GPI Zones Research Articles

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.

Effects of Cu content on the precipitation process of Al–Zn–Mg alloys

Heat treatable Al–Zn–Mg alloys can be strengthened by the precipitation of second phase particles. In this article, four Al–Zn–Mg-based alloys with different Cu contents have been prepared. The effects of Cu content on the aging behaviors of the whole precipitation process of the prepared Al–Zn–Mg alloys have been investigated using hardness tests, transmission electron microscope, selected election diffraction patterns, and their simulations. It has been shown that a certain amount of Cu promotes a rapid hardening response at the early stage of aging and the nucleation of both GP-II zones and the η′ precipitates. However, the addition of Cu above 3 wt% in alloys would decrease the accelerating effect of the transition from the GP-II zone to the η′ precipitates. The η precursor has been observed in Cu-containing alloys and it can retard the transition to the stable phase.

Effect of cohesive energy on thermal expansion behavior of T6 tempered 7A09 aluminum alloy

The dynamic effect of cohesive energy on the CTE of T6 (the highest strength temper) tempered 7A09 aluminum alloy was studied in this paper. Transmission electron microscopy (TEM) was combined with differential scanning calorimetry (DSC) to analyze the phase transitions of T6 tempered 7A09 aluminum alloy taking place in a heating process. The corresponding CTE curve was recorded by a thermal dilatometer. The ion-bonding model was utilized to estimate the variation of cohesive energy caused by the phase transitions. Results indicate that GPII zones and η′ phase are main precipitates in T6 tempered 7A09 alloy. During the heating process, the cohesive energy of alloy system increases with the transition from η′ to η phase and the precipitation of η phase. The dissolution of η phase leads to a decrease of cohesive energy. Although volume changes of alloy system have effects on the CTE, the effect of the cohesive energy change is more obvious than that of volume change after the precipitation of η phase begins in the Ostwald ripening stage. Effects of cohesive energy and volume changes can be expressed in the nonlinearity of CTE curve.

Effect of Microstructure on Thermal Expansion Coefficient of 7A09 Aluminum Alloy

The relationship between microstructure evolution and coefficient of thermal expansion (CTE) of 7A09 aluminum alloy was investigated in this paper. Differential scanning calorimetry (DSC) was combined with transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) to investigate microstructure evolution taking place in 7A09 aluminum alloy during heating and cooling process. The corresponding CTE curves of the 7A09 alloy were recorded by thermal dilatometer. Results indicated that GPII zones and η′ phase were main precipitates in the highest strength tempered (T6) 7A09 alloy. The η phase was the main participate in 7A09 alloy during the cooling process. The nonlinear dependency existed between CTE and temperature in both changing temperature processes. During the heating process, obvious additional contraction of alloy volume was directly caused by phase transition, such as dissolution of η′ phase, transition from η′ to η phase and dissolution of η phase. The additional contraction could slow down the increase of CTE greatly and be expressed in the nonlinearity of CTE curve. Volume and energy changes of alloy system influenced the variation trend of CTE directly, which was caused by the precipitation of η phase during the cooling process. These effects were revealed by the corresponding nonlinear change of CTE.

Precipitates in Al–Cu alloys revisited: Atom-probe tomographic experiments and first-principles calculations of compositional evolution and interfacial segregation

Atom-probe tomography, transmission electron microscopy, X-ray diffraction and first-principles calculations are employed to study: (i) compositional evolution of GPII zones and θ′ precipitates; and (ii) solute segregation at α-Al/ θ′ interfaces in Al–1.7 at.% Cu (Al–4 wt.% Cu) alloys. GPII zones are observed after aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only θ′ precipitates. Most GPII zones and θ′ precipitates are demonstrated to be Cu-deficient at the lower two aging temperatures; only the 533 K treatment resulted in θ′ stoichiometries consistent with the expected Al 2Cu equilibrium composition. For alloys containing ∼200 at. ppm Si we find evidence of Si partitioning to GPII zones and θ′ precipitates. Significant Si segregation is observed at the coherent α-Al/ θ′ interface for aging at 533 K, resulting in an interfacial Si concentration more than 11 times greater than in the α-Al matrix. Importantly, the Si interfacial concentration undergoes a transition from a non-equilibrium delocalized profile to an equilibrium localized profile as the aging temperature is increased from 463 to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning to Cu sites in θ′. Silicon segregation at, and partitioning to, θ′ precipitates results in a decrease in interfacial free energy, and concomitantly an increase in the nucleation current. Our results suggest that Si catalyzes the early stages of precipitation in these alloys, consistent with the higher precipitate number densities observed in commercial Al–Cu–Si alloys.

