GPI Zones Research Articles

Investigation on precipitate behavior and mechanical properties of Al-Zn-Mg-Cu alloys with various Zn/Mg ratios

The effect of Zn/Mg ratio on the microstructure and properties of Al-Zn-Mg-Cu alloys under single-step and double-step aging processes were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), as well as hardness, room temperature tensile, and fracture toughness testing. During single-step aging, as the Zn/Mg ratio increases, the aging response of the alloys accelerates. However, the peak hardness and strength gradually decreased while the average size of precipitates increased. During double-step aging, as the secondary aging time increases, the strength and hardness of the alloys gradually decrease. Additionally, the higher the Zn/Mg ratio, the faster the strength and hardness decline. Under the T74 temper, the alloy with higher Zn/Mg ratios exhibits lower hardness and strength but better fracture toughness and larger precipitate sizes. A simple method was proposed to statistically determine the critical diameter size of GPII zone and η′ phase, which was approximately 4.47 nm. A strength model was developed to predict the yield strength of alloys, considering factors such as solid solution strengthening, grain boundary strengthening, and precipitation strengthening. The model demonstrates good predictive accuracy for both peak-aged and T74 temper alloys, providing valuable guidance for alloy design and microstructure regulation.

Unravelling precipitation behavior and mechanical properties of Al–Zn–Mg–Cu alloy

We investigate the effect of aging temperature on precipitation behavior and mechanical properties of an Al–7.6Zn–2.7Mg–2.0Cu–0.1Zr–0.07Ti (wt.%) alloy by evaluating the matrix's microhardness, electrical resistivity, and tensile properties: additionally, employing X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atom-probe tomography (APT) to characterize this alloy. The nanoprecipitates forming under peak-aging conditions vary with aging temperature, forming coherent GPI zones at 80 °C, GPII zones with minor η' at 120–150 °C, and η'/η with minor GP zones at 180–220 °C. GPI and GPII zones forming at 80–150 °C contain similar concentrations of solute atoms (11Zn–9Mg–(<1.0)Cu (at.%)), whereas the η'/η nanoprecipitates forming at 180 °C contain larger concentrations of solute atoms (28Zn–24Mg–3.4Cu (at.%)). The strength of the peak-aged alloy decreases with increasing aging temperature owing to the increasing size and decreasing number density of the nanoprecipitates. Under peak-aging conditions, precipitation strengthening originates mainly from dislocation shearing at 80–150 °C and from Orowan bypassing at temperatures above 180 °C. The shearable to non-shearable transition of the nanoprecipitates at 180 °C reduces the strain hardening rate, thereby decreasing the alloy's ductility.

Effects of Mg/Zn ratio and pre-aging on microstructure and mechanical properties of Al–Mg–Zn–Cu alloys

The alloy composition and aging process are significant factors determining the type of precipitates in Al alloys. In this work, the effects of Mg/Zn ratio and pre-aging process on the microstructure and mechanical properties of Al–Mg–Zn–Cu alloys were systematically investigated using transmission electron microscopy (TEM), hardness measurements and tensile tests. The results show that reducing the Mg/Zn ratio significantly increases the density of T′-Mg32(AlZnCu)49 precipitates, thereby enhancing and accelerating the aging response. A detailed comparison has been made between Al–Mg–Zn–Cu alloys aged by single-step and two-step artificial aging processes. With the same composition, the alloy exhibits a higher yield strength under the two-stage aging process, resulting from the higher precipitation strengthening effect (Orowan dislocation bypassing mechanism) due to the smaller size and higher density of precipitates. Furthermore, it is particularly interesting that there may be two evolution paths for GPII zones with (111)Al habit plane: one is that the GPII zones are dissolved into the matrix, and the other is that the GPII zones gradually evolve into the T′ precipitates. This work sheds some light on the precipitation sequence in Al–Mg–Zn–Cu alloys and is greatly helpful for designing age-hardening Al–Mg based alloys.

