GPI Zones Research Articles

Revealing the aging time on the precipitation process and stress corrosion properties of 7N01 aluminium alloy

In this paper, the precipitation of second phase particles, hardness and stress corrosion behavior of 7N01 alloys during 120 °C aging are systematically investigated by using micro-hardness tester, transmission electron microscope (TEM), selected area election diffraction patterns (SAEDP) and slow strain rate test (SSRT). The experimental results show that when 7N01 aluminum alloy was aged at 120 °C, the GPII zones formed at first, then gradually dissolved, transformed into η′ phase, and eventually evolved into η′ phase. Accordingly, three peak values appear on the hardness curve. The electrochemical measurements and SSRT results show that the high potential differences and the precipitates distributed continuously in the grain boundary can obviously decrease the stress corrosion cracking of the alloy after aging for 94 h. The narrow PFZ and the precipitates distributed discontinuously in the grain boundary effectively increase the resistance of stress corrosion cracking (SCC) of the alloy after aging for 46 h and 122 h.

Effects of artificial aging on microstructure, mechanical properties and stress corrosion cracking of a novel high strength 7A99 Al alloy

A recently developed 7A99 Al alloy was subjected to hot extrusion, solid solution and artificial aging. The microstructure evolution and precipitation behavior were examined, and their effects on mechanical properties and stress corrosion cracking were analyzed. The results showed that the coarse Mg(Zn, Cu)2 phases were broken and refined by hot extrusion. Most of the phases dissolved into Al matrix during solution, and only insoluble Al7Cu2Fe phase was remained. GPI zones and η′ appeared in the under-aged and peak-aged samples. With the extension of aging time, stable η phase with large size formed and became the dominant precipitate in the over-aged sample. It was concluded that the precipitation of 7A99 Al during aging at 150 °C followed the sequence of solid solution → GPI zones → metastable η′ → stable η. The peak-aged sample owned the high yield and ultimate tensile strength of 588 MPa and 622 MPa, and a low elongation of 10.2%. The under-aged and over-aged samples exhibited relatively lower strength and higher elongations. 2% pre-stretching prior to aging could further improve the tensile properties. Both the under-aged and peak-aged samples were susceptible to the stress corrosion cracking, and the intergranular cracking was observed. In contrary, the over-aged sample showed much better corrosion resistance with a typical ductile fracture, due to the coarsening and separation of grain boundary precipitations.

Effect of Ni additions and hot deformation on precipitation behavior and hardness in Al–Si–Mg aged alloys

Additions of 1–2 Ni (wt. %) and hot deformation on the microstructure and hardness of A356 aged alloy were studied. The results show that the addition of this element generates the formation of Ni31Si12 and NiFe phases with high thermal stability. In addition, the presence of Ni into the Al matrix and precipitates of the second phase affect the precipitation kinetics, transformation sequence, growth rate, and slow loss of hardness in the system after reaching the maximum values. For example, the Ni additions contribute to generating the coexistence of β" precipitate and GP-I zones. Of all the strengthening mechanisms present, hardening by precipitation is the most influential in the hardness.

Formation of solute nanostructures in an Al–Zn–Mg alloy during long-term natural aging

Nature aging (NA) is believed to produce solute nanostructures, including solute clusters and GPI zones in Al–Zn–Mg alloys. Here we report the first observation that GPII (GPη’) zones with an average Zn/Mg ratio near 1.3–1.4 formed in the Al–Zn–Mg alloys during long-term natural aging. High-angle-annular-dark-field scanning transmission electron microscopy and atom probe tomography were used to study the evolution of solute nanostructures during natural aging. Early clusters were found to have a wide spread of Zn/Mg ratios and their number density increased sharply with a dramatic hardening effect during the first month ageing. Further ageing up to 3 months made the Zn/Mg ratios of the solute clusters gradually converge to near 1.2 (GPI zones), but produced no change in the morphology, size distribution and volume fraction of the solute nanostructures, as well as hardness. At room temperature, GPII (GPη’ type) zones formed very slowly and provided stronger hardening effects than early clusters. Importantly, unlike solute-rich clusters, GPI and GPη’ zones are easy to transform into η′ during subsequent artificial ageing, avoiding hardness drop at the early stage of subsequent artificial aging at 120 °C.

