were performed on 2195 Al-Cu-Li alloy ingots in the temperature and strain rate ranges of 250–500 °C and 10−3-1 s−1, respectively. A processing map was established to predict the workability under different conditions. The corresponding microstructures were examined to reveal their hot deformation mechanism. Results demonstrated that the processing map was divided into four distinct domains. The unstable zone was located in the 250–375 °C/10−1.5-1 s−1 and 250–275 °C/10−2.5–10−1.5 s−1, temperature/strain rate regimes, in which deformation bands and flow localization were observed. The deformation bands blocked dislocations and compromised the dynamic recrystallization (DRX) behavior, hence damaging the microstructure. Further, large coarse T 1 phases precipitated in the 325–410 °C/10−2.5–10−3 s−1 regime, which led to the formation of heterogeneities in the microstructure, corresponding to a minimum workability in the processing map. The optimal hot-working domain for the 2195 alloy was determined to be 460–500 °C/10−3–10−2 s−1, and the dominating hot deformation mechanisms were DRX and dynamic recovery (DRV). Specifically, continuous and discontinuous DRX behaviors dominated in the 300–400 °C/10−3 s−1 and 450–500 °C/10−3 s−1 regimes, respectively. DRX gradually weakened as the strain rate increased at 500 °C. Further, the weakening of DRX was only minimally affected by temperature at 400–500 °C during high-speed deformation (1 s−1). The results reported herein can provide valuable insights into the hot working of Al-Cu–Li alloys. • A processing map (PM) was established to accurately predict the hot deformation behavior of the 2195 alloy. • Deformation bands and flow localization were the main microstructure response in the unstable domain. • Large coarse T 1 phases precipitated in the precipitation sensitive domain, leading to heterogeneity deformation. • The optimal hot working domain was 460–500 °C/10−3–10−2 s−1, which was dominated by DRX and DRV. • Within the 300–400 °C/10−3 s−1 regime, CDRX was the main DRX mechanism, whereas DDRX predominant at 450–500 °C/10−3 s−1. [ABSTRACT FROM AUTHOR]

The intergranular corrosion in a partially recrystallized 2A97 Al-Cu-Li alloy at various tempers was investigated. Immersion tests showed that the T6-tempered alloy was susceptible to intergranular corrosion (IGC) and intersubgranular corrosion (ISGC), and local corrosion preferentially initiated and propagated at subgrain boundary. The IGC and ISGC sensitivity decreased notably in the T8-tempered alloy and intragranular pitting dominated the corrosion in this case. The precipitates and solutes distribution at grain and subgrain boundary in T6- and T8-tempered alloy were characterized by TEM, STEM-EDX and APT to address the key influencing factor to the localized corrosion. ● Subgrain boundary is more sensitive to corrosion than grain boundary at T6 temper. ● Locally enriched Al 2 Cu phase on subgrain boundary facilitates local corrosion. ● Pre-stretching decreases the corrosion susceptibility of grain and subgrain boundary. ● Cu-rich film protects grain boundary from being attacked at T8 temper. [ABSTRACT FROM AUTHOR]

Pre-strain deformation before artificial aging is preferred for fabricating high-performance Al-Cu-Li alloy plates. The precipitation and mechanical properties of rapidly solidified Al-Cu-Li alloy as a function of pre-rolling deformation from 4% to 20% were investigated. The increase in pre-rolling deformation leads to higher dislocation density, which provides more nucleation sites for the T 1 phase. As a result, higher density and finer diameter of the T 1 phase in grains and mitigated enrichment of T 1 phase at the subgrain boundaries were obtained. Meanwhile, more T 1 phase consumes more Cu atoms, thus suppressing the precipitation of θ' phase. Due to strain hardening and precipitation strengthening, the yield strength of alloy increases with the pre-rolling deformation, reaches a maximum value of 641.6 MPa after 20% pre-rolling. The strength model shows that increasing the pre-rolling deformation leads to a decrease in the strengthening contribution of the T 1 phase, which results from the failure of the dislocation bypass mechanism. While the increase in yield strength is attributed to larger strain hardening from high levels of pre-rolling deformation. • Precipitation behavior and mechanical properties of rapidly solidified and pre-rolled Al-Cu-Li alloy were studied. • Pre-rolling deformation reduced the diameter of T1 platelets and increased the number density of T1 phase. • Contribution of the precipitation strengthening to yield strength decreased with increasing pre-rolling deformation. [ABSTRACT FROM AUTHOR]