Microstructure evolution and strengthening mechanisms of high strength Al-Zn-Mg-Cu alloy via pre-hardening forming

The precipitating and strengthening behavior of the pre-hardening forming (PHF) process for a high-strength Al-Zn-Mg-Cu alloy is studied. The PHF process obviates the need for artificial aging (120 °C for 24 h) during hot forming. Despite the considerably lower volume fraction of η' (with a maximum of 0.54 %) in the PHF samples compared to that of the T6 samples, the maximum yield strength of the 12 h-5 min tensile specimen is 496 MPa, similar to that of the T6 alloy (490 MPa). It is assumed that the GPII zones have compensated for the insufficient yield strength in PHF samples. Through the integration of small-angle X-ray scattering (SAXS) experiments and the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equations, the strengthening effects attributable to GPII zones and η' have been discerned. Intriguingly, the contribution of GPII zones to yield strength (219 MPa in the 12 h-5 min sample) equals that of η' (167 MPa in the 12 h-5 min sample). A comprehensive yield strength model is subsequently established, including both GPII zones and η' phase, exhibiting a deviation between fitting and experimental results of less than 5 %.

Mechanism for enhanced precipitation strengthening due to the addition of copper to Al-Zn-Mg alloys with high Zn/Mg ratio

The mechanism for enhanced precipitation strengthening due to the addition of copper to Al-9.0Zn-1.5Mg (wt.%) alloys has been investigated by means of tensile test, transmission electron microscopy, anomalous small-angle X-ray scattering and three-dimensional atom probe. With the addition of copper increasing from 0% to 1.0%, and then 2.6%, the strength increment of peak-aged alloys due to precipitation strengthening increases from 274.8 MPa to 329.3 MPa, and then 343.0 MPa. Based on the copper-induced change of the parameters of clusters, GPI zones and η′ precipitates coexisting in the aged alloys, the enhanced precipitation strengthening has been discussed in detail by considering their coherent, modulus and short-order strengthening. In the alloy without copper, the coherent strengthening from η′ precipitates play a main role in precipitation strengthening. In copper-containing alloys, the contribution of clusters to strengthening become significant. The addition of copper decreases coherent strengthening and modulus strengthening from η′ precipitates, but increases modulus strengthening from GPI zones and clusters, and short-range order strengthening from clusters more apparently, and consequently enhances strength of the alloys after aging.

Origin and effect of anisotropy in creep aging behavior of Al–Cu–Li alloy

The precise control of forming in the new generation Al–Li alloy is challenged by the anisotropic creep aging behaviors. This anisotropy is influenced by coupling effect the microstructure and process parameters during creep age forming, where process parameters such as creep aging temperature and stress play a crucial role. Taking the 2195 Al–Cu–Li alloy as an example material, the study aimed to investigate the effect of creep aging temperatures (140–200 °C) and stresses (150–200 MPa) on the anisotropic creep aging behaviors. The findings revealed that with increasing temperature and stress, the in planar anisotropy (IPA) value of creep deformation decreased from 19.1% to 9.4%, the IPA value of strength decreased from 10.3% to 5.1%. The study uncovered the anisotropy of creep aging and its mitigation mechanisms. The analysis revealed the crucial role of the initial texture and precipitation. By regulating the type and size of precipitates, the process parameters can mitigate anisotropic creep aging behavior. The stress-oriented precipitation of GPI (θ″) zones intensifies creep aging anisotropy, while the homogeneous precipitation of T1 phase hinders creep aging anisotropy, the smaller and more uniform the T1 phase, the stronger the hindering effect. The coarsened T1 phase have a relatively small effect on creep aging anisotropy. In conclusion, the optimal combination of low deformation anisotropy (9.4%) and excellent yield strength (527 MPa) can be achieved when the creep aging temperature is set at 170 °C under 200 MPa.

