Guinier-Preston Research Articles

Investigations into Hot Form Quench Conditions on Microstructure Evolution and Bake-Hardening Response for High-Strength Aluminum Alloy

Hot Form Quench (HFQ®) is an advanced technology for forming complex-shaped parts in automobile industry. In the present work, the effects of HFQ® conditions on microstructure evolution and bake-hardening (BH) response of AA7075 were investigated by means of transmission electron microscopy (TEM), x-ray diffraction, electron back-scattered diffraction and microhardness tests. The results show that, with pre-strain increasing, more η and η′ precipitates appear. The size of η′ changes little but η becomes larger. The reason for the high volume fraction of precipitates is the high-density dislocations which can act as the effective nucleation sites. Meanwhile, it is revealed that the effect of precipitation strengthening and dislocation strengthening has been enhanced gradually. Nevertheless, the solid-solution strengthening effect is weakened and the effect of grain boundary strengthening changes slightly. Compared with water quenching condition, more large-sized η phase appears easily in air-cooling condition, which was recognized as the reason for the decrease in hardness. As evidenced in the analysis of HRTEM and TEM, no precipitates existed in the sample with solid-solution state and a large number of Guinier–Preston (GP) zones were found in 10% pre-strain + pre-aging sample, indicating pre-aging enables to promote GP zone nucleation, which may bring an improvement in BH response.

Evolution of microstructure and hardness during artificial aging of an ultrafine-grained Al-Zn-Mg-Zr alloy processed by high pressure torsion

An ultrafine-grained (UFG) Al-4.8%Zn-1.2%Mg-0.14%Zr (wt%) alloy was processed by high pressure torsion (HPT) technique and then aged at 120 and 170 °C for 2 h. The changes in the microstructure due to this artificial aging were studied by X-ray diffraction and transmission electron microscopy. It was found that the HPT-processed alloy has a small grain size of about 200 nm and a high dislocation density of about 8 × 1014 m−2. The majority of precipitates after HPT are Guinier–Preston (GP) zones with a size of ~ 2 nm, and only a few large particles were formed at the grain boundaries. Annealing at 120 and 170 °C for 2 h resulted in the formation of stable MgZn2 precipitates from a part of the GP zones. It was found that for the higher temperature the fraction of the MgZn2 phase was larger and the dislocation density in the Al matrix was lower. The changes in the precipitates and the dislocation density due to aging were correlated to the hardness evolution. It was found that the majority of hardness reduction during aging was caused by the annihilation of dislocations and some grain growth at 170 °C. The aging effect on the microstructure and the hardness of the HPT-processed specimen was compared to that observed for the UFG sample processed by equal-channel angular pressing. It was revealed that in the HPT sample less secondary phase particles formed in the grain boundaries, and the higher amount of precipitates in the grain interiors resulted in a higher hardness even after aging.

Role of Al addition in modifying microstructure and mechanical properties of Mg-1.0 wt% Ca based alloys

In this work, the role of Al addition in modifying the microstructure and mechanical properties of the Mg-1.0Ca-xAl (x = 0.6, 1.0 wt%; termed as XA10 and XA11, respectively) based alloys has been systematically investigated. Microstructural examination showed that, when the Al content is low, the Al and Ca atoms were inclined to segregate to the dislocation lines in the XA10 sample. As a result, the mobility of dislocation was restrained and the dynamic recovery behavior was hindered, which leads to a low degree of dynamic recrystallization. When the Al content is high, in contrast, no apparent Ca/Al segregations along dislocations have been found in XA11-0 extrusion billet during the whole stages of processing, which results in the enhanced dislocation mobility. Consequently, the recovery of dislocations and dynamic recrystallization rate can be largely enhanced at the earlier stage of extrusion. More importantly, subsequently formed high density Guinier-Preston (G.P.) zones and nano-disk Al2Ca phases in the high-Al content Mg sample can play an effective role in inhibiting the growth of subgrains and dynamically recrystallized (DRXed) grains, which enhance the thermal stability of high density low angular grain boundaries (LAGBs) in the later extrusion stage. The high density nano-disk phases, high volume fraction of DRXed grains, as well as the high density LAGBs in the un-DRXed grains, can together contribute to both high strength and good elongation of the XA11 sample. The findings lead to the controllable Mg-Ca based alloy designing strategy that can improve the strength and ductility simultaneously.

