Evolution of mechanical properties,localized corrosion resistance and microstructure of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging

The evolution of mechanical properties,localized corrosion resistance of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging(NIA)was investigated by hardness test,electrical conductivity test,tensile test,intergranular corrosion test,exfoliation corrosion test,slow strain rate tensile test and electrochemical test,and the mechanism has been discussed based on microstructure examination by optical microscopy,electron back scattered diffraction,scanning electron microscopy and scanning transmission electron microscopy.The NIA treatment includes a heating stage from 40℃to 180℃with a rate of 20℃/h and a cooling stage from 180℃to 40℃with a rate of 10℃/h.The results show that the hardness and strength increase rapidly during the heating stage of NIA since the increasing temperature favors the nucleation and the growth of strengthening precipitates and promotes the transformation of Guinier-Preston(GPI)zones toη'phase.During the cooling stage,the sizes ofη'phase increase with a little change in the number density,leading to a further slight increase of the hardness and strength.As NIA proceeds,the corroded morphology in the alloy changes from a layering feature to a wavy feature,the maximum corrosion depth decreases,and the reason has been analyzed based on the microstructural and microchemical feature of precipitates at grain boundaries and subgrain boundaries.

Influence of quench-induced precipitation on aging behavior of Al-Zn-Mg-Cu alloy

The effects of quenching and aging （T6, T7 and RRA） on the microstructural evolution of an A1-Zn-Mg-Cu alloy were investigated by hardness test, optical microscopy （OM）, transmission electron microscopy （TEM） and differential scanning calorimetry （DSC） measurements. It is found that the hardness of T6 aged sample after water-quenching is the highest. The quench sensitivities of T7 and RRA are almost the same, which are 1.2% higher than that of T6. TEM observation shows that the quench sensitivity for the studied alloy is mainly caused by heterogeneous precipitation during slow quenching. Many r/phases precipitate on A13Zr dispersoids inside recrystallized grains and at （sub） grain boundaries, while T and S phases form in the substructure with high density of dislocations and defects. After aging, the η＇ precipitates are coarser in the vicinity of equilibrium r/phase. However, the size and morphology of the precipitates show different characteristics among T6, T7 and RRA treatments. The DSC results are highly consistent with the TEM observation. The DSC curves of T6 aged samples are different from those of T7 and RRA aged samples, which also reflects the differences on the microstructure.

Precipitation behavior and properties of a new high strength Al-Zn-Mg-Cu alloy

The microstructure and the associated hardness, strength and electrical conductivity of a new Al-Zn-Mg-Cu alloy during one-step ageing treatment were systematically studied. The results show that the electrical conductivity of the alloy increased continuously with increasing ageing temperature and ageing time. At the early stage of ageing, the hardness and strength of the alloy increased rapidly and then reached the peak value. When aged at 120 °C, the hardness and strength maintained at high level for a long time after the peak value. The main precipitations are GPI zones, GPII zones and metastable η′ phase. GPI and GPII zones are found in the alloy after ageing for 24 h at 120 °C, which indicates that some stable GP zones can exist through the ageing process. When aged at 160 °C, the hardness and strength decreased rapidly after the peak value. The precipitation process is significantly promoted compared with that aged at 120 °C. Both GPI zones and GPII zones disappeared after ageing for 1 h at 160 °C. The main precipitates are η′ phase when aged at 160 °C for 1 h. The main precipitates are η phase when the ageing time prolongs to 24 h.

Corrosion and electrochemical behaviors of 7A09 Al-Zn-Mg-Cu alloy in chloride aqueous solution

The corrosion and electrochemical behaviors of 7A09 Al？Zn？Mg？Cu alloy were investigated in 3.5% NaCl （mass fraction） solution using complementary techniques such as scanning electron microscopy （SEM）, metallographic microscopy and electrochemical measurements. The results show that both pitting corrosion from or around the intermetallic particles and intergranular corrosion are observed after the immersion test due to the inhomogeneous nature of the microstructure of the 7A09 alloy. The preferential dissolution of the anodic Cu-depleted zone along grain boundaries is believed to be the possible cause of intergranular corrosion. The passivation and depassivation of this alloy show significant dependence of immersion time, owing to the formation and dissolution of various passive films on the sample surfaces. Furthermore, the corrosion process and corrosion mechanism were also analyzed.