Effect of thermal strain on the microstructure evolution and post-aging mechanical properties of Al-Zn-Mg-Cu alloy in simulating hot stamping process

To identify the effect of thermal strain on the microstructure evolution and mechanical properties of Al-Zn-Mg-Cu alloys in simulating hot stamping process, the alloys were strained at elevated temperature and then artificially aged at 120 °C for different time. The post-aging mechanical properties of the alloys were examined, and the microstructures after aging were systematically characterized via using electron backscattered diffraction (EBSD), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Especially, the effect of dislocation on precipitation behavior and its mechanism on properties were analyzed in detail. The results show that an 8.96% rise of ultimate tensile stress (UTS) can be found in solid solution specimens and the peak-aging time shortens by 50% with the increase of thermal strain from 0% to 30%. The deformed grains are slightly refined from 12.62 μm to 10.99 μm when the strain increases to 30%, while aging time has no obvious effect on the grain size and orientation. Experiments demonstrate that dislocations definitely exist after thermal strain, and will not be completely devoid because of recovery. To a certain extent, dislocations can be used as the favorable nucleation core for the precipitates, promote the rapid formation of strengthening phases, and finally result in a shorter peak-aging time with mixed intragranular precipitates of a large amount of η' and a small amount of GPII zones.

Effects of pre-stretching and baking treatment on microstructural evolution and mechanical properties of 7A09 aluminum alloy subjected to pre-hardening forming

In this study, a novel pre-hardening forming process is applied to 7A09 aluminum alloy. Then, the effects of pre-stretching and baking treatment on mechanical properties and microstructural evolution are investigated. Uniaxial tensile tests are conducted to evaluate the evolution law of the mechanical properties of alloys treated using different parameters. Microstructural evolution is revealed by transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and small-angle X-ray scattering (SAXS). Results show that the yield strength (YS) of the alloys pre-aged at 110 ℃ for 12 h (PA) reaches 612–694 MPa after they are pre-stretched by 2–6 %. This strength is 26–43 % higher than that of T6-temper alloy according to the ASTM B221M-21 standard. The elongation of PA alloys with 6 % deformation increases from 2.7 % to 10 % after baking at 215 °C for 1 min. Hence, the subsequent baking treatment compensates for the loss in ductility. The TEM result shows that numerous GPII zones and few η΄ phases co-exist in the matrix of the PA alloys. Simultaneously, η΄ phases serve as the main strengthening phases after pre-stretching the PA alloys by 6 %. Moreover, pre-stretching can increase the density of dislocations and the volume fraction of precipitates. In addition, the average size of precipitates has increased from 6.4 nm to 10 nm with the increase of baking time from 1 min to 10 min, and the transformation of η΄ phases to coarse η phases has appeared during this process. All results indicate that the strength–ductility of the foregoing alloys is better than that of T6-temper alloy.

Enhanced intragranular dislocation capture ability in nano/ultrafine Al-Zn-Mg-Cu alloy by burnishing-assisted two-stage aging treatment-induced dynamic nucleation

The effects of burnishing-assisted two-stage aging (BATSA), including pre-aging, burnishing, and subsequent aging (SA) on the microstructural evolution, precipitation, and hardening behavior of Al-Zn-Mg-Cu alloy were systematically investigated. Transmission electron microscopy, X-ray diffraction, small-angle X-ray scattering, and Vickers microhardness tests were used to analyze the above-mentioned characteristics of the burnished-layer alloys and the matrix-layer alloys, respectively. The BATSA treatment promotes a concentrated distribution of precipitate sizes, delaying phase coalescence and maintaining a relatively dispersed precipitate distribution. Moreover, the persistent nucleation of Guinier-Preston (GP) zones in burnished alloys was observed during SA. These results are conducive to the enhancement of the dislocation storage capacity of nano/ultra fine-grained alloys (the decrease in dislocation density of pre-aged burnished alloy was about 65% that of non-pre-aged burnished alloy). Based on this, dynamic nucleation induced by pre-aging and burnishing resulted in a better synergy of multiple strengthening mechanisms, significantly shortening the time to peak hardness in pre-aged burnished alloys to about 1/3 that in non-pre-aged matrix alloys, and maintaining hardness stability. Moreover, it was found that the critical width of the in situ phase transformation from GPII zone to η' precipitate can be as low as 3–4 at. planes.