The quenching expansion tests in the temperature range 350–500 ℃ and isothermal compression tests in the range 350–500 ℃ & 0.01–10 s−1 of a homogenized Al-Cu-Li alloy were conducted to investigate the flow behavior and thermal expansion behavior. The corresponding microstructures of the tested specimens were characterized by back-scattered electron-scanning electron microscope (BSE-SEM), electron back-scattered diffraction (EBSD) and transmission electron microscope (TEM). Coarse secondary phases including Al 2 CuLi (T 1), Al 2 (Cu-Ag)(Li-Mg) (Ω), Al 2 Cu (θ), Al 2 CuMg (S) dissolve with increasing temperature, which is likely to increase the coefficient of thermal expansion. The temperature rising induced by deformation heat varies from 2.29 ℃ to 38.32 ℃ with increasing strain rate and decreasing temperature. Three different constitutive models were calculated, evaluated and optimized, based on which, the two-step optimized Johnson-Cook model showed the highest precision. At the temperature ≤ 400 ℃, the existences of undeformable coarse phases (especially T 1 or Ω phase) promote particle stimulated nucleation (PSN), which pronouncedly increases the volume fraction of dynamic recrystallization (DRX) and decreases the flow stress. At temperature of 450 ℃, most of undeformable coarse phases dissolved, cutting-off mechanism of S and θ phase was observed, which is no longer beneficial to PSN, and CDRX becomes the main DRX mechanism. • Flow behavior and hot expansion behavior of the homogenized Al-Cu-Li alloy were analyzed. • Microstructures before and after compression at high temperature of the homogenized Al-Cu-Li alloy were compared. • A novel two-step optimized Johnson-Cook model with high prediction precision was proposed. • Cut-off mechanism of Al 2 CuMg phase during deformation were identified. • PSN and CDRX were characterized and the effects of precipitates on them were discussed. [ABSTRACT FROM AUTHOR]

The effects of minor Sc additions on the formation of various precipitates and mechanical properties of a new Al-Cu-Li alloy have been experimentally investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), three dimensional atomic probe (3DAP) analysis and tensile tests. Under T8 temper, the introduction of 0.092 wt% Sc accelerates the aging response of Al-Cu-Li alloy and induces a little reduction in the tensile and yield strength due to the decrease of solid solubility of Cu in Al alloy matrix and number density of T 1 (Al 2 CuTi) strengthening phase, which are closely linked with the formation of insoluble Sc-containing W(AlCuSc) phase by consuming Cu solutes for the precipitation of T 1 phase. Except for T 1 and W phase, Nanoscale Al 3 (Sc,Zr) and S′ phases are also found after minor Sc addition. The 3DAP tomography indicates that the Al 3 (Sc,Zr) precipitates have a core-shell structure with Zr atoms mainly close to the shell and Sc atoms enriched in the core. It is believed that such precipitate is only probably composed of Al and Sc at the initial stage, and then Zr enters into them at the later stage, which hinders the diffusion of Sc in the matrix through the boundary into the interior of the particle and thus makes the coarsening rate of Al 3 Sc decrease. • Minor Sc addition accelerates aging response of Al-Cu-Li alloy and induces change in alloy strength and elongation. • W phases inhibit precipitation of T1 phase by consuming the available Cu solutes for T1 phase. • Length range of T1 phase slightly increases and its number density obviously decreases after Sc addition. • Zr aggregation at the interface inhibits the diffusion of Sc from alloy matrix to the precipitate interior. • Zr aggregation also makes coarsening rate of Al3(Sc, Zr) particle decrease. [ABSTRACT FROM AUTHOR]

The corrosion behaviour and its relationship with microstructure, especially the distribution of precipitates at grain boundaries and within grains, of two Al-Cu-Li alloys with different Cu/Li ratio were investigated using accelerated immersion testing, advanced microscopy characterization and quasi-in-situ corrosion observation. The Al-2.23Cu-0.96Li alloy with low Cu/Li ratio exhibited excellent intergranular corrosion resistance and its corrosion modes unusually remained consistent intragranular corrosion regardless of the ageing stage. The corrosion behaviour of the counterpart Al-4.03Cu-0.91Li alloy with high Cu/Li ratio was found to follow the general evolution regulation, from intergranular corrosion to intragranular corrosion with the ageing time. Microstructure characterizations demonstrated that the coarsening and discontinuous distribution of grain-boundary precipitates played the key role in lowering the intergranular corrosion susceptibility of the two Al-Cu-Li alloys. Further quasi-in-situ corrosion observation of the Al-2.23Cu-0.96Li alloy showed that the dealloying process of the grain-boundary precipitates led to the dissolution of the precipitate-free zone, which meant that the broadening of the precipitate-free zone along the grain boundary could reduce the intergranular corrosion resistance of the alloy. Besides, the thin T 1 precipitates that were uniformly distributed within the grains did not suffer preferential dissolution, but they still affected the intragranular corrosion propagation at the nanoscale. Specifically, the corroded T 1 precipitates could become {111} Al -oriented sidewalls of the intragranular corrosion channel. • The Al-2.23Cu-0.96Li alloy kept low IGC susceptibility regardless of the ageing stage. • Intragranular T 1 precipitates do not preferentially dissolve due to dealloying. • Nanoscale corrosion behaviour was investigated through quasi-in-situ TEM experiment. [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]