Low cycle fatigue behavior of T4-treated Al-Zn-Mg-Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting(GC) and squeeze casting(SC) T4-treated Al-7.0Zn-2.5Mg-2.1Cu alloys were employed to investigate the microstructures,mechanical properties and low cycle fatigue(LCF) behavior.The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing(SDAS).Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy.GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes(less than 0.4%) and cyclic hardening at higher total strain amplitudes.The degree of cyclic hardening of SC samples is greater than that of GC samples.Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them,while the crack initiation sites for SC samples are slip bands,eutectic phases and inclusions at or near the free surface.

Microstructure and mechanical properties of high strength Al-Zn-Mg-Cu alloys used for oil drill pipes

Three Al?Zn?Mg?Cu alloys used for oil drill pipes (Alloy A: Al?6.9Zn?2.3Mg?1.7Cu?0.3Mn?0.17Cr; Alloy B: Al?8.0Zn?2.3Mg?2.6Cu?0.2Zr, Alloy C: Al?8.0Zn?2.3Mg?1.8Cu?0.18Zr) were studied by hardness tests, tensile tests and transmission electron microscopy (TEM). The results show that the ultimate tensile strength, yield strength and elongation for Alloys A, B and C are 736 MPa, 695.5 MPa and 7%; 711 MPa, 674 MPa and 12.5%; 740.5 MPa, 707.5 MPa and 13%, respectively after solid solution treatment ((450 °C, 2 h)+(470 °C, 1 h)) followed by aging at 120 °C for 12 h. The dominant strengthening phases in Alloy A are GPII zone andη′ phase, the main precipitate in Alloy B isη′ phase, and the main precipitates in Alloy C are GPI zone, GPII zone andη′ phase, which are the reason for better comprehensive properties of Alloy C. The increase of zinc content leads to the improvement of the strength. The increase of copper content improves the elongation but slightly decreases the strength. Large second-phase particles formed by the increase in the manganese content induce a decrease in the elongation of alloys.

Microstructure evolution of Al-Zn-Mg-Cu alloy during non-linear cooling process

The microstructure evolution and properties of an Al-Zn-Mg-Cu alloy were investigated under different non-linear cooling processes from the solution temperature, combined with in-situ electrical resistivity measurements, selected area diffraction patterns （SADPs）, transmission electron microscopy （TEM）, and tensile tests. The relative resistivity was calculated to characterize the phase transformation of the experimental alloy during different cooling processes. The results show that at high temperatures, the microstructure evolutions change from the directional diffusion of Zn and Mg atoms to the precipitation of S phase, depending on the cooling rate. At medium temperatures, q phase nucleates on A13Zr dispersoids and grain boundaries under fast cooling conditions, while S phase precipitates under the slow cooling conditions. The strength and ductility of the aged alloy suffer a significant deterioration due to the heterogeneous precipitation in medium temperature range. At low temperatures, homogeneously nucleated GP zone, η′ and η phases precipitate.

Microstructure evolution in cooling process of Al-Zn-Mg-Cu alloy and kinetics description

The microstructure evolution of Al-Zn-Mg-Cu alloy was studied by differential scanning calorimetry （DSC） and transmission electron microscopy （TEM） during different rate cooling processes. Based on the DSC results, the kinetics analysis was carried out. The results indicate that the precipitation of η phase is the predominant transformation for the alloy during the cooling process after the solution treatment. And the η phase nucleates on dispersoids and at grain boundaries. The amount of η phase decreases with increasing cooling rate, and reduces by 75% as the cooling rate increases from 5 to 50 ℃/min. The kinetics of the precipitation of η phase can be described by the Kamamoto transformation model when the cooling rate is a constant.

