GP Zones Research Articles

Precipitation behavior and mechanical properties of Mg-Nd-Sm-Zn-Zr alloy

A novel Mg-Nd-Sm-Zn-Zr alloy with promising age-strengthening effects has been successfully developed to meet the urgent needs of lightweight in the aerospace industry. The precipitation behavior and elastic properties of the nano-precipitates in the alloy have been revealed using transmission electron microscopy and first-principles calculation. The Guinier-Preston (GP) zones, lamellar γ′ precipitates, and granular β′ precipitates formed in the α-Mg matrix after aging at 190 °C for 3 h (under-aging stage). Notably, the alloy reached the peak-aging stage and achieved an excellent ultimate tensile strength of 266 MPa after aging for 18 h. These precipitates further transformed into the lamellar β′ and β1 precipitates, and the enclosed nano-strengthening structure was constructed by γ′, β′ and β1 precipitates. The precipitation sequence of the aged alloy was determined as the supersaturated solid solution (SSSS) → GP zones → β′/γ′ → β1. First-principles calculations and 3D visualization approach demonstrated that the addition of mixed light RE elements can effectively improve the elastic modulus and reduce the elastic anisotropy of β′ nano-precipitates. The Young's modulus (E), shear modulus (G), and anisotropy index (AU) of β′-Mg7(Nd,Sm) precipitates were 48.38 MPa, 18.79 MPa and 2.29, respectively. This study provides a theoretical and experimental basis for further research and development of novel high-performance Mg-light RE alloys.

On the Use of a Cluster Identification Method and a Statistical Approach for Analyzing Atom Probe Tomography Data for GP Zones in Al-Zn-Mg(-Cu) Alloys.

Early-stage clustering in two Al-Mg-Zn(-Cu) alloys has been investigated using atom probe tomography and transmission electron microscopy. Cluster identification by the isoposition method and a statistical approach based on the pair correlation function have both been applied to estimate the cluster size, composition, and volume fraction from atom probe data sets. To assess the accuracy of the quantification of clusters of different mean sizes, synthesized virtual data sets were used, accounting for a simulated degraded spatial resolution. The quality of the predictions made by the two complementary methods is discussed, considering the experimental and simulated data sets.

The influence of natural aging on the precipitation behavior of the low-alloy content Al-Zn-Mg aluminum alloys during subsequent artificial aging and related mechanisms

The age hardening and precipitation behavior of the low-alloy content and widely used Al-Zn-Mg alloy during artificial aging was studied adopting methods of Vickers hardness testing and transmission electron microscopy. To meet with practical application of the production line conditions, the influence of natural aging (NA, at room temperature) on the precipitation behavior during subsequent artificial aging (AA, at 130 °C∼150 °C) and related mechanisms are discussed based on the classical nucleation theory. The results show that precipitates like solute clusters and GP zones will form during the NA process, which have a significant influence on the subsequent precipitation behavior during AA. These NA precipitates will dissolve at the initial stage of AA resulting in abundant localized supersaturated regions which are the favorable nucleation sites for η′ phase. This positive effect is highly dependent on the temperature adopted during the AA process and the duration of NA. Higher AA temperatures result in a more noticeable increase in hardness compared to the samples without NA but require a longer duration of AA to achieve a relatively high and stable peak-aging hardness. However, an excessive long NA period can lead to a significant drop in hardness in the over-aging status. In the low-alloy content Al-Zn-Mg alloys, the actual nucleation rates of precipitation during the early stages of AA vary significantly with the AA temperature, which serves as the underlying cause of the observed NA effect. These results provide rather an important guidance for the production process of the 7003alloy and other aluminum alloys with similar chemical composition.

Effects of Sn addition on precipitation of a pre-naturally aged Al–Zn–Mg alloy during artificial aging

The effect of Sn addition and pre-natural aging on the artificial aging behavior of Al–Zn–Mg alloy was investigated using both experimental approaches and first principle study. The result shows that Sn retards hardness increase at the initial stage of artificial aging by inhibiting the formation of Guinier-Preston zones (I). Atom probe tomography and density functional theory have revealed that the high binding energy of Mg-Sn disturbs Mg-Zn clustering and delays the formation of Guinier-Preston zones (I). However, the retardation effect of Sn on the artificial aging was found to be suppressed when the pre-natural aging was applied to the Al–Zn–Mg–Sn alloy: hardness at the initial stage increased rapidly compared to the Al–Zn–Mg alloy with pre-natural aging. The reason for the rapid increase of hardness in the Al–Zn–Mg–Sn alloy with pre-natural aging is that Guinier-Preston zones (II) was directly formed from vacancy-rich clusters in the Al–Zn–Mg–Sn alloy in contrast to the Al–Zn–Mg alloy with pre-natural aging. This means Mg-Sn and Zn-rich cluster formed during pre-natural aging can accelerate precipitation kinetic by providing nucleation site for Guinier-Preston zones (II) during artificial aging.