Multiple nano-phases and nano-complexes feature the typical microstructures of current commercial Al-Li alloys. We present a combined first-principles and scanning transmission electron microscopy study of various nano-features in an aged Al-Cu-Li-Zr model alloy, in attempt to elucidate the fundamental aspects of their formation and interaction. Our results suggest that highly-diffusive excess Li can segregate to the early-precipitated β'(L1 2 -Al 3 Zr) nano-phase, to develop a tentative δ'(L1 2 -Al 3 Li) shell at the early-aging stage. The δ' shell promotes GP-zone precipitation along the (100) facets but is metastable only, and can eventually dissolve into β' with time. Consequently, GP-zones can evolve into θ" and θ' independently under the prolonged aging. Excess Li also weakly segregates to θ' interfaces to develop an "in-phase" or "anti-phase" δ' shell. The gained insights help clarify various experimental observations, and their implications for Al-Li alloy design are also discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

Formability is the key to restrict the widespread industrial application of Al–Cu–Li alloy. And low-temperature forming has been considered to be a promising material formation technique. This work investigates the origin of the simultaneous increase in strength and ductility that takes place in Al–Cu–Li alloys at low-temperature. We found that as the temperature decreased (ranging from 298 to 77 K), both the strengthening and ductility of Al–Cu–Li alloys with full annealing (O-state) and water quenching (W-state) increased simultaneously, producing a better strength-ductility synergy compared to room temperature. The dislocation slip is the primary deformation mechanism of two different state alloys at low-temperature and room temperature. The improved ductility at low-temperature results from the activated slip systems, promoted multisystem slip, prolonged elastic-plastic transition, and larger strain gradient, which induces the deformation to be more homogeneous. The improved strength is the result of dynamic recovery inhibition, which induces an increased dislocation density. This work indicates that Al–Cu–Li alloys have good ductility at low-temperatures and promise low-temperature applications. • Lower deformation temperature strategy to achieve an excellent combination of ductility and high strength in Al–Cu–Li alloys. • The dislocation density increases significantly and the interactions of dislocations between different slip systems hinder the further movement of dislocations at low-temperature. • The number of activated slip systems increases, the number of slip bands increases, and the deformation are more uniform at low-temperature. [ABSTRACT FROM AUTHOR]

Effects of Ce and Zr additions (combinative addition and single addition) on the microstructures and tensile properties of novel Al-5.8wt%Cu-1.3wt%Li alloys with an elevated Cu/Li ratio were investigated comparatively by a microscopy method and tensile test. The microstructural observation shows that the intermetallic dispersoid in the case of combinative addition of Ce and Zr is more significantly refined from coarse polygonal shape to fine irregular particles compared to that in the case of single addition of Ce or Zr. Due to the refinement and the modification of intermetallic dispersoid, the corresponding fracture mode changes from brittle intergranular fracture to ductile transgranular fracture during tension. In addition, microstructural analysis reveals that the addition of Ce promotes the precipitation of the T 1 phase which is the predominant strengthening phase in Al-Cu-Li alloy. In comparison to Ce containing Al-Cu-Li alloy, Ce+Zr-containing Al-Cu-Li alloy raises the degree of supersaturation of Cu in matrix due to less Cu atoms being trapped in finer AlCuCe dispersoid after solution treatment. Thus it is found that the ultimate tensile strength (UTS) and yield strength (YS) for the later alloy are increased by 19.6% and 16.1%, respectively. And the elongation (El) is similar, owing to decreasing of the phase size, changing of precipitation type to T 1 phase only and further grain refinement. [ABSTRACT FROM AUTHOR]

The microstructure evolutions of novel Al-Cu-Li alloys with rare earth cerium (Ce) additions (0, 0.1, 0.3, and 0.5, wt%) were investigated. The as-cast samples were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction meter and electron probe microanalysis with wavelength dispersive spectroscopic. The microstructural observation shows that the grains are significantly refined with increasing the addition of Ce. Mg+Ag-rich Al 7 Cu 4 Li intermetallic phase can be observed in all experimental alloys except for the one with addition of 0.5wt%Ce, which an extra gray CuSi(AgMg) phase emerges. Furthermore, the coarse rodlike Al 2 CuLi phase is completely absent upon Ce level up to 0.3wt%. The analysis shows that the Ce addition introduces changes in the precipitation sequence and consequently in the solidification behavior of the alloy. It is deduced that the grain refinement is mainly the result of the primary Al 8 Cu 4 Ce promoting α (Al) nucleation and further prohibiting the grain growth for the experimental alloys with cerium addition. [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 microstructure and mechanical properties of an in-situ TiB 2 particles reinforced Al–3.3Cu–1.0Li–0.60Mg–0.40Ag–0.14Zr matrix composite were investigated in the naturally aged condition, together with those of the unreinforced alloy following an identical processing and aging history. With the introduction of sub-micro/nano-sized TiB 2 particles, the precipitation of δ′ (Al 3 Li) was promoted in the composite in comparison with the unreinforced alloy. The composite exhibited the smaller grain size, higher elastic modulus, enhanced yield strength, ultimate tensile strength, and superior Vickers hardness over the unreinforced alloy. [ABSTRACT FROM AUTHOR]