Precipitation and aging phenomena in an ultrafine grained Al-Zn alloy by severe plastic deformation

The post-deformation aging response and underlying precipitation kinetics in ultrafine-grained, age-hardened alloys processed by severe plastic deformation are critical and heretofore poorly understood phenomena that ultimately influence mechanical behavior. In this study, a supersaturated solid solution 7075 Al alloy was processed by equal-channel-angular pressing (ECAP) route Bc at 250 °C for 6 passes to attain ultrafine grained (UFG) structures. Thermal and mechanical analyses revealed that there are three sequential phase transitions that govern post-deformation aging: first, dissolution reactions of Guinier-Preston (GP) zones, metastable ηp and η′ precipitates which correspond to slight decreases in strength and ductility; second, afresh precipitating of GP zones, metastable ηp and η′/stable η phases corresponding to an increase in strength with significant decrease in ductility; and third, subsequent η transformation of the metastable η′ and ηp as well as coarsening of stable η phases corresponding to a dramatic decreases in strength. Microstructural analysis suggests that the significant decrease in ductility may be attributed to grain boundary precipitation of the metastable ηp and η′/stable η phases.

Friction stir welding of 2060–T8 Al[sbnd]Li alloy. Part I: Microstructure evolution mechanism and mechanical properties

2 mm thick rolled 2060–T8 AlLi alloy plates were subjected to friction stir welding (FSW) under rotation rates of 400–1200 rpm and welding speeds of 50–200 mm/min. In the nugget zone (NZ), FSW resulted in the dissolution of the precipitates and the formation of δ′ (Al3Zr) precipitates, as well as the formation of Guinier–Preston (GP) and Guinier–Preston–Bagaryatsky (GPB) zones except for at 400 rpm–200 mm/min. In the heat affected zone (HAZ), FSW led to the dissolution and coarsening of T1 (Al2CuLi) and S′ (Al2CuMg), and the subsequent formation of new θ′ (Al2Cu) and σ (Al5Cu6Mg2) precipitates. The lowest hardness zones (LHZs) of the FSW joints were located at either the NZ or the HAZ under varied welding parameters. Different from the FSW joints of conventional precipitation–hardened aluminum alloys, the ultimate tensile strength of FSW 2060–T8 joints increased as the rotation rate increased from 400 to 800 rpm, but was unchanged with further increasing the rotation rate from 800 to 1200 rpm under a constant welding speed of 200 mm/min. On the other hand, the joint strength increased as the welding speed increased from 50 to 200 mm/min under a constant rotation rate of 1200 rpm. While the FSW joints fractured at the LHZs under 400 rpm–200 mm/min and 1200 rpm–50 mm/min, abnormal fracture at the thermo–mechanically affected zone (TMAZ) was observed under 800 rpm–200 mm/min and 1200 rpm–200 mm/min.

Influence of particulate B4C size on microstructural evolution and mechanical behavior in nanostructured Al matrix during heat treatment

We reported the influence of particle size (micro-B4C and nano-B4C) on microstructural evolution and mechanical properties in nanostructured Al matrix during heat treatment. The results indicated the incorporation of nano-B4C could significantly inhibit the grain growth of Al during heat treatment. The segregation layer in Al/B4C interface increased after heat treatment for all the composites samples. High density of Guinier-Preston (GP) zones and platelet η' could be observed in grain interior of all samples, while some coarse η phases (MgZn2) could be only detected in grain boundaries of AA7075 alloys and the composites with micro-B4C. The yield strength of the composites with the incorporation of nano-B4C could reach up to 958 ± 28 MPa after heat treatment, which could be ascribed to the Hall-Petch strengthening and dislocation strengthening.