Grain Refinement Enhanced Tensile Strength, Fracture Toughness and Fatigue Crack Propagation Resistance of a High Zn-Containing Al-Zn-Mg-Cu Alloy

In present work, a high Zn-containing Al-Zn-Mg-Cu alloy with different grain sizes was fabricated by extrusion and related precipitation characteristics and mechanical property were investigated after uniform heat treatments. The results showed that precipitation characteristics for the three alloys were almost the same. Matrix precipitates were GPII zone and η' phase and possessed small size and dense distribution while grain boundary precipitates exhibited discontinuous distribution. The rank of strength and fracture toughness for the three alloys are SG&gt;MG&gt;LG. Tearing ridges had been found on all the fracture surface while only LG alloy possess obvious dimple characteristics. The a-N curve showed that crack length list is MG&gt;LG &gt;SG under a same cycle number. The da/dN-ΔK curve also proved that fatigue crack propagation (FCP) rate of MG alloy is slightly larger than that of LG alloy, both were apparently larger than that of SG alloy. The width of fatigue striations on FCP fracture surface also backed it. Besides, obvious transgranular cracking characteristics and apparent secondary cracks were found on the FCP fracture surface.

Effect of variable rate non-isothermal aging on microstructure and properties of Al-Zn-Mg-Cu alloy

Variable rate non-isothermal aging (VR-NIA) treatment by changing the heating and cooling rates is proposed and applied to 7055 aluminum alloy to optimize the mechanical properties and corrosion resistance. The investigated 7055 aluminum alloy undergone two-stage solid solution is heated from 100 °C to 160 °C at a rate of 20 °C/h, then heated to 210 °C at a rate of 60 °C/h, and finally cooled to 100 °C according to the symmetrical temperature route, the hardness and tensile strength can reach 173.3 HV and 654.6 MPa, respectively, and the electrical conductivity is able to reach 38.6 %IACS. At the same time, improved corrosion resistance is obtained, the maximum depth of intergranular corrosion (IGC) is 29.4 μm, the 48 h exfoliation corrosion (EXCO) grade is EA, and the stress corrosion sensitivity index (ISSRT) is 1.8%. The results of electrochemical tests show that the open circuit potential (OCP) of VR-NIA alloy is −764.2 mV, the corrosion potential (Ecorr) is −715.3 mV, and the corrosion current density (Icorr) is 4.6 × 10−8 A/cm2. Microstructure observation shows that GPI zones, GPII zones and a small number of η’ phases are formed during the heating process of 100 → 160 °C, which enhances the mechanical properties of the alloy; in the process of 160 → 210 → 160 °C, small-sized precipitates dissolve back, and part of the large-sized η’ phases grow and transform into η phases. At the same time, the grain boundary precipitates (GBPs) change from continuous to discontinuous distribution, forming a precipitation-free zone (PFZ) with a suitable width, which greatly improves the electrical conductivity and corrosion resistance of the alloy. In the process of 160 → 100 °C, the GPI zones are re-precipitated, and more η phases are formed from the η’ phases without remarkable coarsening. Enhanced performance can be achieved in a relatively shorter time through VR-NIA, behaving with greater advantage over constant-rate non-isothermal aging.