Effect of pre-ageing treatment on second nucleating of GPII zones and precipitation kinetics in an ultrafine grained 7075 aluminum alloy

To clarify the influence of a pre-ageing treatment on precipitation behaviors in the AA7075 aluminum alloys with ultrafine grain (UFG) structure, the alloys were pre-aged prior to a room temperature rolling process (PA + RTR). The precipitation kinetics in PA + RTR alloys during isothermal heat treatment at 100 °C were systematically investigated by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and Vickers hardness measurements compared to its counterpart without pre-ageing treatment (i.e. RTR). Results reveal the pre-ageing treatment can lead to a higher dislocation density (22.8 × 1014 m−2) and a larger concentration of Mg (~12.85 at.%) and Zn (~8.33 at.%) atoms in the vicinity of dislocation, which causes a considerable increase in the size and volume fraction of the precipitates in PA + RTR alloy. Specially, our study shows, for the first time, that many GPII zones come from second nucleating in PA + RTR alloy at 108 h, which may ascribe to the pre-ageing treatment. More importantly, the pre-ageing treatment not only makes both two hardness peaks of PA + RTR alloy appeared ahead of but also higher than those of RTR alloy, by virtue of the enhanced precipitation kinetics in PA + RTR alloy.

Research on the effect of aging time on the microstructure of 7055 aluminum alloy

7055 aluminum alloy is a typical age-hardening alloy whose properties change with microstructural evolution as a result of heat treatment. In this paper, the microstructural evolution of the alloy after solid solution treatment and manual aging is examined using SEM and TEM. Results show that, as aging proceeds, the precipitation phase gradually enlarges and thickens following the precipitation sequence of supersaturated solid solution (SSSS) → GPI zone → GPⅡ zone → η" phase → η' phase; some η phases formed during aging, which have two different nucleation mechanisms: homogeneous nucleation and heterogeneous nucleation, the S phases of the latter being marginally larger in size than those of the former. There is a transition in the transformation of (Al3Cu) in GPII zone into η' -phase.

Measurement and Theoretical Calculation Confirm the Improvement of T7651 Aging State Influenced Precipitation Characteristics on Fatigue Crack Propagation Resistance in an Al–Zn–Mg–Cu Alloy

Precipitation characteristics influencing fatigue crack propagation contained matrix precipitate, grain boundary precipitate and precipitate free zone for Al–Zn–Mg–Cu alloys. Over-aging treatment could effectively regulate precipitation and then to be able to change fatigue crack propagation behavior compared with the peak aging state. In the current work, typical T651 and T7651 aging tempers of the alloy were extracted via hardness, electrical conductivity and mechanical properties under one-step and two-step aging treatments. Fatigue crack propagation (FCP) rate under them was tested and corresponding precipitation characteristics and fracture morphology were observed. The results indicated that fatigue crack propagation resistance for the T7651 temper possessed an obvious improvement on the side of that for the T651 temper, which was also supported by fracture appearance, including tearing ridge, tearing dimple and fatigue striation. The precipitation observation showed that the T651 alloy contained GPI zone, GPII zone and ηʹ phase while the T7651 alloy possessed ηʹ phase and η phase. Compared with the T651 temper, matrix precipitate for the T7651 temper distinctly owed an expanding of size distribution and an enlargement of average size while cuttable phase still remained the dominance for both tempers. Grain boundary precipitate and precipitate free zone manifested no obvious difference between the two aging tempers. Cut and bypass mechanisms of dislocation–precipitate interactions were used for explanation and it revealed the reinforced cuttable phase was in favor of enhancing fatigue crack propagation resistance. A theoretical model which directly correlated FCP rate with matrix precipitate characteristics was employed to calculate FCP rate by substituting quantitative precipitate characteristics and the calculation results were vaguely consistent with the experimental measurement, which proved its reliability and feasibility.

Shock wave determination of the strengthening of commercial aluminum alloy 6061 by point defects

The strengthening of aluminum alloy 6061 (AA6061) by different point defects was determined experimentally using a shock wave technique. Decay of the amplitude of elastic precursor wave τel with propagation distance h was studied in four groups of AA6061 samples, namely, that in the super-saturated solid solution state (SSSS), and those strengthened by atomic clusters (short-range order), by Guinier-Preston zones I and by Guinier-Preston zones II after, respectively 7.5, 240 and 960 min aging of the SSSS samples at 145 °C. The dependences τel(h) were found to be kinked at stress τ*, corresponding to the transition of the control of dislocation motion from phonon viscous drag at τel>τ* to thermally activated obstacle passage at τel<τ*. The values of strengthening by atomic clusters, GP-I, and GP-II zones were found equal to τsro=67, τGP-I=67, and τGP-II=124MPa, respectively. Activation volumes corresponding to the interaction of dislocations with the studied defects, estimated based on τel<τ* segments of the measured dependences τel(h), were found to be in reasonable agreement with existing concepts of dislocation/defect interactions.