Strengthening effects of GP zone induced by SiCp size variations in Al–Si–Mg matrix composites

In this work, SiCp reinforced Al–Si–Mg matrix composites were prepared by stirring casting with different SiCp sizes. The effect of SiCp sizes on the microstructure and mechanical properties was investigated, and different strengthening effects were systematically calculated and analyzed. With the decrease of SiCp sizes, the microstructure of composites is refined and the plasticity is improved significantly. The refined SiCp also leads to a more intense grain refinement strengthening and thermal expansion mismatch strengthening. However, an abnormal decrease of yield strength is discovered with the refinement of SiCp, which is due to the influence of SiCp on GP zones strengthening in the matrix. As SiCp sizes decreasing, the increased SiCp/Al interfaces capture more Mg atoms, and then inhibits the formation of GP zones in the α-Al. Therefore, the refined SiCp weaken the precipitation strengthening in composites. These acicular Mg–Si GP zones exhibit a preferentially growing direction along <100>Al and present a same structure as α-Al with coherent interfaces which pin dislocations and provide precipitation strengthening. The high density of GP zones also increases nano-hardness and elastic modulus of α-Al, which improves the wear resistance of SiCp/Al–Si–Mg composites. Therefore, it is suggested that the interaction between the SiCp and precipitates in matrix should also be considered to obtain a suitable strengthening effect in the design of SiCp/Al alloy composites.

Effects of cooling rate in homogenization on microstructure, hot deformation resistance and subsequent age-hardening behavior of an Al–Mg–Si alloy

In this paper, the microstructure evolution in an Al–Mg–Si alloy during the soaking and cooling of homogenization was investigated. Moreover, the effects of the cooling rate on the hot deformation and subsequent age-hardening behavior were studied. During the soaking, the eutectic Mg2Si is dissolved, the solute segregation is eliminated, and the Fe-bearing phase is refined. During subsequent cooling, no precipitate is formed in the water-quenched sample, while β'' and β-Mg2Si are the predominant phases formed in the air- and furnace-quenched samples, respectively. The experiments agree with the simulated continuous cooling transformation (CCT) curves. The cooling rate in homogenization has a significant effect on the flow stress when the compression is performed at 450 °C. The air-quenched sample exhibits the highest flow stress due to the formation of nanoscale β-Mg2Si particles. The effect of the cooling rate diminishes as the compression temperature increases. At a compression temperature of 500 °C, the water-quenched sample exhibits a slightly higher flow stress due to the precipitation of micrometer-scale β-Mg2Si particles and the Mg and Si solutes retained in the matrix. The compressed samples have nearly similar thermal effects and peak aging hardness increments after compression at 500 °C, indicating that the age-hardening potential of the alloys is equivalent. The same types of precipitates are formed in all compressed samples at peak aging, including the GP zone, β'', β′ and B′, where β'' is the main strengthening phase. The relationship between the precipitation behavior and the increments of hardness, electrical conductivity (EC) was analyzed.

Influence of temperature on the precipitation phase transformation behavior of RR350 aluminum alloy during artificial aging and related mechanisms

This study investigates the influence of artificial aging temperature on the evolution of precipitation phases and age hardening behavior of the RR350 aluminum alloy. The manner of nucleation and growth of different kinds of precipitates and phase transformation between them are analyzed, along with the corresponding mechanisms are discussed. The results reveal that the aging temperature affects little the precipitation process of precipitates which still follows the sequence of GP zones - θ″- θ′ in the RR350 aluminum alloy during artificial aging, but influences significantly the precipitation speed and phase composition, morphology, size and size distribution of the resultant precipitates in the alloy. An imperceptible relationship between the hardness of the sample and the composition of the precipitates were discovered. As the artificial aging temperature (160–180–200 °C) increases, the peak-aging hardness of the samples decreases and the time for reaching the peak-aging status shortens gradually. When the temperature is low, 160 °C for example, the precipitates in the peak-aging status are mainly composed of θ″ phase and some minor θ′ phase. As the temperature increases to 200 °C θ′ phase becomes prevalent. As a result, the alloy shows lower peak-aging hardness since the strengthening effect of the finer θ″ phase is much stronger than that of the θ′ phase. Increasing the aging temperature accelerates the competitive processes of nucleation of GP zones, growth of all precipitates, and the phase transformation. At higher temperatures, phase transformation from θ″ to θ′ overweighs the formation of GP zones, growth of both θ″ and θ′ phases, resulting in more amount of θ′phase in the peak-aging and over-aging statuses.