Phase transformations in novel hot-deformed Al–Zn–Mg–Cu–Si–Mn–Fe(–Sc–Zr) alloys

Precipitation stages in the hot-deformed Al-5.28wt.%Zn-3.21wt.%Mg-1.48wt.%Cu-0.24wt.%Si-0.24wt.%Mn-0.14wt.%Fe(−0.23wt.%Sc-0.19wt.%Zr) alloys were investigated. Positron annihilation spectroscopy was employed for a study of vacancy assisted clustering of solutes on the atomic scale, type and concentration of open volume misfit defects and local chemical surroundings of defects. Microhardness testing, thermal analysis and microstructure observations were also used. The alloys contain the eutectic grain boundary phases: T-phase and α-Al (Mn,Fe,Si) phase. Positrons are trapped at Zn,Mg-containing Guinier-Preston (GP) zones with no open volume defects and at (precursors of) the η′-phase particles. Above ~150 °C precipitation of non-eutectic T-phase and coarsening of the S-phase particles are observed. Precipitate-matrix interfaces are characterized by vacancy-like misfit defects decorated by Si solutes. Binding energies of vacancies to various solutes (Si, Zn, Cu, Mn, Sc and Zr) obtained by ab-initio calculations were computed. Si solutes exhibit the highest vacancy binding energy among the constituents of the studied alloys. Other solutes (especially Mg, Sc, Zr) repel vacancies. The alloy with Sc,Zr-addition contains incoherent primary Al3(Sc,Zr)-phase. Layered primary particles with different morphological features such as square and polygonal shapes contain Sc,Zr-rich layers.

Enhanced Age-Hardening by Synergistic Strengthening from Mg-Si and Mg-Zn Precipitates in Al-Mg-Si Alloy with Zn Addition

The accelerated age-hardening response and the enhanced peak-hardness in Al-Mg-Si with Zn addition was investigated by three-dimensional atom probe and transmission electron microscopy. The results showed that Zn addition significantly increased the number density of solute aggregates by stimulating the formation of more Mg-Si precipitates and resulted in a higher Mg/Si ratio because of the formation of Mg-Zn precipitates. A synergistic strengthening from Mg-Si and Mg-Zn-(Si) solute aggregates was estimated to accelerate the age-hardening response at the early aging stage, and that from β phases for Mg2Si system and Guinier-Preston (GP) I zones/η′ phases for MgZn2 system enhanced the peak-harness.

Atomistic mechanism and probability determination of the cutting of Guinier-Preston zones by edge dislocations in dilute Al-Cu alloys

The interaction between a ${}^{1}/{}_{2}\left[\overline{1}10\right]\left(111\right)$ edge dislocation and a (001) Guinier-Preston (GP) zone in dilute Al-Cu alloys is studied via atomistic modeling. In stark contrast to the previously reported Orowan looping mechanism where the GP zone remains intact after yield, we discover a competing mechanism where the dislocation cuts the GP zone into two pieces. We identify the key atomic process triggering the cutting mechanism and calculate its activation barrier at various strains. In further conjunction with the transition state theory, the occurrence probability of a trigger event is mapped out over a broad range of $T\text{---}\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\varepsilon}}$ parameter space. The predictions of the so-constructed mechanism map are validated by parallel MD simulations. The implications of our findings regarding the discrepancies between the existing age hardening model and experiments are also discussed.

High strength and excellent ductility of dilute Mg-0.68Al-0.32Ca-0.50Mn (wt%) extrusion alloy obtained by T6 treatment

A dilute Mg-0.68Al-0.32Ca-0.50Mn (AXM070305, wt%) magnesium (Mg) alloy with high strength and excellent ductility was developed by extrusion and subsequent T6 heat treatment. The alloy exhibits a yield strength (YS) of 248 MPa, ultimate tensile strength (UTS) of 288 MPa and elongation to failure (EL) of 21.0%, after T6 peak-aged treatment. The high strength is mainly attributed to the presence of high number density of nanosized β-Mn precipitates, a large number of mono-layered Guinier-Preston (G.P.) zones and planar Al2Ca phase. The excellent ductility is mainly due to the fully recrystallized fine grains with weak basal texture. The peak microhardness of this alloy was reached 67.9 HV after aging at 200 °C for 16 h, due to the precipitation of mono-layered G.P. zone, nanosized particle β-Mn phase and planar Al2Ca phase.