Precipitation sequence of η phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy during artificial aging

The precipitation sequence of η（MgZn2） phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 ℃ for various times from 5 min to 6 h. High resolution transmission electron microscopy （HRTEM） observations and energy dispersive X-ray spectroscopy （EDX） analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution （SSS）→vacancy-rich clusters （VRC）→GP Ⅱ zones→η＇→η. Based on the theory of non-equilibrium grain boundary segregation （NGS） and non-equilibrium grain boundary co-segregation （NGCS）, the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP Ⅱ zones, η＇ phase as well as η phase.

Microstructure of spray formed Al-Zn-Mg-Cu alloy with Mn addition

With the aim to improve the strength of Al-Zn-Mg-Cu alloy,the alloy billet containing Mn was produced by spray forming method,and the microstructural features were investigated using X-ray diffraction（XRD）,optical microscopy（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM） and differential scanning calorimetry（DSC）.The results show that the billet mainly consists of fine equiaxial grains of MgZn2 and Al6Mn with size ranging from 5 μm to 25 μm.Nano-scaled MgZn2 is dispersed in the as-sprayed alloy,primary Al6Mn particles are precipitated at grain boundaries with an average size of 5 μm.A few CuAl2,Al3Zr and eutectic are also found in as-sprayed Al alloy.The volume fraction of the porosity is about 12%.DSC result indicates that most of the solutes are precipitated during spray forming process,and no obviously thermal effects occur below 450 ℃.Both matrix grains and Al6Mn particles grow monotonously with the increase of annealing temperature,but the growth rate of Al6Mn particles is markedly lower than that of Al grains,and the matrix grains grow rapidly when the annealing temperature is above 375 ℃.

Synergistic effect of gradient Zn content and multiscale particles on the mechanical properties of Al-Zn-Mg-Cu alloys with coupling distribution of coarse-fine grains

This study investigated the influence of graded Zn content on the evolution of precipitated and iron-rich phases and grain struc-ture of the alloys,designed and developed the Al–8.0Zn–1.5Mg–1.5Cu–0.2Fe(wt%)alloy with high strength and formability.With the increase of Zn content,forming the coupling distribution of multiscale precipitates and iron-rich phases with a reasonable matching ratio and dispersion distribution characteristics is easy.This phenomenon induces the formation of cell-like structures with alternate distribu-tion of coarse and fine grains,and the average plasticity–strain ratio(characterizing the formability)of the pre-aged alloy with a high strength is up to 0.708.Results reveal the evolution and influence mechanisms of multiscale second-phase particles and the corresponding high formability mechanism of the alloys.The developed coupling control process exhibits considerable potential,revealing remarkable improvements in the room temperature formability of high-strength Al–Zn–Mg–Cu alloys.

Mechanical behavior of Al-Zn-Mg-Cu alloy under tension in semi-solid state

In order to study the hot fractures in relation to the semi-solid processing, the tensile tests of an extruded 7075 aluminum alloy which is based on Al?Zn?Mg?Cu system were carried out in the high temperature solid and semi-solid states at different strain rates. The results show that the tensile behavior can be divided into three regimes with increasing the liquid fraction. The alloy first behaves in a ductile character, and as the temperature increases, the fracture mechanism changes from ductile to brittle which is determined by both of liquid and solid, and lastly the fracture mechanism is brittle which is totally dominated by liquid. At strain rates of 1×10?4, 1×10?3 and 1×10?2 s?1, the brittle temperature ranges are 515?526, 519?550 and 540?580 °C, respectively. Two equations which are critical for tensile behavior are proposed.