Atomistic study of crack-tip plasticity in precipitation hardened monocrystalline aluminum

Understanding the atomistic mechanisms of dislocation-based plasticity ahead of a crack-tip in precipitation hardened alloys is a challenging problem due to the complexity of the interactions between the precipitates in the microstructure and the variety of defects nucleated at the crack-tip, such as dislocations, stacking faults and micro-twins. In this paper, we use classical molecular dynamics simulations to perform a comprehensive atomistic analysis of the factors that influence the motion of dislocations ahead of a crack-tip in precipitation hardened aluminum. Specifically, the effects of planar copper GPII zones on the motion of dislocations emitted at the crack-tip of an aluminum crystal in four different crystal orientations under constant strain-rate loading were investigated. By placing the precipitates close to the crack-tip, it was found that they did not affect the nucleation of the first dislocation significantly unless they were located immediately ahead. Moreover, in some crystal orientations, subsequent nucleations were appreciably delayed due to the shielding effect of the first dislocation interacting with the precipitate. Following emission, the interaction between the emitted dislocations and the precipitates consisted of different mechanisms, including shear cutting, Orowan looping, and cross-slip, depending on the crystal orientation. The resistance to dislocation motion caused by the precipitates was quantified by determining the interaction time between each dislocation and the precipitates. It was found that although the applied load in each unit cell was high, the dislocations could be significantly slowed down in some of the crystals. This resulted in less dislocation activity ahead of the crack-tip, especially in the crystals for which micro-twinning was the dominant driver of plasticity. The results of this work pave the way for the development of accurate models to predict the evolution of plasticity in metallic materials by providing a quantified assessment of dislocation motion in complex alloy microstructures.

Effects of Three-Step Aging on Microstructure and Stress Corrosion Cracking of 7N01 Aluminum Alloys

The precipitation of 7N01 alloy after three-step ageing treatment at 65°C, 100°C and 150°C has been investigated. The SAED results show that at 65°Cpre-aging the GPI zones were precipitated from the quenched solid solution and aging at 150°C results in evolution of the GPI zones formed during under ageing into the η’ phase. It was found that compared with the two-stage aging(T74) and T6 aging, pre-aging at 65°C significantly increased number density of η’ phase. The modified microstructure results in tensile properties comparable to that of the T6 temper, but with significantly improved the resistance of stress corrosion cracking performance.

Microstructural characterization of interrupted aging on an AA7050 aluminum alloy

Interrupted aging heat treatments are used to maximize the gain in mechanical properties in 7xxx aluminum alloys through the production of a microstructure with a high density of fine precipitates, which are homogeneously dispersed in the aluminum matrix. In an interrupted aging heat treatment, the first stage, at higher temperature, aims at creating sites for subsequent nucleation of precipitate particles, while the second stage, at lower temperature, aims at the nucleation and growth of precipitate particles. The Interrupted aging heat treatment has been increasingly studied because it confers resistance levels similar to, or even higher than conventional heat treatments (e.g. T6), in addition to producing a more stable microstructure. In this study, a microstructural characterization was carried out using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) in an AA7050 aluminum alloys that was submitted to aging heat treatments T7451, T6 and T6I4-65, seeking a better understanding of the microstructure produced in each heat treatment and the reasons of the gain in mechanical properties. The results indicated that the T6I4-65 interrupted aging heat treatment was the most efficient in creating sites for nucleation of phases in the aluminum matrix, when compared with other conditions, due to the higher density of particles of the phases η′, η and T, in addition to being the only condition to have GPII zones and S phase in its microstructure.

Pre-strain-dependent natural ageing and its effect on subsequent artificial ageing of an Al-Cu-Li alloy

The effects of pre-strain and natural ageing (NA) on the precipitation hardening behavior in an Al-Cu-Li alloy have been systematically investigated. Applying pre-strain significantly restricts the solute clustering-induced formation of fine GPI zones (single Cu-layer) and thus hardness increase during NA. NA before artificial ageing (AA) has a marginally positive effect on the age hardening in samples without pre-strain, giving rise to a strength improvement of 15–20 MPa without loss of ductility. NA slightly enhances the precipitation of T1 phase during AA while shows no obvious effect on θ′, σ, δ′ and X phases. The X phase hasn't been reported and is isostructural to the θ′ phase, but the habit plane is (011)Al instead of (001)Al. With the increase of pre-strain level, the T1 plates become dominant and finer. However, NA after pre-strain has no appreciable effect on the age hardening and microstructure of the alloy. The analysis by complementary methods suggests that the low stability of NA clusters and their inhibition by pre-strain are responsible for the observed NA effect on AA in Al-Cu-Li alloy.