On the age-hardening of an Al–Zn–Mg–Cu alloy: A vacancy perspective

Positron annihilation spectroscopy was performed to study the microstructure of peak aged Al–Zn–Mg–Cu alloy. Applying artificial ageing at different temperatures, the solute content around vacancies is found to be increased compared to as-quenched state. The similarity of vacancy surroundings of all artificially aged states, except for over-aged ones, suggests that peak strength is caused not only by η′ precipitates but also by GPII zones. Moreover, η′ precipitates produced by artificial aging at 403 K are apparently coherent with the Al matrix.

Effect of Aging Treatment on Properties and Microstructure of an Al-7.5Zn-1.3Mg-1.4Cu-0.12Zr Alloy

In this study, the effect of various aging treatment (T6 and T7 treatment) on the mechanical properties, electrical conductivity and the microstructure of an Al-7.5Zn-1.3Mg-1.4Cu-0.12Zr alloy has been investigated. The results show that with elevating the aging treatment temperatures, the aging response rate is greatly accelerated. When T6 temper is performed at 140°C for 12h, as compared to peak aging for 24h at 120°C, the UTS and the corresponding Elongation values keep the same level, whereas the TYS and the electrical conductivity obviously increase by 5% and 9%, which is up to 560 MPa and 22.6 MS/m, respectively. And there are clear PFZs along the grain boundary and slightly coarser precipitates inside the grain. GPI zones, GPII zones and η' phases are major precipitates for the alloy under T6 condition. When T7 temper is performed on the alloy, the main precipitates are GPII zones, η′ and η phases. The coarser precipitates inside the grain and discontinuous grain boundary precipitates are favorable to electrical conductivity, which decrease the strength of 5~17% compared to T6 treatment. After T76 treatment (i.e., 110°C/6 h + 160°C/6 h), the UTS, TYS, Elongation and electrical conductivity values were 540 MPa, 510 MPa, 16.7% and 23.5 MS/m, respectively.

Precipitate Behavior and Microstructure Characterization of An Al-Zn-Mg-Cu Alloy Prepared by Spray Forming Process

A high Zn content Al-Zn-Mg-Cu alloy was prepared by spray forming process and the precipitate behavior and microstructure of the extruded alloy were also investigated. The precipitate sequence of the spray-formed alloy could be described as “α-solid solution → GPI zone → GPII zone (also called Metastable ′ )→ Stable  (MgZn2)” during artificial ageing treatment. In the early stage of artificial ageing treatment, the GPI zone was the main strengthening phase and kept coherent relationship with the matrix. With the increasing of ageing time, ′ phase dominate strengthening phase and kept semi-coherent relationship with the matrix. With the further increasing of ageing time,  phase took the place of ′ phase, and dominated the strengthening phase in the alloy. The grain size of the spray deposit is finer than that of cast alloys. The ultimate tensile strength of the alloy is over 810MPa in peak ageing condition.

Characterization of precipitate evolution in an artificially aged Al–Zn–Mg–Sc–Zr alloy

The precipitates of an age-hardenable Al–Zn–Mg–Sc–Zr alloy aged at 140°C have been studied by transmission electron microscopy (TEM). The results are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable η′ hardening precipitate and the stable phase η-phase (MgZn2). At early stages of ageing, the GPI zones act as nucleation sites for the plate-like η′ phase which precipitates on the {111}Al plane. The transformation from η′ to η occurs gradually at longer ageing times. The η particles mainly consist of η1 and η2 phase, with small amounts of η4 type and minor quantities of η′ type structures. Furthermore, while a fine dispersion of coherent Al3(Sc,Zr) precipitates is known to have a beneficial effect on the strengthening of the alloy, the formation of GPII zones is suppressed in the presence of zirconium that leads to a reduction in the amount of η′ formed during subsequent ageing.

Microstructure and mechanical properties of spray-deposited Al–Zn–Mg–Cu alloy processed through hot rolling and heat treatment

In this study, Al–10.8Zn–2.8Mg–1.9Cu alloy was synthesized by the spray atomization and deposition technique. Microstructure and mechanical properties of the spray-deposited alloy processed through hot rolling and heat treatment were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction and tensile test. The results indicate that the microstructure of the spray-deposited alloy is mainly composed of the Al matrix and the Mg(ZnCu) 2 compounds. The fragmentation of the Mg(ZnCu) 2 phases in the alloy has been regarded as one of the main feature during hot rolling. After solution treating, the microstructure is found to be composed of the Al matrix and a spot of Al 7Cu 2Fe, Al 9FeNi compounds. The spray-deposited Al–Zn–Mg–Cu alloy in the T6 temper displays superior strength due to the presence of two types of coherent GP zones (spherical GPI zones and thin platelet GPII zones).