This paper presents a systematic study where the distribution of precipitate microstructures is mapped in the cross-section of a friction stir weld made with an AA2050 Al–Cu–Li alloy in the naturally aged temper, as well as the evolution of this microstructure during subsequent post-welding heat treatment (PWHT). This study is carried out using spatially resolved small-angle X-ray scattering, supported by transmission electron microscopy, differential scanning calorimetry and microhardness mapping. The as-welded microstructure is dominated by solute clusters, while very little precipitation has taken place during the welding operation. During PWHT, the precipitation kinetics in the different zones of the weld is mainly controlled by the local dislocation density inherited from welding, and by the amount of solute available for precipitation, which depends on the volume fraction of welding-induced intermetallics. Pre-deforming the weld before the PWHT results in a very effective strength recovery and a nearly homogeneous distribution of hardness. [ABSTRACT FROM AUTHOR]

The Al-Cu-Li alloy is welded by using the vacuum electron beam welding (EBW). The microstructure of joint under the as-welded (AW) condition and the effect of post-weld heat treatment (PWHT) on the microstructure of joint are investigated. The results show that under the AW condition, the microstructure of weldment is a typical dendrite, and there are a lot of eutectic structures which are mainly composed of α+θ′（Al2Cu） phase distributed in the dendrite boundary. The quantity of strengthening phase is less in the weld center. The microstructure of joint obviouslychanges after PWHT. The dendritic structure in the center of weldment is transformed into equiaxed grains. The segregation phenomenon of grain boundary is eliminated under the AW condition. Many strengthening phases precipitate in the weldment, such as spherical δ′(Al3Li) phase and fine needle-like T1(Al2CuLi) phase, and the tensile strength of welded joint is greatly improved. The tensile strength of joint is enhanced from 348 MPa under the AW condition to 423 MPa after PWHT, and the tensile fracture of joint presents the characteristic of ductile fracture. [ABSTRACT FROM AUTHOR]

In this study, a novel Al–5Cu-0.6Li-0.5Mn-0.3Mg-0.15Ti alloy was developed, in which Li is as low as 0.6 wt% and Ti is added, no Ag, Zn, Zr, RE, etc., and high-performance Al–Cu–Li alloy casting billets were prepared by squeeze casting (SC) with ultrasonic treatment (UT). The results show that compared with traditional gravity casting (GC), the porosity and compositional segregation in the SC billets are significantly reduced, and the grain size is also reduced from 80 μm to 38 μm, especially the microstructure of UT + SC billets is further optimized. After UT + SC, the ultimate tensile strength (UTS), yield strength (YS) and elongation of Al–Cu–Li alloy billets are 300 MPa, 190 MPa and 16.5%, which are 27.1%, 35.7% and 230% higher than those of the GC billets, and these as-cast properties are increased by 13.2%, 15.2% and 37.5%, compared with Li-free Al–5Cu-0.5Mn-0.3Mg-0.15Ti alloy billet prepared by UT + SC, respectively. The improvement of the strength and elongation of Al–Cu–Li alloy billet prepared by UT + SC is attributed to the reduction of porosity, grain refinement and uniform distribution of second phases. [ABSTRACT FROM AUTHOR]

Corrosion behaviour of 2A97-T6 Al-Cu-Li alloy was investigated by anodic polarization, immersion test and quasi-in-situ analysis with corrosion morphology characterized by a range of electron microscopy techniques. It is revealed that selective dissolution of grain interior tends to occur in grains of high stored energies, resulting in corroded bands associated with dense precipitate bands in the alloy. The evolutions of corrosion morphology during immersion test and quasi-in-situ analysis are similar, which both exhibit well-defined crystallographic pits. By contrast, crystallographic feature was less evident for localized corrosion induced by anodic polarization, which may be associated with the extensive Cu re-distribution. • Corroded bands are confined within individual grains of high stored energies. • Corroded bands are related to precipitates. • Cu re-distribution promotes the localized corrosion development but compromises its crystallographic features. [ABSTRACT FROM AUTHOR]

