Al7Cu2Fe Particles Research Articles

Surrogate model-based assessment of particle damage behaviour of Al[sbnd]Zn[sbnd]Mg alloy

Recent research has found some intermetallic compound particles with even stronger hydrogen trapping capacity (e.g., Al7Cu2Fe) than the age-hardening precipitates that are reported to be the origin of hydrogen embrittlement in aluminium. Such intermetallic compound particles can reduce hydrogen concentration at the interface between the precipitates and aluminium by absorbing hydrogen in their interiors, thus preventing the hydrogen embrittlement of aluminium. However, this cannot be achieved if the particles, which have absorbed large amounts of hydrogen, are damaged due to hydrogen embrittlement. In this study, the hydrogen embrittlement of aluminium was observed in situ by X-ray CT, and the damage behaviour was analysed of all the particles that were located in the gauge section of a single tensile specimen. After exhaustive quantification of the size, shape, and spatial distribution of the particles, coarsening processes identified highly correlated design variables. Subsequently, particle damage behaviour was analysed utilizing a surrogate model using a support vector machine. The damage to Al7Cu2Fe particles could be described only by design variables representing size and shape, while those representing spatial distribution were removed through the coarsening processes. No change was observed in the damage behaviour of Al7Cu2Fe particles with increasing hydrogen concentrations, and it was concluded that the dispersion of Al7Cu2Fe particles is effective in preventing hydrogen embrittlement of aluminium. The contribution of damaged particles to the formation of fracture surfaces and the damage behaviour of Mg2Si particles, where damage is accelerated by hydrogen, were also analysed.

Evading the strength-ductility trade-off dilemma in AA2024 alloy by short-term natural re-aging after T351 temper

In this research, the influence of short-term natural re-aging after T351 temper on the microstructural evolution and mechanical behavior of AA2024 aluminum alloy was investigated. Grain growth occurred in the microstructures of the natural re-aged sample and a large number of Al7Cu2Fe particles were located inside the alpha grains. At the re-aging time of 1440 min, the peaks of XRD were shifted strongly to the right due to the formation of θ′′, S″, θ′, and S′. The results revealed that the precipitation rate was high in the AA2024 alloy during natural aging. With increasing the re-aging time, texture parameters remained almost unchanged. The hardness increased slowly within the first 60 min, then enhanced rapidly between 60 and 2880 min, and finally became stable at around 139 HV between 2880 and 11520 min. When the natural re-aging time increased from 240 to 2880 min, the strengthening trended speed up, viz, the yield strength increased from 226.6 to 357.3 MPa, and the ultimate tensile strength enhanced from 452.2 to 535.5 MPa. Compared to the as-received sample (T351 temper), the ultimate tensile strength of the re-aged sheet improved from 455.5 to 535.5 MPa, the ductility remained unchanged, and the hardness increased from 128.8 to 138.2 HV, which was owing to the acceleration of the precipitation caused by the presence of high-content Al7Cu2Fe particles in the interior of the alpha-aluminum grains in the natural re-aged sample. It was found that the Portevin-Le Chatelier instability of AA2024 alloy was effectively postponed after natural re-aging. With increasing the natural re-aging time, the strain hardening rate of the AA2024 sheet increased. The strengthening of the natural re-aged sample for 1440 and 2880 min was a result of a synergistic effect of precipitation hardening due to the formation of θ′′, S″, θ′, and S′ phases, elimination of Portevin Le-Chatelier instability, and highly efficient load transfer from alpha-aluminum to Al7Cu2Fe. Finally, to use the A2024 alloy produced by natural re-aging for 1440 or 2880 min, two methods were proposed.

Initial localized corrosion induced by multiscale precipitates in the new generation high-strength Al-Zn-Mg-Cu alloy

Initial localized corrosion behavior induced by nanoscale Mg (Zn, Al, Cu)2 and micron-scale Al7Cu2Fe precipitates were systematically investigated, respectively. Opposite to the traditional view of considering Mg-rich phase as micro-anodes, experimental and calculation results both suggest that Mg (Zn, Al, Cu)2 precipitates exhibit higher Volta potential than the matrix due to the doping of Cu atoms. Meanwhile, dealloying of Mg (Zn, Al, Cu)2 particles via Mg atoms dissolution would facilitates the formation of nano pits on the surface in aggressive media. Besides, localized corrosion initiation induced by Al7Cu2Fe particles occurs earlier ascribed to a greater Volta potential difference.