The evolution of precipitates in an Al–Zn–Mg alloy

The precipitation sequence in Al–Zn–Mg alloys has been subject to many revisions over the years as more structural details of the early-stage Guinier-Preston (GP) zones and precipitates have been uncovered. To further investigate this, a 7003 aluminium alloy naturally aged for one year was subjected to artificial ageing at 140 °C to investigate the evolution of early-stage precipitates. Scanning transmission electron microscopy was coupled with atom probe tomography and hardness measurements to characterise the precipitate structure and chemistry of the alloy at different stages in the heat treatment. The naturally aged condition contained a dense population of GPI zones, co-existing with a smaller distribution of η′ precipitates. After artificial ageing for 10 min, the hardness decreased from 121 HV to 88 HV. The number densities of clusters were similar in the two conditions, while the average size of the clusters had increased during the first 10 min of artificial ageing. The average Zn/Mg ratio of the clusters decreased from 2.0 to 1.7 and the clusters no longer exhibited the GPI zone atomic structure, indicating that the GPI zones had dissolved. Concurrently, the fraction of precipitates having η′ structure increased from 2% to 18%. During further ageing, the hardness increased again, reaching a peak after 5 h. In this condition, η′ was the dominant phase, co-existing with η1, η2 and T′ phases. Atomically resolved scanning transmission electron microscopy images in addition to precession electron diffraction patterns of the disputed T′ phase are presented and compared to literature.

Enhanced mechanical properties and corrosion resistance of 7055 aluminum alloy through variable-rate non-isothermal aging

The effect of variable-rate non-isothermal aging (VR-NIA) on the properties of 7055 aluminum alloy was investigated by hardness, electrical conductivity, tensile tests and immersion corrosion, stress corrosion and electrochemical corrosion experiments. A four-stage VR-NIA process is proposed and applied to 7055 alloy, and a good combination of mechanical properties and corrosion resistance can be obtained in a relatively short time. TEM and HRTEM images show that GPⅠ zones, GPⅡ zones and η' phases are formed during the heating process of 100→180 °C, which enhances the mechanical properties; in the process of 180→210→180 °C, GPⅠ zones, part of the small-sized GPⅡ zones and η' phases are redissolved into the matrix, while part of η' phases grows. Meanwhile, the grain boundary precipitates (GBPs) change from continuous to discontinuous, forming the proper width of the precipitate free zone (PFZ), which greatly improves the electrical conductivity and corrosion resistance. In the cooling process of 180→100 °C, "secondary precipitation" of the GPⅠ zones occurs, and the η' phases grow up, with a small part transforming into the η phases, but no remarkable coarsening occurs. During the process of VR-NIA, the content of Zn, Mg, and Cu elements in GBPs is increasing continuously.

Early-stage clustering and precipitation behavior in the age-hardened Al–Mg–Zn(-Cu) alloys

T-Mg32(AlZnCu)49 phase-strengthened Al–Mg–Zn–Cu alloys have shown a promising age-hardening response. However, the age-hardening mechanism underlying the different heat treatments is not clear during the early-stage precipitation. Additionally, the lack of information on the early-stage precipitation impedes the development of this series alloys. In this study, the formation of the Guinier-Preston (GP) zone and T′-phase of the novel Al–Mg–Zn(-Cu) alloys during the early-stage precipitation has been investigated using transmission electron microscopy and atom probe tomography analysis. Investigation revealed that the Cu content and thermal treatment had significant effects on the microstructure evolution of the alloys. GP zones were observed to act as sites for the formation of the T′-phase. The higher number density of Cu-incorporated GP zones led to higher strength and bake-hardening response after pre-aging. Occasionally, some GPI zones were observed to dissolve into the Al matrix without further transforming into T′ precipitates during natural aging (NA). Pre-aging prior to NA facilitated stable GP zone formation and mitigated the negative effect of NA. Furthermore, the precipitates were grown to an appreciable size after single-step aging at 180 °C, which adversely affected the mechanical properties of the novel alloys. The present findings provide an efficient guideline for designing the T-phase in the crossover Al–Mg–Zn-(Cu) alloys.