The effects of surface finishing on the corrosion behavior and electrochemical activity of AA2098-T351 (Al–Cu–Li alloy) were investigated on the basis of the correlation between surface chemistry, microstructure and electrochemical activity. The alloy was evaluated in the as-received and polished conditions. The morphology of the two types of surfaces was investigated using confocal laser scanning microscopy (CLSM), optical microscopy and optical 3D profilometry. The surface chemistry was analyzed by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX). Scanning microelectrochemical techniques (namely, localized electrochemical impedance spectroscopy (LEIS), the scanning vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM) in potentiometric mode) were used to examine the electrochemical activity of the surfaces. The results showed that on the as-received surface, the near surface deformed layer (NSDL), which is composed of Mg-rich bands, influenced the corrosion activity of the alloy. Higher electrochemical activity and greater susceptibility to severe localized corrosion were related to the polished surface condition compared to the as-received one. [Display omitted] • Surface finishing effects on chemical activity of 2098-T351 Al–Cu–Li alloy studied using scanning microelectrochemical techniques. • The effect of the near-surface deformed layer (NSDL) on 2098-T351 Al–Cu–Li was investigated for different surface finishes. • A solid contact magnesium ion-selective microelectrode (Mg2+ ISME) employed to monitor Mg2+ distributions over the surfaces. • SVET, LEIS, and SECM in potentiometric operation produced high spatial resolution images of reactive sites on the surfaces. • The surface chemistry, microstructure and electrochemical activity of the alloy with and without NSDL were correlated. [ABSTRACT FROM AUTHOR]

The Al-Cu-Li alloys with different grain sizes were manufactured through different routes and treated with T34 before the creep aging process. Their creep-aging behaviors at 160 °C under 175 MPa were investigated and their corresponding mechanical properties were tested. Their microstructure was characterized by EBSD and HAADF-STEM. The results show that the total creep strains increase with increasing grain size. The creep strain of specimens with an average grain size of 53.4 µm is twice as that with an average grain size of 12.2 µm. However, their yield and tensile strengths are grain size independent. This appearance could be explained by the large creep strain achieved before the secondary primary creep stage, which is contributed by dislocation motion and increases with increasing grain size. When strengthening precipitates start to nucleate and grow, the creep resistance of dislocations sharply increases and Harper-Dorn creep becomes the main creep mechanism. The alloys with different grain sizes have similar precipitates and substructures in their peak-creep-aged state, leading to their similar yield and tensile strengths at room temperature. • A new thermomechanical treatment was used to fabricate sheets with fine grains. • The creep mechanism of the complex four-stage creep age test in the Al-Cu-Li alloy has been clarified. • Grain refinement was proven to have a negative influence on the total creep strain in this work. • GND densities was used to characterize the distribution evolution of dislocations during creep age test. [ABSTRACT FROM AUTHOR]

The anodized AA2099-T83 Al-Cu-Li alloy was post-treated in a lithium nitrate-based solution in order to improve its corrosion resistance. The microstructure and corrosion resistance of the alloy after various post-treatments were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET) and neutral salt spray test (NSST). It was found that, under the optimized pH of 10, the corrosion resistance of the anodized alloy treated at 25 °C and 90 °C could withstand 500 h and 1000 h of NSST, respectively. The improved corrosion resistance was ascribed to sealing effect of the pseudo-boehmite, boehmite and/or layered double hydroxides formed in the anodic film. Further, SVET analysis suggests that the anodized alloy after the post-treatment exhibited self-healing capacity. • The anodic film on Al-Cu-Li alloy was effectively sealed in a lithium salt solution. • The sealing treatment could be performed within a wide temperature window. • The alloy treated at 90 °C for 30 min passed 1000 h of neutral salt spray test. • The treated alloy showed self-healing characteristics when exposed to NaCl solution. [ABSTRACT FROM AUTHOR]

Creep ageing of Al–Cu–Li alloys induces precipitation of T 1 (Al 2 CuLi) and a high fraction of low-angle grain boundaries (LAGBs) and dislocations in grain interior, factors potentially contributing to corrosion. Qualitative/quantitative analysis of precipitates and quasi-in-situ EBSD observations of stress free and stress added alloy reveal that fine dense precipitation of T 1 in grain interior and suppressed precipitation along grain/subgrain-boundary induced by stress adding make the corrosion mode evolve from intergranular to intragranular, and grain orientation displays as the most relevant metallurgical parameter for the localised corrosion although the fraction of LAGBs and dislocations have been greatly improved by stress loading. • Al-Cu-Li alloys are formed by creep deformation during ageing. • Precipitation of T 1 is an important factor for localised corrosion in these alloys. • The formation of T 1 , LAGBs and dislocations are improved by stress adding. • Corrosion depends chiefly on the orientation of the grains. • As stress increases, corrosion changes from inter- to intra-granular. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation of a 2A97 Al–Cu–Li alloy under a pulsed bi-polar constant current regime has been carried out in electrolytes containing 5, 32 and 56 g l − 1 NaAlO 2 . Wear resistant coatings, containing relatively large amounts of α-Al 2 O 3, were formed using concentrated solutions. The best protection was obtained with a relatively compact coating, generated with 32 g l −1 NaAlO 2 . The coating growth rate increased from ~ 1.5 to 11.3 μm min −1 between 5 and 56 g l −1 NaAlO 2. The high rates indicate that non-Faradaic processes contribute to the coating formation. Increased concentrations of NaAlO 2 reduced the energy required for coating formation; e.g. to the range ~ 1 to 5 kWh m − 2 μm −1 for coatings formed with 32 and 56 g l −1 NaAlO 2 . [ABSTRACT FROM AUTHOR]