Microstructural and ageing response assessment of eutectic Al-Cu alloy due to isothermal heat treatment in semisolid state

An as cast Al-33%Cu eutectic alloy has been processed by semisolid heat treatment, with the ultimate objective of developing a ductile and strong eutectic composite material for probable application as bearing materials and other sliding wear components. In this phase of work only microstructural characterises has been attempted. The as cast sample of Al–Cu alloy is subjected to isothermal heat treatment at a temperature of 560 °C for varying periods of time and followed by water quenched. The microstructures of the semisolid heat treated (SSH) samples are analysed using an optical microscope and scanning electron microscopy (SEM). The microstructures of the quenched Al-33%Cu alloy samples show lamellar to lamellar + dendritic transition after SSH treatment. The quenched microstructure after 20 min of holding time primarily consists of copper aluminide dendrites + supersaturated α–Al grains. A massive copper-aluminide (>80%Cu) intermediate phase is observed after 3 to 6 min SSH. The sample configuration did not change after semisolid heat treatment and water quenching. This suggest that complete remelting and overburning did not occur during the semisolid heat treatment. The SSH sample shows maximum hardness, after 10 min holding time compared to solution treated (at 560 °C) and aged (at 150 °C) sample of a standard Al-4.5%Cu cast alloy. The Transmission electron microscopy (TEM) micrograph of the as quenched 10 min. SSH sample shows rod shaped CuAl2 (θ) precipitates, a dislocation network, cellular subgrains and an extremely fine lamellar eutectic. Solute cluster probably Guinier–Preston (GP) zones, precipitates and fine α –Al grains was observed in the peak aged sample. The x-ray diffraction (XRD) analysis of aged SSH treated sample shows several intermetallic copper-aluminide phases in each sample.

Texture Hardening Observed in Mg–Zn–Nd Alloy Processed by Equal-Channel Angular Pressing (ECAP)

The addition of Nd significantly improves the mechanical properties of magnesium alloys. However, only limited amounts of Nd or other rare earth (RE) elements should be used due to their high price. In this study, a low-alloyed Mg–1% Zn–1% Nd (ZN11) alloy was designed and processed by hot extrusion and subsequent equal-channel angular pressing (ECAP) in order to achieve a very fine-grained condition with enhanced strength. The microstructure, texture, and mechanical properties were thoroughly studied. The microstructure after 8 passes through ECAP was homogeneous and characterized by an average grain size of 1.5 µm. A large number of tiny secondary phase precipitates were identified as ordered Guinier–Preston (GP) zones. Detailed analysis of the Schmid factors revealed the effect of the texture on deformation mechanisms. ECAP processing more than doubled the achieved yield compression strength (YCS) of the ZN11 alloy. Significant strengthening by ECAP is caused by grain refinement and the formation of ordered Guinier–Preston zones and particles of a secondary γ-phase.

Evolution of Guinier-Preston zones in cold-rolled Al0.2CoCrFeNi high-entropy alloy studied by synchrotron small-angle X-ray scattering

The synchrotron small-angle X-ray scattering (SAXS) technique was used to investigate the morphological evolution of Guinier-Preston (GP) zones in 80% cold-rolled Al0.2CoCrFeNi high-entropy alloy aged at 550 °C for different times. The hardness profile exhibited double-peaked aging behavior. HRTEM observations revealed the appearance of the GP zones in specimens aged for two peak aging times. The SAXS analysis demonstrated that, according to their evolutions of relative volume fraction and size, the two groups of GP zones could be divided into GP1 and GP2 zones from 1 h of aging. The corresponding peaks in volume fraction and in size of GP1 and GP2 zones were correlated to the first and the second peak aging, respectively, indicating precipitation hardening by GP1 and GP2 zones. In addition, mechanical properties measured by tensile tests showed a trend similar to that of the hardness variation. The increasingly large particles after 1 h of aging detected by SAXS were also observed by TEM in 100 h over-aged specimens and identified as an AlNi-rich L12 phase. Based on the combined investigations of SAXS and TEM, the precipitation sequence in 80% cold-rolled Al0.2CoCrFeNi alloy aged at 550 °C is GP1 → GP2 → L12.