Microstructure evolution and mechanical properties of an ultrahigh strength Al-Zn-Mg-Cu-Zr-Sc (7055) alloy processed by modified powder hot extrusion with post aging

A modified powder hot extrusion including gas atomization, pre-compaction and hot extrusion was used to fabricate an ultrahigh strength Al-Zn-Mg-Cu-Zr-Sc (7055) alloy. The results reveal that a homogeneous microstructure containing fine grains and tiny second phases is formed after extrusion. The solution-treated alloy aged at 393 K for 24 h achieves peak hardness of 177 HB due to the common effect of GPI zone and η′ phase. The peak-aged alloy exhibits an ultrahigh ultimate tensile strength of 734 MPa and a good elongation at fracture of 9.8%, which is mainly attributed to fine grains and high-density nanoscale second phases (GP, η′, and Al3(Sc/Zr)). Micropore aggregation fracture is the main fracture mechanism in all the tested alloys states. The cracking or debonding of coarse second phases is the main cause of the limited strength of as-extruded alloy, while the coarse phases and PFZs at the grain boundaries are responsible for the decrease in the elongation of peak-aged sample. Compared with the 7055 alloys fabricated by other techniques, it is suggested that this modified powder hot extrusion may be an effective approach to develop high strength Al alloys.

Grain structure and precipitate variations in 7003-T6 aluminum alloys associated with high strain rate deformation

The variation of the grain and precipitate structure plays a rather important role on the response of the 7003-T6 Al-Zn-Mg-Cu alloys to high speed deformation but related reports are limited. This paper represents a detailed investigation about the flow behavior and microstructure evolution of these alloys under high strain rate impacting. Methods such as split Hopkinson pressure bar testing, optical and TEM microscopy, Vickers hardness testing, etc. were employed. Features such as plastic compression, tilting, distortion, elongation and refinement manners of the grains as deformation proceeds were studied. The changes of the precipitates in phase type, size and density, and their contribution to the flow stress during this thermo-mechanical process was investigated. The results show that the motion of dislocations and temperature rising induced by the impact work were responsible for these microstructure changes. Deformed shear bands with elongated grains formed in the diagonal direction of the sample when total strain reached 0.17. Prototype of the refined grain structure in these bands was constructed when the dislocation walls accumulate in the triangle grain boundary and form closed cells. With the assist of localized temperature rising and further increase of the strain to above 0.19, the deformed shear band was finally changed into transformed shear band with fine recrystallized grain structure in the center. Dynamic re-precipitation including phase transformation from GPII zones to ƞ′ and then to ƞ, coarsening in size and then dissolving was promoted. Elevated temperature and multiplied dislocation facilitated these processes. Precipitation coarsening in the matrix and dissolving in the ASB played a significant role of softening in the flow behavior as a counterbalance of the deformation and geometric hardening. A schematic model considering both of the grain structure and precipitate evolutions was established and discussed.

Distribution and evolution of aging precipitates in Al-Cu-Li alloy with high Li concentration

Abstract The evolution and distribution of the aging precipitates in 1460 Al-Li alloy with high Li concentration (2.14%, mass fraction) during T6 aging and two-step T8 (4% predeformation) aging were investigated through TEM. The aging precipitates include δ′ (Al3Li) and T1 (Al2CuLi) phases, of which the δ′ phases are formed first in grain interiors. A lot of δ′/GPI/δ′ composite precipitates in which GPI zones are flanked with a pair of δ′ phases, are formed at 145 °C of T6 aging, which are thermally stable. At 160 °C and 175 °C of T6 aging, many T1 phases nucleate first at subgrain boundaries and grain boundaries, and then form and grow within grains. As to the T8 aging, the δ′/GPI/δ′ composite precipitates are formed during the first-step aging at 130 °C for 20 h, which are thermally stable during the second-step aging at 160 °C. The plastic predeformation accelerates T1 nucleation within grains during the second-step aging at 160 °C.