The high-temperature deformation behaviour and microstructure evolution of an extruded Al-Cu-Li alloy were investigated by compression tests conducted at various temperatures (613, 673 and 733 K) with various strain rates (0.001, 0.01, and 0.1 s -1 ). The results indicated that the deformation activation energy increased from 208.7 kJ/mol to 255.7 kJ/mol with an increase in strain from 0.1 to 0.7. The electron backscatter diffraction maps indicated that a dynamic recrystallisation occurred during the high-temperature deformation. Two types of recrystallisation mechanisms, grain boundary bulging and a grain boundary transformation from low misorientation to high misorientation, were considered as the mechanisms for controlling the formation of the recrystallised grains. A new dynamic recrystallisation model containing these two mechanisms was proposed to describe the microstructure evolution of the extruded Al-Cu-Li alloy. At the early stage of the deformation, the recrystallised grains were formed by grain boundary bulging along the original grain boundaries. With increasing strain, recrystallised grains were gradually generated in the deformed grains due to the transformation from low angle boundaries to high angle boundaries. [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]

The application prospect of 2195 alloy in aerospace structure parts has received widespread attention. The high strain rate forming combined with heat treatment process is a potential way to produce the high performance Al-Cu-Li alloy parts. In this work, the mechanical properties and microstructure evolution of the as-annealed and solution treated 2195 alloy specimens under dynamic compression were studied and compared. The dynamic compression tests were performed at the strain rates of 1400 s−1, 2200 s−1, 3400 s−1, and 4500 s−1 by split Hopkinson pressure bar. The results indicate that the mechanical properties of the two kinds of heat-treated specimens have been improved by dynamic compression, an increase of 59.7 % and 63.1 % of hardness respectively. The main strengthening mechanisms of the specimen under dynamic compression conditions are fine grain strengthening, dislocation strengthening, and low angle grain boundaries strengthening, which differs depending on the heat-treated state and the strain rate. Besides, it is found that the stable-end orientation of grain rotation in the two kinds of heat-treated specimens after dynamic compression is <110 > //CD (CD: compression direction). Different from the as-annealed specimen, the solution treated specimen shows insufficient grain rotation behavior. The mechanism of insufficient grain rotation is revealed by the Schmid factor. The Schmid factor of as-annealed specimen increases by 7.5 % than that of solution treated specimen. The lower Schmid factor makes the plastic deformation resistance of solution treated specimen higher, which contributes to the insufficient grain roration behavior. [ABSTRACT FROM AUTHOR]

• An Al-Cu-Li alloy was rolled at 25 °C, − 100 °C, −190 °C respectively. • Heterogeneous grain structure occurs in samples by cryorolling at − 100 °C. • Sheets by cryorolling at − 100 °C have higher strength and better ductility. • A back stress model was used to explain the highest strength of cryorolled sheets. An Al-Cu-Li alloy was selected as the model material to get insight into the mechanism of deformation strengthening at cryogenic temperatures. The mechanical properties and microstructure evolution of the alloy fabricated by symmetric rolling and asymmetric rolling (the rolling ratio was 1.3) at different temperatures (25 °C, − 100 °C, − 190 °C) were investigated. Smaller grains and denser dislocations were observed in the cryorolled samples, leading to strength enhancement compared to the samples processed at room temperature. The heterogeneous grain structure was made of coarse and fine grains in the samples processed by cryorolling (CR) at − 100 °C, which resulted in higher strength and better ductility than those processed by CR at − 190 °C and room temperature rolling. Finally, a new back stress model was employed to calculate the yield strength of the heterogeneous structure, which could explain the highest strength of samples processed by CR at − 100 °C. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) of an Al-Cu-Li alloy has been carried out with a pulsed-bipolar current regime in alkaline sodium aluminate electrolytes. Thick coatings can be developed under plasma discharges in electrolytes of 5g l-1 NaAlO2+1g l-1 KOH and 24g l-1 NaAlO2+1g l-1 KOH. In contrast, the alloy exhibits a conventional anodization behavior with the occurrence of galvanoluminescence in concentrated aluminate electrolyte of 32g l-1 NaAlO2+1g l-1 KOH, resulting in extremely thin nanoporous films and macropits on the alloy surface. It is proposed that, in the latter case, chemical attack from the electrolyte caused severer thinning of the original oxide film, paticularly at the weak sites of anodic constituent particles or precipitates, which then triggers strong field-assisted dissolution of the oxide film, preventing the further growth of the coating. However, after the formation of a thin pre-existing film(~ 1-2μm) on the alloy in less concentrated electrolytes, then thick coatings develop more rapidly in 32g l-1 NaAlO2+1g l-1 KOH, since the field-assisted dissolution is only prevalent in extremely thin films and retreats quickly as the film thickens. Long-term immersion in aluminate electrolytes can eliminate all the weak points on the alloy surface and the oxide film is repaired, thus thicker PEO coatings can also be developed in concentrated electrolytes. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution in situ synchrotron X-ray laminography combined with digital volume correlation (DVC) is used to measure the damage and plastic strain fields ahead of a notch introduced within a 2198 Al–Cu–Li alloy sheet. Synchrotron laminography is a technique specifically developed for three-dimensional (3-D) imaging of laterally extended sheet specimens with micrometre resolution. DVC is carried out using the 3-D image contrast caused by iron-rich intermetallic particles present in the alloy. The alloy is recrystallized and tested in T8 artificial ageing condition involving relatively low work hardening. Inclined strain localization bands are shown to develop at ∼800μm from the notch prior to the onset of damage. Damage in this region results mainly from the nucleation of voids on micrometric intermetallic particles and occurs at a very late stage of deformation, followed by very limited void growth. The accumulation of strain in the slanted localization band is found to be a steady process, whereas the crack propagation is a sudden process. Standard 3-D FE calculations using either von Mises plasticity or Gurson’s model do not capture the plastic localization process. [Copyright &y& Elsevier]