Microstructural effects on the spall failure of 7085 aluminum alloy

Designing aluminum alloys for spall resistance requires an understanding of the active failure mechanisms under dynamic loading. However, it is time-consuming and expensive to obtain sufficient data to investigate these mechanisms from conventional plate impact spall experiments. Here we use a high-throughput laser-driven micro-flyer plate impact technique to connect the spall failure of aluminum alloy Al7085-T711 to its microstructure. By conducting tests at four impact velocities, we observe the full range of behaviors from incipient spall to complete spall failure. The spall strength of Al7085-T711 increases with both increasing strain rate and peak shock stress, as is typically the case in aluminum alloys. Examination of recovered samples indicates that incipient spall voids initiate primarily at Al7Cu2Fe second-phase particles. To further explore the effect of microstructure on spall failure, we annealed some specimens at 500°C to increase the aluminum grain size while retaining the Al7Cu2Fe particles, which had only a minor effect on spall strength. Solutionizing at 600°C to eliminate the Al7Cu2Fe second-phase particles, on the other hand, increases the spall strength significantly. Our results suggest that spall failure of Al7085-T711 is dominated by the presence Al7Cu2Fe second-phase particles, and that eliminating these particles could result in improved spall resistance of commercial alloys.

Effect of retrogression temperature on the fracture toughness of 7136 aluminum alloy

7136 aluminum alloy was treated by retrogression and re-aging (RRA) at different retrogression temperatures in this study. The effect of retrogression temperature on the fracture toughness was investigated and the influence mechanism was discussed in detail. When the retrogression temperature is 175 °C, the KIC value of 7136 aluminum alloy reaches a peak of 47.3 MPa m1/2, increasing 59.2% compared to 171 °C. The fracture of the RRA-treated 7136 aluminum alloy is a combination of trans-granular crack and inter-granular crack, while the width of fatigue strips and the volume fraction of dimples are different. Al7Cu2Fe particles distributing along the grain boundary reduce the fracture toughness by weakening the interface bonding force and cracking themselves. TEM results show that retrogression temperature affects the fracture mode by changing the strength difference between the grain boundary and the grain. The grain boundary with a complex dislocation structure makes the dislocation movement more difficult, which is benefit to the fracture toughness of 7136 aluminum alloy.

Suppression of Hydrogen Embrittlement due to Local Partitioning of Hydrogen to Dispersed Intermetallic Compound Particles in Al–Zn–Mg–Cu Alloys

Recent studies have revealed that hydrogen embrittlement in Al–Zn–Mg alloys appears to be dominated by hydrogen partitioning to MgZn2 precipitates. A method has recently been proposed for reducing the hydrogen concentration at MgZn2 precipitates by adding specific intermetallic compound particles that have high hydrogen trap energy. In the present study, the effectiveness of Al7Cu2Fe particles on suppression of hydrogen embrittlement in Al–Zn–Mg–Cu alloys was evaluated using X-ray microtomography. Quasi-cleavage cracks were found to be initiated in regions where local volume fractions of the Al7Cu2Fe particles were relatively low. Hydrogen partitioning to the MgZn2 precipitate interface was suppressed, even in high hydrogen concentration material, by adding Al7Cu2Fe particles. However, the fractional area of the quasi-cleavage fracture in the material with high hydrogen concentration was higher due to insufficient hydrogen diffusion inside the Al7Cu2Fe particles and at the interface between the aluminum matrix and the particles. It appears that finely distributed small Al7Cu2Fe particles might effectively suppress hydrogen embrittlement.