Microstructure and mechanical properties of a large billet of spray formed Al–Zn–Mg–Cu alloy with high Zn content

In this study, a large billet of Al–12Zn–2.4Mg–1.1Cu–0.5Ni–0.2Zr alloy was prepared by spray deposition technique, the billet was subsequent processed by hot extrusion, solid solution treatment at 758 K and aging at 393 K for 20 h. The aged alloy was then subjected to tensile testing, it was shown that yield strength, tensile strength and elongation rate of the alloy reached 689 MPa, 750 MPa and 11%, respectively. The composition, microstructure and fracture characteristics of the tensile sample were explored by EDS (energy dispersive spectrometer), optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. A spherical morphology of the primary α-phase was observed in the spray-deposited Al alloy, dispersed and fine-scale L1 2–Al 3Zr phases, GPII zones and η′ precipitates, were also evident in the microstructure of the aged alloy. Fracture analysis of the tensile sample exhibited transgranular fractographic features. We suggest that precipitate-phase hardening and solid solution strengthening were responsible for reinforcing of the spray-deposited alloy.

Investigation of precipitation behavior and related hardening in AA 7055 aluminum alloy

The precipitation behavior and related hardening in AA 7055 aluminum alloy aged at 120 and 160 °C was investigated in detail. GPI zones were the dominant phase in the alloy upon ageing at 120 °C for 60 min. The metastable η′ phase begins to precipitate in the alloy after being aged at 120 °C for 60 min, and turns to be the main phase after ageing for 300 min. When the alloy was aged at 160 °C, the precipitation was significantly promoted. The results also revealed that the transformation of small GPI zone to η′ phase is the dominant mechanism for η′ formation. Formation and growth of GPI zones and η′ phases led to the increase of the yield strength, while formation and coarsening of η resulted in the decrease of the strength. η′ is responsible for the peak hardening of this alloy.

Secondary ageing in an aluminium alloy 7050

Secondary precipitation takes place in alloy 7050 at 65 °C after underageing at 130 °C and quenching (T6I4-65 temper) and results in a significantly increased number density of the η′ platelets, the precipitates also formed in the T6 temper. The modified microstructure results in tensile properties comparable to that of the T6 temper, but with significantly improved fracture toughness. A combined transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) study has shown that secondary ageing at 65 °C results in evolution of the GPI zones formed during underageing into the η′ phase. Ageing at 65 °C alone results in the formation of GPII zones, which provide lesser strengthening than the η′ platelets. The DSC study revealed six exothermic reactions corresponding to the formation of six different types of precipitate during the DSC scan.

Effect of Quenching Temperature on Age Hardening of AA6016 Sheets

Heat treatable AlMgSi alloys are being used increasingly for automotive skin sheet. AA6016 sheets produced from direct chill (DC) cast ingots are the material of choice in Europe. However, manufacturing of sheets using twin-roll casting (TRC) is a cost effective alternative to DC casting. The effect of processing parameters on the age hardening of TRC AA6016 sheets was assessed. The influence of quenching temperature after solution annealing and subsequent pre-aging at 100 °C was studied. Sheets in the artificially aged condition were prepared using a simulated car-body paint baking procedure (180 °C/30 min). The precipitation kinetics, resulting dispersion of the agehardening phase and their correlations with properties were investigated by differential scanning calorimetry, transmission electron microscopy and tensile tests. It is demonstrated that pre-aging results in increased age hardening effect, especially in the material quenched to 20 °C after solution annealing. The differences in strength are ascribed to the differences in the nucleation rate of β'' precipitates during stabilisation and the formation of stable GP-I zones in naturally aged specimen. The differences in precipitate microstructure inherited from the pre-treatment affect precipitation kinetics during paint baking and result in different precipitate dispersion and sheet strength.

Microstructure and Mechanical Properties of the Spray-Deposited Al-10.8Zn-2.8Mg-1.9Cu Alloy at Various Aging Conditions

In this paper, Al-10.8Zn-2.8Mg-1.9Cu alloy was synthesized by the spray atomization and deposition technique. The microstructure and mechanical properties of the spray deposited alloy at various aging conditions (T6, T73 and RRA treatment) were studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII zones are major precipitates for the alloy under peak-aged condition. The peak UTS and YS values achieved at an ageing temperature of 120 °C was 818 MPa and 793 MPa. Under two-step aging condition, the GP zones and η’ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa(about 7%) comparing to the peak-aged condition. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.