Effect of One-Step and Two-Step Aging Treatments on Microstructure and Properties of an Al-9.0Zn-2.0Mg-2.0Cu Alloy

Aging treatments of an Al-9.0Zn-2.0Mg-2.0Cu alloy, which belongs to high strength aluminum alloy widely used in aerospace industry, are investigated by various techniques, including hardness, electrical conductivity, mechanical properties, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The result shows that hardness and conductivity for one-step aging treatment increase with aging time prolongs while those for two-step aging treatment exhibit increment and decrement, respectively. Besides, the ultimate tensile strength (UTS) and yield strength (YS) for one-step and two-step aging treatments show slow increase and obvious decrease, respectively. Based on these, typical T6 and T76 aging regimes are extracted for microstructure observation. The matrix precipitates for the T6 alloy have small size and dispersive distribution while that for the T76 alloy has big size and sparse distribution. The grain boundary precipitates for both exhibit discontinuous distribution and the T76 alloy has larger size and broader precipitate free zones. The selected area diffraction patterns and HREM observations reveal that main precipitates for the T6 alloy are GPI zone, GPII zone and η' phase while for the T76 alloy are η' phase and η phase.

Microstructural Comparison of an Al-8.0Zn-1.8Mg-2.0Cu Alloy with Typical T6 and T76 Tempers

In order to analyze aging behavior of an Al-8.0Zn-1.8Mg-2.0Cu alloy, the microstructure of the alloy subjected to T6 and T76 states are investigated by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Based on the precipitate observations, precipitate size distributions and average precipitate size are extracted from bright-field TEM images projected along 〈110〉Al orientation with the aid of an imaging analysis. The results indicate that the main precipitates are GPI zone, GPII zone and η' phase in the T6 alloy while η' phase and η phase in the T76 alloy. The bright-field TEM observations reveal that the matrix precipitates for the T6 alloy have small size and dispersive distribution while that for the T76 alloy has big size and sparse distribution. Both have discontinuously distributed grain boundary precipitates. Quantitative structural information including precipitate size distribution and average precipitate size has been calculated by an image analysis based on the bright-field TEM images projected along 〈110〉Al orientation. The results show that the T6 alloy has a narrower precipitate size range than the T76 alloy and thus the T6 alloy possesses a smaller average precipitate size than the T76 alloy.

Defective GP-zones and their evolution in an Al-Cu-Mg alloy during high-temperature aging

Nano-precipitates at the two-stage hardening peaks of an Al-5.0Cu-0.4 Mg (wt%) alloy under isothermal aging at 210 °C were thoroughly characterized using aberration-corrected scanning transmission electron microscopy and first-principles energetics calculations. It was observed, for the first time, that individual one-dimensional GPB1 units can form with a high number density in the matrix, due to the stabilization of substitutional defects. The GPII zones at the early-stage peak aging, as well as the θ′ nano-precipitates at the second-stage peak aging, are also frequently found to be defective in structure. Further analyses suggest that the high-temperature age hardening involves a complex association and evolution of the defective GP-zones.

RETRACTION: A Study on the Nucleation Mechanism of Nano-Scale Precipitates of 7055 Aluminum Alloy

7055 aluminum alloy is a typical age-hardening alloy whose properties change with microstructural evolution as a result of heat treatment. In this paper, the microstructural evolution of the alloy after solid solution treatment and manual aging is examined using SEM, TEM and HADDF. The precipitation, nucleation and growth mechanisms of θ′ and θ′′ phases are also analyzed. Results show that, as aging proceeds, the Al–Cu phase gradually enlarges and thickens the following precipitation sequence of supersaturated solid solution (SSSS) → GPI zone → GPII zone →θ′′ phase →θ′ phase; some S phases formed during aging, which have two different nucleation mechanisms: homogeneous nucleation and heterogeneous nucleation, the S phases of the latter being marginally larger in size than those of the former. There is a transition in the transformation of (Al3Cu) in GPII zone into θ′-phase (Al2Cu). The orientation relation between θ′ phase and Al matrix is (100)Al//(100)θ′, (010)Al//(010)θ′, (001)Al//(001)θ′. According to...

Single-stage aging behaviour and precipitate evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy

ABSTRACTProperty evolution in a high Zn-containing Al–9.78Zn–2.02Mg–1.76Cu alloy treated at 120°C, with the microstructure subjected to under-aging (UA), peak-aging (PA) and over-aging (OA), is investigated. Precipitate size distributions and distances between neighbouring precipitates are determined. Results indicate that a certain time is required to reach peak hardness and yield strength, and that peak values can be sustained for a relatively long time. As the aging time increases, the conductivities increase persistently. As the alloy temper varies from UA to PA to OA, the main matrix precipitates change from ‘GPI zone and GPII zone and η′ phase’ to ‘GPII zone and η′ phase’ and then to η′ phase. Meanwhile, the precipitate size distribution becomes broader, and the average precipitate size increases.