Effect of laser shock peening on corrosion behaviors of ultra-high strength Al-Zn-Mg-Cu alloys prepared by spray forming and ingot metallurgy

In the present work, laser shock peening was applied to the ultra-high strength Al-Zn-Mg-Cu alloys prepared by ingot metallurgy and spray forming. The microstructural evolution, pitting corrosion, intergranular corrosion resistance and electrochemical analysis were systematically investigated. The depth and number of corrosion pits caused by Al matrix, Al7Cu2Fe and Al2CuMg particles can be successfully reduced by laser shock peening. The depth of intergranular corrosion and the width of intergranular cracks also decreased. Cyclic polarization and electrochemical impedance spectroscopy tests also indicated the improved corrosion resistance caused by laser shock peening. The relationship between microstructural evolution and corrosion resistance was studied.

In-situ 3D observation of hydrogen-assisted particle damage behavior in 7075 Al alloy by synchrotron X-ray tomography

We directly captured, classified, and evaluated 3D particle debonding and fracture behavior in a H-charged 7075 Al alloy throughout the entire tensile deformation using synchrotron X-ray tomography and microstructural feature tracking techniques. The effects of particle size, shape, spatial clustering and stress state on strain-dependent particle damage were identified and isolated from each other. Moreover, state-of-the-art imaging and tracking techniques enabled the establishment of spatially and time-resolved hydrogen distributions during deformation. Based on realistic hydrogen partitioning among various nanoscopic trap sites, the contributions of particles to hydrogen trapping and the hydrogen effect at individual damaged particles were assessed quantitatively. Fracturing of coarse and irregular Al7Cu2Fe particles was found to be the predominant particle damage mode due to the spatial clustering and brittleness of these particles, but a hydrogen effect was not observed. The debonding of Mg2Si particles seemed to be the result of competition between hydrogen and clustering-induced stress localization, but detrimental effects of hydrogen on ductile fracture induced by accelerating interfacial debonding were found to be limited. The quantitative evaluation of particle damage in the present model material clarified a viable strategy for mitigating hydrogen embrittlement, which involves introducing and modifying intermetallic particles with strong hydrogen trapping capacities.

Localised corrosion of intermetallic particles on aluminium AA2099-T8

ABSTRACT Specimens of AA2099-T8 were immersed for different times ranging from 2 to 180 min in 0.1M NaCl. The development of corrosion around intermetallic particles was studied by scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDXS). The degradation evolution of each major intermetallic particle after exposure of the alloy to electrolyte allowed to determine which particle corroded during the first stages of immersion. The corrosion of lithium-containing particles was also investigated by transmission electron microscopy (TEM). The earliest stages of attack started with localised corrosion of Al2CuMg (S-phase) particles resulting in dealloying which was followed by trenching around these particles. After 10 min, trenching was observed around Al7Cu2Fe(Mn) particles and then progressed to AlCuFeMnSi particles after 90 min.

Effects of critical defects on stress corrosion cracking of Al–Zn–Mg–Cu–Zr alloy

The effects of critical defects on stress corrosion cracking (SCC) of Al–7.82Zn–1.99Mg–2.41Cu–0.12Zr alloy under under-ageing (UA), peak-ageing (PA) double-ageing (DA) and retrogression and reageing (RRA) were investigated. It is found that anodic dissolution of grain boundary precipitates (GBPs) for UA sample generates critical defects due to low Cu content of GBPs, whereas the dissolution of Al, Mg and Zn elements within the grain interior is responsible for the formation of critical defects for PA, DA and RRA samples. The high distribution continuity of GBPs promotes the continuous propagation of stress corrosion crack along grain boundary, leading to intergranular SCC for UA sample. Coarsened GBPs which are distributed discontinuously could hinder anodic dissolution along the grain boundary. As a result, the cracks propagate along the grain interior, causing transgranular SCC for DA sample. And large size and low density of matrix precipitates (MPTs) decease SCC susceptibility. Al7Cu2Fe particles promote the dissolution of the surrounding matrix, inducing local stress concentration, and thus leading to the generation of stress corrosion cracks.