Effect of cyclic ageing on the early-stage clustering in Al–Zn–Mg(-Cu) alloys

The cyclic application of mechanical stress to aluminum alloys at room temperature, referred to as cyclic ageing, continuously injects vacancies, enabling dynamic precipitation of a fine distribution of solute clusters. These are uniformly distributed throughout the bulk and are responsible for enhancing the mechanical properties to comparable values as T6 conditions obtained by conventional artificial aging. The atomic structure of the clusters in two Al–Zn–Mg (-Cu) alloys is studied using atomically resolved transmission electron microscopy, and their size, volume density and chemistry are investigated using atom probe tomography. It was found that cyclic ageing yields a high number density of solute clusters exhibiting structural similarities to the GPI zones commonly observed after low temperature ageing in such alloys. A subsequent 10-day natural ageing allows both cluster nucleation and further cluster growth, without altering their atomic structure. As compared to natural ageing, the strengthening caused by the solute clusters is accelerated during cyclic ageing, due to dynamic precipitation. The copper containing alloy also had higher amounts of Mg and Zn. Still, a slightly lower number density of smaller clusters with lower Zn/Mg ratio formed in this alloy, both after cyclic aging and after natural aging. The higher strength of this alloy is attributed to a higher strength contribution from each cluster.

Atomic-scale study on the precipitation behavior of an Al–Zn–Mg–Cu alloy during isochronal aging

• The phase evolution of an Al−Zn−Mg−Cu alloy during isochronal aging at three different heating rates—3, 10 and 30 K/min is revealed. • The characteristic DSC peaks related to the formation of GPI and GPII zones have been identified. • A novel type of metastable phase η 1 ’ as the precursor of η 1 is identified. A pathway for the formation of η 1 via η 1 ’ is demonstrated. It is found that the precipitation kinetics of η’ cannot be evaluated solely by one characteristic DSC peak, because η’, η 1 and η 1 ’ contribute to the same DSC peak. • The role of quenched-in dislocation in promoting the precipitation of η phase is revealed. • Based on quantitative analysis, the influence of heating rate and ending temperature on the cross section and number density of phases is further discussed. The precipitation behavior of a 7075 Al alloy during isochronal heat treatments at three different heating rates has been studied using differential scanning calorimetry, high-angle-annular-dark-field scanning-transmission-electron microscope and density functional theory calculation. In the early stage of aging, GPI and GPII zones form sequentially and cause two characteristic DSC peaks. Subsequently, the formation of η 1 precipitates takes place concurrently with η’. A novel type of metastable phase η 1 ’ is identified as the precursor of η 1 , which can lower the lattice misfit between η 1 and Al matrix along the direction of [10 1 ¯ 0] η1 // [001] Al . Accordingly, a pathway for the formation of η 1 via η 1 ’ is demonstrated. Precipitates η’ together with η 1 and η 1 ’ contribute to the third DSC peak. With the further increase of temperature, η precipitates become prevailing. Based on the quantitative analyses, the influence of the heating rate and ending temperature on the cross section and number density of phases formed is discussed.

Nano-Treating Promoted Natural Aging Al-Zn-Mg-Cu Alloys

Natural aging reduces the cost of alloy manufacturing while saving input energy but takes too long to complete for most Al-Zn-Mg-Cu alloys. Research has proved that nano-treating can facilitate precipitation in heat-treatable alloys. In this study, nano-treated Al-6.0Zn-2.6Mg-xCu samples containing different Cu contents were fabricated to investigate the influence of nano-treating on natural aging. TiC nanoparticles were used for nano-treating. Three cooling conditions after solution treatment (water quenching, air cooling, and as-cast) were investigated to check their quench sensitivities. The study shows the alloy’s microstructure was modified by nano-treating, and the growth of dendritic arms was inhibited. Compared to the control samples, nano-treating also increased both the microhardness and tensile strength of the alloy after natural aging. Out of the three different solution treatments, the air-cooled samples presented the highest UTS and microhardness values. The precipitation process was sped up by nano-treating by approximately 50%, and a higher volume fraction of GPII zones were formed in the nano-treated samples. HRTEM results also confirm the formation of more GPI and GPII zones in a nano-treated samples. With the help of natural aging, the Al-6.0Zn-2.6Mg-0.5Cu alloy reached a UTS of 455.7 ± 40.2 MPa and elongation of 4.52 ± 1.34% which makes it a great candidate for a naturally aged Al-Zn-Mg-Cu alloy.