Multistage-aging process effect on formation of GP zones and mechanical properties in Al-Zn-Mg-Cu alloy

The tensile properties and fatigue behavior of an Al-Zn-Mg-Cu alloy were investigated by performing tensile tests and fatigue crack propagation （FCP） tests. The tensile results show that lower aging temperature modified retrogression and re-aging （RRA） process enhances the elongation, but reduces the strength of the alloy, as compared to conventional RRA process which employs peak aging temperature. Both ductility and strength, however, are increased by employing a natural aging prior to re-aging based on the former modified RRA process. Fatigue test results show that both routes reduce FCP rate. Especially, the lower re-aging temperature modified RRA process obtains the lowest FCP rate. Natural aging treatment could enhance the nucleation rate of GP zones. A large amount of GP zones could be cut by dislocations, which is responsible for the highest tensile strength and elongation, as well as lower FCP rate.

Thermodynamic calculation of high zinc-containing Al-Zn-Mg-Cu alloy

Phase fraction and solidification path of high Zn-containing Al-Zn-Mg-Cu series aluminum alloy were calculated by calculation of phase diagram （CALPHAD） method. Microstructure and phases of Al-9.2Zn-1.7Mg-2.3Cu alloy were studied by X-ray diffraction （XRD）, differential scanning calorimetry （DSC） and scanning electron microscopy （SEM）. The calculation results show that η（MgZn2） phase is influenced by Zn and Mg. Mass fractions of η（MgZn2） in Al-xZn-1.7Mg-2.3Cu are 10.0%, 9.8% and 9.2% for x=9.6, 9.4, 8.8 （mass fraction, %）, respectively. The intervals of Mg composition were achieved for θ（Al2Cu）＋η（MgZn2）, S（Al2CuMg）＋η（MgZn2） and θ（Al2Cu）＋S（Al2CuMg）＋η（MgZn2） phase regions. Al3Zr, α（Al）, Al13Fe4, η（MgZn2）, α-AlFeSi, Al7Cu2Fe, θ（Al2Cu）, Al5Cu2MgsSi6 precipitate in sequence by no-equilibrium calculation. The SEM and XRD analyses reveal that α（Al）, η（MgZn2）, Mg（Al,Cu,Zn）2, θ（Al2Cu） and Al7Cu2Fe phases are discovered in Al-9.2Zn-1.7Mg-2.3Cu alloy. The thermodynamic calculation can be used to predict the major phases present in experiment.

Influence of sub-grain boundaries on quenching process of an Al-Zn-Mg-Cu alloy

The effects of sub-grain boundaries on the quenching sensitivity and the precipitation behavior in Al-7.01Zn-1.26Mg- 1.43Cu alloy were investigated by an end-quenching test. Specimens were solution treated at 440 ℃ and 480 ℃ to get different recrystallization fractions, respectively. The results show that the maximum hardness value of the Al-Zn-Mg-Cu alloy can be improved by the sub-grain boundaries, but the depth of age-hardening layer decreases significantly. The precipitation temperature and the activation energy are reduced by the changes of surface energy, which is induced by sub-grain boundaries. So, the precipitation process from η phase to η phase becomes much easier. In this way, an increase in the number of sub-grain boundaries promotes the precipitation of MgZn2 particles, especially η＇-MgZn2.

Correlations among stress corrosion cracking,grain-boundary microchemistry,and Zn content in high Zn-containing Al-Zn-Mg-Cu alloys

The correlations among the corrosion behaviour,grain-boundary microchemistry,and Zn content in Al-Zn-Mg-Cu alloys were studied using stress corrosion cracking(SCC)and intergranular corrosion(IGC)tests,combined with scanning electron microscopy(SEM)and high-angle angular dark field scanning transmission electron microscopy(HAADF-STEM)microstructural examinations.The results showed that the tensile strength enhancement of high Zn-containing Al-Zn-Mg-Cu alloys was mainly attributed to the high density nano-scale matrix precipitates.The SCC plateau velocity for the alloy with 11.0 wt.%Zn was about an order of magnitude greater than that of the alloy with 7.9 wt.%Zn,which was mainly associated with Zn enrichment in grain boundary precipitates and wide precipitates-free zones.The SCC mechanisms of different Zn-containing alloys were discussed based on fracture features,grain-boundary microchemistry,and electrochemical properties.