Effect of multi-directional hot forging process on the microstructure and mechanical properties of Al–Si based alloy containing high amount of Zn and Cu

Abstract In this study, the hypoeutectic Al–7Si based alloy containing Zn and Cu was severely deformed with multi-directional forging (MDF) process at 200 °C. The samples were placed into an open die allowing the free deformation in only one direction and forged with hydraulic press at cumulative strains of 2.07, 4.14 and 6.21. The microstructural examinations were performed with X-ray diffractometer, optical, scanning and transmission electron microscopies while their strength properties were determined by tensile and hardness tests. MDF led to a refinement in α-Al grains, hard Si, Al7Cu2Fe and Al2Cu particles of crystallization origin. This process also provided a method for formation of secondary precipitates within the alloy's microstructure. It was determined that MDF increased the tensile properties of the alloy but decreased its hardness. On the other hand, this process converted brittle type fracture of homogenized alloy to ductile type. These results were explained in terms of grain boundary strengthening, precipitation hardening, refining of hard particles, and softening in matrix due to dynamic recovery and recrystallization mechanisms.

Microstructure and mechanical properties of thixoforged complex box-type component of 2A12 aluminum alloy

Complex box-type components of 2A12 aluminum alloy were thixoforged successfully, and their microstructures and mechanical properties were studied. Full filling status without surface defects was acquired by thixoforging a trapezoidal semisolid billet at the female die temperature of 420 °C and punch temperature of 340 °C. The maximum yield strength, ultimate tensile strength and elongation of the thixoforged component under optimized T4 treatment condition were 263.4 MPa, 414.2 MPa and 36.0%, respectively. Liquid segregation phenomenon happened at the upper edge of the component due to liquid flow and flow of liquid incorporating solid particles. Liquid fraction at the bottom part of the component was little, therefore severe plastic deformation mechanism of solid particles along with dynamic recrystallization was dominant. Dislocations, dislocation loops and dislocation bands of thixoforged 2A12 product were observed through transmission electron microscope (TEM). Deformation textures (Brass, Goss and S), recrystallization textures (Cube and R) and shear texture (rotated Cube) existed at the bottom region. Dispersoids Al20Cu2Mn3, Al7Cu2Fe particles and intermetallic phase S (Al2CuMg) were speculated in the microstructure of thixoforged 2A12 component with T4 heat treatment. Dislocations strengthening, refinement strengthening and strengthening of the second phase codetermined the final mechanical properties.

Influence of Homogenization Heat Treatment and Cooling Modes on Microstructure of AA7475 Aluminum Alloy Ingot

Microstructure evolution during the homogenization heat treatment of an Al-Zn-Cu-Mg (AA7475, which is typically used for the manufacture of aircraft design) alloy, was investigated using a combination of light microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ingots after different types (one-or two-steps treatments) of temperatures (from 380 to 510 °C) of homogenization and cooling conditions (cooling with an air or quenching to water) were investigated. The results show that the microstructure of ingot presents a typical microstructure with some isolated Al7Cu2Fe particles, which after homogenization almost remains in both the size and morphology. The structure ingot after homogenization below 400 °C contains secondary phases, based on η (MgZn2), S (Al2CuMg) and T (Al2Mg3Zn3) are distributed along the grain boundary. In the T (Al2Mg3Zn3) phase copper dissolves up to 30 wt.%. Then the increase in temperature and the complication of heat treatment of homogenization, which led to the complication of the kinetics of the evolution of inter-dendritic phases, were found. The two-steps homogenization has a better effect than a single homogenization, as its completely dissolution of non-equilibrium phases was established.

Phase transformation and dispersoid evolution for Al-Zn-Mg-Cu alloy containing Sn during homogenisation

Optical microscope (OM), field emission scanning electron microscope (FESEM), energy dispersive X-ray Spectroscopy (EDS), transmission electron microscope (TEM) differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were employed to investigate the evolution of intermetallic phases during homogenisation at 465°C for 0–48h for Al-Zn-Mg-Cu-Sn alloy. Evidentially, casting is accompanied by severe dendritic segregation. The primary phases appeared during casting consisted of α (Al), eutectic mixture of η (Mg (Zn, Cu, Al)2), θ (Al2Cu) and coarse Al7Cu2Fe particles. After 6h of homogenisation, two eutectic phases namely S (Al2CuMg) and Cu3Sn were formed. The appearance of dendrites and their consequent dissolution with the progress of homogenisation time was modelled using a kinetic equation taking interdendritic spacing and temperature as two variants. TEM micrographs revealed the presence of high density of fine dispersoids after 6h of homogenisation following almost complete dissolution of the dendrites into the matrix after 48h of homogenisation. Coarsening of the said dispersoids was observed with further homogenisation treatment following Ostwald ripening mechanism and consequently lowering of Zener pinning pressure (ZPP). Crystallographic texture analysis revealed the formation of strong γ fibre in both cast and homogenised samples. However, strong α fibre along with Goss, Brass, P, and Copper components was also noticed in the cast sample.