Characterization of precipitation and phase transformations in Al–Zn–Mg alloy by the differential dilatometry

The phenomena of phase transformations are very complex particularly in the case of the Al–Zn–Mg alloys because of the various parameters which influence them (heat treatments, chemical composition, types of elements of addition, relationship between different chemical constituents…). In order to contribute to the comprehension of these phenomena we have made a study of phase transformations by using differential dilatometry with some microhardness measurements. The sequence of precipitation for this system of alloys is as follows: solid solution → Guinier–Preston zones (ZGP) → intermediate η′ phase → equilibrium η phase. The analysis of the results suggests principally the formation of GPII zones on one hand and, on the other hand, at the beginning of its formation the equilibrium η phase consists of several types of precipitates and for sufficiently long time there remains one type.

Investigation of microstructural evolution and mechanical properties during two-step ageing treatment at 115 and 160 °C in an Al–Zn–Mg–Cu alloy pre-stretched thick plate

The microstructure evolution and age-strengthening characteristics of an Al–Zn–Mg–Cu alloy thick plate during a two-step ageing treatment at 115 °C and 160 °C were studied. The results show that the first step ageing treatment contributed to a major increase of tensile strength, the UTS value achieves 569 MPa after ageing for 7 h at 115 °C, both the GPI and GPII zones are dominant precipitates under this ageing condition. During the second step ageing at 160 °C, the peak strength value is achieved after a short ageing time, and then the strength decreases obviously, the UTS value reaches 493 MPa after ageing for 36 h. The microstructure in the matrix of the alloy at this ageing stage contains mostly metastable η′ phase and stable η phase. The grain boundary precipitates in two-step aged alloy were coarse and sparsely distributed, and clear precipitation free zone was observed.

Regular and high resolution transmission electron microscopy observations on the precipitates in a naturally aged Al–Mg–Si alloy AA6022

The precipitates in the Al–Mg–Si alloy AA6022 naturally aged for 2–3 years were investigated by regular and high resolution transmission electron microscopy (TEM and HRTEM) and by selective area electron diffraction (SAED). The shape of the particles was generally either nearly circular or circular with one or more bulges. They had an approximate log-normal size distribution with a peak diameter d p = 2.75 nm. Their crystal structure matched that of the c-centered monoclinic β // phase (GP-II Zone) reported in the literature for aging of this alloy type at elevated temperatures.

Kinetic Monte Carlo simulation of clustering in an Al–Zn–Mg–Cu alloy (7050)

A kinetic Monte Carlo technique using a simple vacancy jump for atomic diffusion has been employed for the first time to simulate early-stage precipitation in an Al–Zn–Mg–Cu (7050) alloy. Using interaction parameters deduced from the matrix compositions in over-aged samples measured using 3-dimensional atom probe, the simulation has successfully reproduced early-stage metastable GPI zone precipitation behaviour including evolution of the cluster chemistry, the cluster number density and cluster size. The simulation has revealed the effect of strong Mg–vacancy interaction on the composition of small clusters and has indicated that there is Zn enrichment of some clusters, which further explains previous experimental observation of smaller GPI zones transforming to η′ precipitates.

The effect of cold work on the precipitation kinetics of AA6111 aluminum

In the automotive industry, the drive to meet Corporate Average Fuel Efficiency (CAFE) standards has resulted in the need to reduce vehicular weight in new automobile designs. One effective method has been to use lighter materials such as aluminum alloys in outer body panels. Heat treatable AA6111 is one of the materials being employed in outer body panels due to its unique combination of formability, paint bake strengthening and superior corrosion resistance characteristics. For a formed and painted automobile panel, the final mechanical properties involve a combination of strength components arising from cold work, strain aging, precipitation and recovery. The challenge is to find a method of quantifying each of these strength components. In this study, the effect of cold work on the precipitation kinetics of AA6111 has been evaluated, by means of tensile testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results show a considerable improvement in yield and tensile strength with increasing level of cold work. DSC showed acceleration in the precipitation kinetics of the alloy with increasing level of cold work. The Avrami-Johnson-Mehl approach, further developed by Mittemeijer and co workers was employed to determine the activation energies for dissolution of GPI zones and β″ formation. The activation energy of dissolution of GPI zones is observed to increase with increasing level of cold work while that for formation of β″ decreases with increasing level of cold work. As expected, TEM showed strong interaction of strengthening precipitates with dislocations. The density of dislocation tangles is shown to increase with increasing degree of cold work.