The effects of different cerium contents on the mechanical properties of a novel Al-Cu-Li alloy under the peak aged conditions of T6 and T8 were studied by conventional tensile test and Kahn Tear Test. In addition, the grain boundary precipitates and fractography were observed by transmission electron microscopy (TEM) and scanning electron microscope (SEM). The results show that the UIE and the strength for alloys under T6 condition are generally lower than those for the alloys under T8 condition, while strength-toughness combination has been obtained in alloy 2#(0.15% Ce) under T8 (pre-stretched + artificially aged) condition. Meanwhile, a statistic analysis of the microscopic data by Image-Pro Plus (IPP) software, taking into account the effect of cerium content and pre-stretch was carried out. The analysis shows that the larger area fraction of the combination of dimples and intermetallic particles is corresponding to the higher value of UIE. The changes in the precipitate free zone (PFZ) width, grain boundary precipitate number density and grain boundary precipitate effective size govern the fracture resistance and the crack path of the test alloy under the conditions of the different Ce contents and heat treatment. [ABSTRACT FROM AUTHOR]

In order to study the workability of a Al-Cu-Li alloy, isothermal hot compressive deformation was investigated in the temperature range of 350-500 °C at strain rates in the range of 0.01-10/s up to a true strain of 0.9 on Gleeble-1500 mechanical testing machine. The flow stress increased rapidly to a peak value. The peak stress decreased with increasing deformation temperature and decreasing strain rate. The effects of strain rate and temperature on hot deformation behavior can be represented by a Zener-Hollomon parameter including an Arrhenius term. The influence of the strain has also been incorporated in the constitutive equation and four material constants α, n, A, and the activation energy Q were calculated by compensation of strain. The proposed constitutive equation (considering the compensation of strain) gives an accurate description for the flow stress of the Al-Cu-Li alloy. [ABSTRACT FROM AUTHOR]

Abstract: The hot deformation behavior of an Al–Cu–Li alloy was studied by isothermal compression in the temperature range of 573–773K with strain rates of 0.01–10s−1 on a Gleeble-3500 thermo-mechanical simulator. The microstructural evolution during hot deformation was investigated by optical microscope (OM) and transmission electrical microscope (TEM). The results show that the flow stress increases with decreasing deformation temperature and increasing strain rate. Processing maps at the strains of 0.3–0.8 are obtained by isothermal compression data and exhibit two flow instability domains (573–648K at 0.014–1.5s−1 and 698–773K at 0.3–10s−1) and two peak efficiency domains (723–773K at 0.01–0.1s−1 and 573–623K at 0.01–0.03s−1). The strain has a significant effect on the peak efficiencies, and the peak efficiencies increase almost linearly as the strain increases. Dynamic recrystallization takes places at high temperatures (673–773K) and low strain rates (0.01–0.1s−1). The second phases precipitating dynamically during hot deformation pin dislocations and boundaries of subgrains and restrain the occurrence of dynamic recrystallization, resulting in a higher temperature for dynamic recrystallization. According to the processing maps and microstructural observation, the optimized processing condition of hot deformation for Al–Cu–Li alloy is at 723–773K and 0.01–0.1s−1. [Copyright &y& Elsevier]