Effect of Copper on the Localized Corrosion and Stress Corrosion Resistance of High Strength Aluminum Alloys

High strength to weight ratio of the 7xxx aluminum alloys offer an exciting alternative to advanced high strength steels in automotive structural applications. Recently, Novelis has developed AdvanzTM 7000 series alloys that not only has higher strength to weight ratio but also improved localized corrosion resistance compared to conventional 7075. In this work, we have studied the effect of copper content on the stress corrosion cracking resistance of the AdvanzTM 7000 in peak-age (T6) conditions. ASTM G110 test showed pitting corrosion morphology in both low and high copper alloys compared to intergranular corrosion in 7075. Electrochemical polarization showed that increase in copper content increased the cathodic activity and pushed the free corrosion potential in noble direction. Slow strain rate tests were carried out according to ASTM G129 in ASTM D1384 solution containing hydrogen peroxide and in silicone oil. The fractured samples were subjected to scanning electron microscopy (SEM) and metallography cross section. These results indicated that the SCC cracks were initiated from the pitting corrosion sites in AdvanzTM 7000, while the intergranular corrosion site acted as crack nucleation site for 7075. The failure mechanism is discussed in terms of shift in corrosion morphology and the role of Al7Cu2Fe particles that had increased with the copper content of the alloys.

Relationship of particle stimulated nucleation, recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys

The variations of coarse intermetallic particles in hot-extruded 7055 aluminum alloys with 0.041 wt% Fe and 0.024 wt% Si increasing to 0.272 wt% Fe and 0.134 wt% Si were investigated. The particle stimulated nucleation (PSN) behaviors for different kind of coarse particles were detailly analyzed by EBSD. Moreover, the effect of PSN responding to Fe and Si contents on recrystallization and tensile properties of 7055 alloys was evaluated. With increasing Fe and Si contents, the size and number density of coarse η/S particles are reduced, while the number densities of coarse Al7Cu2Fe and Mg2Si particles are both increased and the coarse Al7Cu2Fe particles transform from rod-like to irregular. More PSN recrystallized grains with predominant orientations deviated from the extruded fiber textures are stimulated by the irregular Al7Cu2Fe and Mg2Si particles, because a higher degree of local non-uniform deformation is produced. The rod-like Al7Cu2Fe particles cause the greatest degree of local non-uniform deformation owing to the largest aspect ratio, but the shape also restricts the area of particle deformation zone (PDZ) resulting in fewer PSN recrystallized grains. The irregular η/S particles give rise to the lowest degree of local non-uniform deformation and fewest PSN recrystallized grains with the major orientations close to the extruded fiber textures. Consequently, despite the number and size of coarse η/S particles are reduced, the proportion of high angle grain boundaries (HAGBs) is increased and the extruded fiber textures are weakened with Fe and Si contents increasing, because of the increased Al7Cu2Fe and Mg2Si particles. The strength is slightly declined by the weakened <111>//ED (extrusion direction) fiber texture, while the elongation is reduced for a larger number of coarse particles and more HAGBs with higher Fe and Si contents.

Localized corrosion behavior associated with Al7Cu2Fe intermetallic in Al-Zn-Mg-Cu-Zr alloy

The localized corrosion behavior of Al-7.82Zn-1.99Mg-2.41Cu-0.12Zr alloy was investigated. At the early stage of corrosion, the sample surface only suffers slight local pitting corrosion. While as the immersion time extends, intergranular corrosion occurs, accompanied with surface exfoliation. At the initial stage of pitting corrosion, Al and Mg elements in the matrix around Al7Cu2Fe particles are dissolved preferentially, forming Al and Mg deplete zones. Furthermore, the dissolution of Al and Mg elements triggers the selective dissolution of Al element in Al7Cu2Fe, remaining Cu-rich and Fe-rich cluster. Atomic force microscopy (AFM) results show that this Cu-rich and Fe-rich cluster has higher potential difference relative to the matrix than the uncorroded Al7Cu2Fe particle.