Studies on softening behavior and mechanism of heat-affected zone of spray formed 7055 aluminum alloy under TIG welding

In this investigation, fine micro-zone specimens in the heat-affected zone (HAZ) of TIG welded spray formed 7055 aluminum alloy were obtained by heat treatment method. The softening behavior and mechanism of HAZ of this alloy under single pass TIG welding were investigated. Severe non-uniformities of microstructures and mechanical properties were found, according to which the HAZ was divided into the solid solution zone and the over-aging zone. The most heavily softened area of HAZ occurred in the over-aging zone at the position corresponding to the peak thermal cycling temperature of 230–350 °C. The lowest microstructures value was 105 HV, the lowest tensile strength was about 410 MPa. The elongation was slightly higher than that of the base material. The main causes of softening in the over-aging zone were the dissolution of the coherent precipitate η′ and the growth and coarsening of the non-coherent precipitate η.Thermal cycle temperature in the solid solution zone was about 350–536 °C. Mechanical properties in this zone were higher than those of the over-aging zone. After natural aging, mechanical properties recovered to more than 90% of the base material. Microhardness reached 180 HV and tensile strength reached 570 MPa when the thermal cycle temperature was 450 °C. At the same time, the elongation in the solid solution zone was higher than that of the base material. The high mechanical properties were the results of the combined action of the increased ratio of large angle grain boundaries, the increased solid solubility and the formation of coherent GPI zone.

Studying GPI zones in Al-Zn-Mg alloys by 4D-STEM

A new methodology has been developed to study the fine details of GP zones in age-hardenable aluminium alloys. It is complementary to atomic resolution high-angle annular dark-field scanning transmission electron microscopy imaging, and combines scanning precession electron diffraction with diffraction simulations. To evaluate the method, data was collected from an Al-Zn-Mg alloy in a condition with a dense distribution of GPI zones. Diffraction patterns were recorded in the 〈001〉Al orientation, capturing GPI zones in three projections: along the unique [001]GPI axis, and along the two other mutually orthogonal orientations. The GPI zones viewed along [001]GPI revealed how the truncated octahedron units of the GPI zones were connected in multi-unit GP zones, while the two orientations normal to [001]GPI highlight the internal structure. The stability of the atomic models developed based on the experimental results was verified by density functional theory calculations.

Influence of laser pre-precipitation on the corrosion resistance of Al-Zn-Mg-Cu alloy with deep cryogenic treatment

A recently developed AA7075 was subjected to solid solution, high temperature laser surface treatment, deep cryogenic treatment, and artificial aging. The microstructure evolution and precipitation behavior were examined, and their effects on corrosion behaviour were analyzed. The results showed that coarse η phases disappeared and a large number of smaller η phases were formed after high temperature laser surface treatment. Most of the phases dissolved into Al matrix during solution and fine η phases precipitated in laser treatment. η′ phases appeared in the high temperature laser surface treatment samples. Mg element precipitated in deep cryogenic treatment. It was concluded that the precipitation of AA7075 during heat treatment followed the sequence of solid solution → GPI (Guinier Preston) zones → metastable η′ → stable η. Reasonable laser scanning power owned the best corrosion resistance. The reasonable sample which the laser power was 1000 W showed much better corrosion resistance due to the coarsening and separation of grain boundary precipitations.