Effects of multi-stage aging treatments on the precipitation behavior and properties of 7136 aluminum alloy

The solutionized 7136 Al alloy was heat treated by one-step aging, two-step aging, retrogression and re-aging (RRA) and RRA plus further aging treatment, respectively. The influences of different aging treatments on the precipitation behavior, electrical conductivity and tensile properties were investigated. The results showed that one-step aging treatment forms continuous grain boundary precipitates (GBPs), coherent Guinier-Preston (GP) zone and semi-coherent η’ grain interior precipitates (GIPs). By two-step aging treatment, the GIPs are coarsened and partly transformed into incoherent η phase, which result in a reduction of strength but an enhancement of ductility and electrical conductivity. RRA treatment makes the GBPs to be discontinuous and forms precipitates free zone (PFZ) near grain boundary. The RRA alloy shows similar strength but higher electrical conductivity in comparison with one-step aging. When the RRA alloy further aged at 160 °C, the strength keeps almost constant until 10 h, while the electrical conductivity is increased with increasing aging time. The PFZ is widened and the GIPs become coarser by further aging at high temperature. By contrast, the PFZ is eliminated, accompanying with the formation of GP(II) as further aged at low temperature of 70 °C, therefore a higher strength and lower electrical conductivity are obtained.

The Effects of Heat Straightening Temperature on the Microstructure and Properties of 7N01 Aluminum Alloy.

The 7N01 aluminum alloys are always used in vehicles, but heat treatment can deteriorate mechanical properties and corrosion resistance, which limits its utilization. In this paper, the influences of the temperature of heat straightening on the corrosion behavior and mechanical properties of 7N01 aluminum alloy are investigated. Most of the initial Guinier Preston (GP) zones dissolve into the matrix during heat treatment process, while the grain boundary precipitates have no obvious change. The precipitates of the samples after heat treatment mainly consist of high density GP zones due to the natural aging effect, which result in the recovery of micro-hardness. Although heat treatment decreases the mechanical properties of 7N01 aluminum alloy, there is no obvious difference in mechanical properties of the specimens after different heat treatment conditions. The corrosion resistance of heat treatment samples decreases significantly compared with the base metal, which is attributed to enhancing the difference between the potential of the alloy matrix and the grain boundary.

Age hardening responses of as-extruded Mg-2.5Sn-1.5Ca alloys with a wide range of Al concentration

This article aims to explore the age hardening responses of both as-extruded and as-aged Mg-2.5Sn-1.5Ca-xAl alloys (x = 2.0, 4.0 and 9.0 wt%, termed TXA322, TXA324 and TXA329, respectively) through microstructural and mechanical characterization. Results indicate that grain size of as-extruded TXA322, TXA324 and TXA329 alloys were ∼ 16 μm, ∼ 10 μm and ∼ 12 μm, respectively. A number of <a> and <c+a> dislocations were observed in all the as-extruded samples. Guinier – Preston (GP) zones were evidently identified in TXA322 alloy, while only a small number of Mg17Al12 phases existed in both TXA324 and TXA329 alloys. An aging treatment facilitated the precipitation of a high number density of GP zones within the matrix of TXA322 alloy. In contrast, no obvious nano-precipitates were in as-aged TXA324 alloy. Numerous nano-Mg17Al12 phases were formed through a following aging treatment in TXA329 alloy. In terms of mechanical properties, it is apparent that an increment in ultimate tensile strength of ∼ 46 MPa and ∼ 40 MPa was yielded in peak-aged TXA322 and TXA329 alloys, while no obvious variations in UTS were present in peak-aged TXA324 alloy, in comparison with the as-extruded counterparts.