• Corrosion occurs preferentially in non-recrystallized regions. • Subgrain boundaries decorated with T 1 precipitates are preferred sites for corrosion initiation. • Local corrosion susceptibility is strongly correlated to grain stored energy. • Cu-enriched layer is developed at the crevice wall of attacked grain boundaries. In the present study, correlative corrosion testing and electron microscopy, including quasi-in-situ electron microscopy, are employed to investigate the corrosion behaviour of 2A97-T8 Al-Cu-Li alloy extrusion in 3.5 wt% NaCl solution. It is found that the corrosion behaviour of the alloy is influenced by synergetic effect of the plastic deformation introduced by the thermomechanical processing during alloy fabrication and the precipitation of T 1 phase during T8 treatment. Localized corrosion initiates and propagates preferentially within non-recrystallized regions of the partially recrystallized alloy. Corrosion initiated at coarse constituent intermetallic particles is confined to its periphery, which doesn't develop further when the particles are disconnected from alloy matrix. [ABSTRACT FROM AUTHOR]

To facilitate the processing and manufacturing of heat-treatable aluminum alloy components, the initial temper of the billet material is usually adjusted to the as-quenched (AQ) temper by solid solution treatment. However, for most as-quenched aluminum alloys, the obvious natural aging (NA) may occur and cause sensitive time scheduling for the subsequent forming processes. Taking the 2195 Al-Cu-Li alloy as the case material, from the aspects of Portevin-Le-Chatelier (PLC) effect, properties and microstructure evolution, this study deeply and systematically explores the NA behaviors and reveals the underlying mechanism. The combined interaction of Cu and Li solute atoms with mobile dislocations is the major mechanism for the PLC effect in this alloy. The NA induced decreasing of the interaction of solutes with the dislocations makes the number of dislocations in the PLC bands reduce, resulting in its character to change from type B to type A during NA. The Al-Cu-Li alloy exhibits a strong response at the early stage of NA, viz., the tensile strength increases rapidly in the initial 4 h and then reaches the plateau 24 h later. Major sources of strengthening in the NA state are the δ′/β' precipitates, GP zones and solute atoms. The extended age hardening model were used to predict the evolution of the YS and microstructure variables of the alloy during NA. The results indicate that the contribution of solute atoms (σ ss), δ′/β' precipitates (σ δ ′) and GP zones (σ GP) to strength in NA state is ~161 MPa, ~80 MPa, and ~ 141 MPa, respectively. The electrochemical measurement results indicate that the NA state has a higher corrosion resistance than the AQ state. The pitting pores form associating with the negative potential of the spherical δ′ phase in NA state. • The reduction in the number of dislocations in the PLC bands causes its character to change from type B to A during NA. • The contribution of σ ss , the σ δ ′ and σ GP to YS in NA state is ~161 MPa, ~80 MPa, and ~ 141 MPa, respectively. • The pitting pores form associating with the negative potential of the spherical δ′ phase in NA state. [ABSTRACT FROM AUTHOR]

The influence of pre-strain (2%, 5%, 8% and 12.5%) and aging state (under, peak and over aging) on mechanical properties and stress corrosion behavior of 2297 Al–Cu–Li alloy was studied. The peak microhardness of the alloy reaches 156.3 HV at 15 h with 12.5% pre-strain, and that of the alloy is only 115.6 HV at 31 h without pre-strain. The slow strain rate testing results indicate the low stress corrosion cracking factor for the samples without pre-strain and high stress corrosion cracking factor for the samples with pre-strain, and the stress corrosion cracking factor of the peak-aged sample with 12.5% pre-strain reaches to 33.3%. From the transmission electron microscopy characterization, it can be inferred that the quantity and distribution of T 1 (Al 2 CuLi) precipitates significantly affect the stress corrosion behavior of 2297 Al–Cu–Li alloy. The over-aged sample exhibits poor stress corrosion resistance, resulting from the large numbers of coarse T 1 precipitates and precipitation free zone at the grain boundary. [ABSTRACT FROM AUTHOR]

Highlights: [•] PEO processing of an Al–Cu–Li alloy in silicate electrolytes is investigated. [•] Brownish-red coatings, due to cuprite species, improved the corrosion resistance of the alloy. [•] Bi-layered coatings formed in dilute electrolyte gave superior wear resistance. [•] Both the outer and inner layers of the coatings contribute to the wear resistance. [•] A modified model of PEO was proposed for coating formation in dilute silicate electrolyte. [ABSTRACT FROM AUTHOR]

Abstract: The inherent properties of lithium, such as high reactivity and toxicity, relatively low density, low melting point, along with its high cost requires a special technological approach to cast Al–Cu–Li alloy AA2195 as compared to the conventional Direct Chill (DC) casting of aluminum alloys. This paper describes the processing requirements for melting and casting of 200kg of Al–Cu–Li alloy in a Vacuum Induction Melting (VIM) furnace under dynamic inert atmosphere. The as-cast billets have been homogenized to remove microsegregation as well as to avoid incipient melting, and subsequently subjected for secondary metal processing operations viz., forging and rolling. The product in the form of 4mm thick sheets was subjected to various heat treatments in T8 (Solution Treatment+WQ+CW+Aging) condition. Mechanical properties were evaluated at room temperature and were correlated with microstructures of the sheets processed under different conditions using transmission electron microscopy (TEM). [Copyright &y& Elsevier]