Structures and tensile properties of Sc-containing 1445 Al-Li alloy sheet

The strength and precipitates after T8-aging (second-step aging at 170 °C following first-step aging at 150 °C for 4 h after 6% predeformation) and grain structures of 1445 Al-Li alloy (Al-1.7Li-1.5Cu-0.9Mg-0.1Mn-0.1Zr-0.1Ag-0.1Ti-0.08Sc-0.07Ni) cold-rolled sheet were investigated. The main precipitates are δ′ (Al3Li) and S′ (Al2CuMg). It is critical that nano-sized Al3(Sc,Zr) dispersoids are formed, and the Cu-enriched and Sc-containing W phases (AlCuSc) are not found. Due to the strong pinning effect of the nano-sized Al3(Sc,Zr) dispersoids on sub-grain boundaries and dislocations, the solutionized 1445 Al-Li alloy sheet displays non-recrystallization characteristics, which corresponds to a high volume fraction of deformation textures of Copper {112}<111>, Brass {011}<211> and S {123}<634>. This non-recrystallization feature makes the 1445 Al-Li alloy suitable for thin sheet. In addition, due to the addition of minor Ni, Al9FeNi particles instead of Al7Cu2Fe particles are formed.

The effects of aging treatments on mechanical property and corrosion behavior of spray formed 7055 aluminium alloy

The relationship among microstructures, mechanical properties and corrosion behaviors of spray formed 7055 aluminium alloy has been investigated upon peak-aging (T6), double-aging (DA) and retrogression and re-aging (RRA). The research is estimated by hardness test, tensile test, immersion test, detailed microstructure observation and potentiodynamic polarization under different aging treatments. Results demonstrate that the corrosion resistance rank of aging treatments is as follow: DA > RRA > T6. The continuous grains boundary precipitations (GBPs) and coarse Al7Cu2Fe particles increase the corrosion susceptibility. The appearance of the pit cavity attributes to coarse grains and the considerable potential difference between the matrix and GBPs or precipitate-free zones (PFZs) during intergranular corrosion (IGC) process. The wedging stress from hydrogen cracks and the accumulation of the corrosion products help induce the exfoliation corrosion (EXCO). Anodic dissolution and hydrogen embrittlement cause stress corrosion cracking (SCC) of the alloy under different aging treatments. The strengthening mechanism attributes to the distribution of matrix phases (MPs), η′ phase and Al3Zr particles.

Effect of Cu Content and Aging Conditions on Pitting Corrosion Damage of 7xxx Series Aluminum Alloys

The corrosion behavior and microstructure of AA7055 alloy with a high Cu content, and a Cu-free Al-Zn-Mg alloy in various aging conditions were studied. Polarization measurements and ASTM B117 salt-spray exposure were used to assess the corrosion characteristics. Transmission electron microscope and atom probe tomography were used to evaluate the microstructure and the composition of precipitates. The matrix composition of AA7055 changed with aging because of precipitation. Thus, the galvanic relationship between Al7Cu2Fe particles and the surrounding matrix correspondingly changed, which affected the susceptibility to pitting corrosion attack. The susceptibility to pitting corrosion damage of AA7055 was found to decrease according to: over-aged samples > under-aged samples > peak-aged sample. However, aging had only a small influence on pitting corrosion damage for the Cu-free Al-Zn-Mg alloy. Furthermore, despite the more active breakdown potential of the Cu-free Al-Zn-Mg alloy, this alloy exhibited less severe pitting corrosion damage than AA7055, in particular for the over-aged samples. This was attributed to the weaker galvanic coupling between Al3Fe particles and the surrounding matrix. Although Cu in the intermetallic particles is detrimental to pitting resistance, the galvanic effect can be reduced by increasing the matrix Cu content.