Effect of Alternating Magnetic Field on the Fatigue Behaviour of EN8 Steel and 2014-T6 Aluminium Alloy

The application of an alternating magnetic field (0.54 T) was observed to lead to an improvement in the fatigue endurance and an increase in Vickers microhardness and tensile strength of both EN8 steel and AA2014-T6 alloy. Fractography using scanning electron microscopy showed evidence of more ductile fracture features after treatment in contrast to untreated samples. The results of X-ray diffraction indicated formation of more compressive residual stresses following treatment; while examination by transmission electron microscopy showed evidence of fewer dislocations. In the case of the AA2014-T6 alloy; Guinier-Preston (GP) zones were also generated by the alternating magnetic field. However; the temperature increase during the treatment was too low to explain these observations. The results were attributed to the non-thermal effect of the alternating magnetic field treatment that led to depinning and movement of dislocations and secondary precipitation of copper.

Temperature-dependent nucleation kinetics of Guinier-Preston zones in Al–Cu alloys: An atomistic kinetic Monte Carlo and classical nucleation theory approach

Guinier-Preston (GP) zones, which form in Al–Cu alloys, exhibit characteristic patterns of Cu-rich, disk-shaped atomic clusters along the {100} planes. GP zones have received much attention not only for their precipitation-hardening effect on the Al matrix, but also from fundamental interest in the physics and chemistry of the nanoscale organization of solute atoms. In this study, we established an atomistic kinetic Monte Carlo (kMC) modeling technique for exploring the nucleation and formation processes of GP zones in Al–Cu alloys by constructing an effective on-lattice multibody potential for a dilute Al–Cu-vacancy system by density functional theory calculations. Our model can describe the clustering of Cu atoms via successive exchanges with vacancies and reproduce the characteristic planar nanoprecipitates along the {100} planes in a manner consistent with the crystallographic nature of the GP zones in the early-stage aging of Al–Cu alloys. The time evolution and critical nucleus size of Cu clusters were characterized at various temperatures based on the kMC results. Consequently, we predicted the existence of an optimum temperature (i.e., nose temperature) for the formation of Cu clusters at which the cluster growth was maximized, which was attributable to the interplay between the critical nucleation barrier and the diffusion rate. In addition, the critical nucleus size and temperature for cluster formation were examined based on classical nucleation theory along with the developed multibody potential. These provided an insight into the competition between the enthalpic and entropic effects on the formation of GP zones in the Al–Cu system.

Influence of Nd addition on microstructures and mechanical properties of a hot-extruded Mg−6.0Zn−0.5Zr (wt.%) alloy

Influence of 1.5 wt% Nd addition on microstructures and mechanical properties of the as-extruded Mg−6.0Zn–0.5Zr (ZK60) alloy was studied. The results illustrate that Nd addition clearly refines dynamic recrystallized grains but has almost no influence on texture. Additionally, the intermetallic phases were significantly altered by Nd addition. In ZK60 alloy, the coarse intermetallic phases are MgZn2, Mg21Zn25, Mg4Zn7, MgZn and I-phase, while MgNdZn3 (W), Mg43Zn51Nd6 (T), MgZn2 and Nd-enriched MgZn after Nd addition. Also, three kinds of new orientation relationships (ORs) between MgZn2 or its twin domain and Mg were revealed in this work while both MgZn2 and W were found to be coherent with T following identical ORs. Furthermore, the fine precipitates are with sizes of ∼20 nm and were confirmed as MgZn2, MgZn2 + MgZn and Mg4Zn7 + MgZn2 in ZK60 alloy while new spherical Guinier Preston (G.P.) zones with sizes of ∼5 nm after Nd addition. Finally, the above microstructural modifications because of Nd addition result in ultra-high strength for ZK60 alloy, with tensile yield strength being approximately 408 MPa.

Coherent strain of Guinier–Preston II zone in an Al–Zn–Mg–Cu alloy

The Guinier-Preston (GP) II zone in an Al–Zn–Mg–Cu alloy was investigated by using a high-resolution electron microscopy (HREM). We used Geometric Phase Analysis (GPA) technique to measure the experimental HREM image and the simulated HREM image to obtain the strain distribution maps. The results show converging strain fields in the GP II zone, and the maximum compressive strain can reach −7.2% and −6.7%, respectively. The distribution of the experimental strain field was in consistence with that of the simulation result. The average strain of the alloy measured using XRD is 0.191%.