Creep aging behavior and performance of Al-Zn-Mg-Cu alloys under different parameters in retrogression aging treatment

A study was conducted to better understand how different parameters, namely, regression aging time and regression aging temperature, affect the creep aging properties, i.e., the creep deformation and performance of Al-Zn-MgCu alloy during regressive reaging. The corresponding creep strain and mechanical properties of samples were studied by conducting creep tests and uniaxial tensile tests. The electrical conductivity was measured using an eddy-current conductivity meter. The microstructures were observed by transmission electron microscopy(TEM). With the increase in regression aging time, the steady creep strain first increased and then decreased, and reached the maximum at 45 min.The steady creep strain increased with the increase in regression aging temperature, and reached the maximum at 200 ℃.The level of steady creep strain was determined by precipitation and dislocation recovery. Creep aging strengthens 7B50-RRA treated with regression aging time at 190 ℃ for 10 min, and the difference in the mechanical properties of alloy becomes smaller. The diffusion of solute atoms reduces the scattering of electrons, leading to a significant improvement in electrical conductivity and stress corrosion cracking(SCC) resistance after creep aging. The findings of this study could help in the application of creep aging forming(CAF) technology in Al-Zn-Mg-Cu alloy under RRA treatment.

Microstructural evolution and mechanical properties of an ultrahighstrength Al-Zn-Mg-Cu alloy via powder metallurgy and hot extrusion

In this work,a novel ultrahigh-strength Al-10Zn-3.5Mg-1.5Cu alloy was fabricated by powder metallurgy followed by hot extrusion.Investigations on microstructural evolution and mechanical properties of the fabricated samples were carried out.The results show that the grain size of sintered samples matches with the powder particles after ball milling.The relative densities of sintered and hot extruded samples reach 99.1%and 100%,respectively.Owing to the comprehensive mechanism of grain refinement,aging and dispersion strengthening,the ultimate tensile strength,yield strength and elongation of the Al-10Zn-3.5Mg-1.5Cu alloy after hot extrusion and subsequent heat treatment achieve 810 MPa,770 MPa and 8%,respectively.

Ageing behavior of an Al-Zn-Mg-Cu alloy pre-stretched thick plate

The ageing behavior of a pre-stretched thick plate of Al-Zn-Mg-Cu alloy was systemically studied including one-step ageing, two-step ageing, and retrogression and reageing treatment （RRA）. One-step ageing of the alloy resulted in peak ultimate tensile strengths of 595 and 575 MPa after 22 and 6 h at 120 and 135°C, respectively. The strengthening phase in peak aged （T6 temper） alloy contained GP zones and the η′ phase predominantly. After two-step ageing, the electrical conductivity was increased markedly, but the pre-stretched thick plate sacrificed a great loss of strength. RRA treatment provided a method for maintaining the strength close to that obtained by T6 temper and for obtaining the high electrical conductivity close to that obtained by T7 temper; the ultimate tensile strength and electrical conductivity were 583 MPa and 21.0 MS/m, respectively. TEM analysis of T7 and RRA specimens revealed two types of precipitates that contributed to age strengthening i.e. the η′ and η phases.

Microstructure and properties of an Al-Zn-Mg-Cu alloy pre-stretched plate under various ageing conditions

The microstructures after various ageing treatments and their relation to the strength, fracture toughness, and corrosion behavior of an Al-Zn-Mg-Cu alloy pre-stretched plate were investigated. The results show that retrogression and reaging （RRA） treatment led to a combination of high strength and stress corrosion cracking （SCC） resistance of the alloy. The TEM microstructure of the RRA-treated alloy is a distribution of very fine precipitates in the aluminum matrix grains, similar to that obtained under T6 condition, and the distribution of coarse η MgZn2 precipitates on the grain boundaries similar to that obtained by T7 temper. SEM observations revealed that most of the intergranular fracture characteristics were present on the fracture surface of both the T6 and RRA-treated specimens. On the contrary, the fractographs of the T7 treated specimens mainly consisted of dimple-type ductile transgranular fracture with minor intergranular cracking.