A Novel Precipitation‐Strengthened Conductive Copper Alloy: Cu–Hf–Si Alloy

The Cu–1.1 wt% Hf–0.1 wt% Si alloy containing various precipitates is studied as a novel precipitation‐strengthened alloy with high strength and high electrical conductivity. The Cu–Hf–Si alloy prepared by solution, cold rolling, and aging processes exhibits a hardness of 238.92 HV, a tensile strength of 691.01 MPa, a breaking elongation of 15.09%, and an electrical conductivity of 62.34% International Annealed Copper Standard. The optimal peak aging parameter for Cu–Hf–Si alloy is 430 °C for 90 min. The precipitation of Hf and Si atoms from the Cu matrix is helpful to enhance the electrical conductivity. The precipitation of Hf can produce fibrous Guinier Preston zone and γ' precipitate, as well as spherical Cu10Hf7 phases. In addition, the coprecipitation of Si and Hf can also produce Hf3(Cu2Si)2 and HfCuSi phases. The high strength of Cu–Hf–Si alloy is mainly due to the multiprecipitates synergistic strengthening.

Mechanisms related to the influence of natural aging on the precipitation behavior of the A357 aluminum alloy during artificial aging

This study investigated the influence of natural aging (NA) on the precipitation behavior of the main precipitate β″ and hardness evolution during the subsequent artificial aging (AA) process of the precipitation strengthened A357 aluminum alloy. The corresponding mechanisms were analyzed. The results demonstrate that natural aging has adverse influence on the precipitation process during artificial aging. The underlying mechanism is that small GP zones are formed during natural aging but cannot become nucleation sites for the β″ phase during artificial aging, which spontaneously consumes the supersaturated solute atoms in the Al matrix, increases the nucleation barrier of the β″ phase, and also consumes the vacancies produced after solid solution and quenching, thereby reducing the nucleation rate of the β″ phase during artificial aging, and leading to an un-expected increase in the size of the precipitated phase, thus reducing the peak aging hardness of the alloy. This effect becomes more pronounced with increasing natural aging time. Increasing the artificial aging temperature can improve the nucleation rate of the β″ phase during artificial aging, thereby to some extent reducing the unfavorable influence of natural aging on the artificial aging by reducing the sizes and increasing the amount of the β″ precipitates.

Electron microscopic investigation of precipitation hardening in Al-Si based alloys

The present work was performed on Al-8.5%Si-1.75%Cu-0.55%Mg-0.2%Ti-0.32%Zr-0.015%Sr alloy (coded as 354 alloy) following T6 (at 160 °C) and T7 (at 240 °C) artificial aging conditions using high resolution transmission electron microscopy (HR-TEM). The main results inferred from the present study are the precipitation of GP zones in samples aged for 20 h at 160 °C along with S-Al2CuMg phase particles. Aging at 240 °C was carried out for different times, ranging between 30 min and 200 h. The quenched structures revealed a series of precipitations mainly GP zones at low aging times to Θ’-Al2Cu after 200 h together with the S-phase. No coherency was observed between the S-phase and the aluminum (Al) matrix. The growth of the S- phase takes place by precipitation of S2 type phase on the advancing S1-type phase. Mismatching was clearly observed at particle/matrix interfaces. High magnification images indicated the dissolution of some of the Θ” and Θ’ particles to enrich the nearby particles (coarsening). The start of incoherency is observed when the alloy is aged at 240 °C for 200 h.

Detailed investigation on microstructure and strengthening contribution of Al-xCu-1.3Li-X alloy sheets

The microstructure-mechanical properties connection and strengthening contribution of Al-xCu-1.3Li-X (x = 3.8, 4.2, 4.5 wt%) alloy sheets subjected to various heat treatments were systematically investigated. After solid-solution treatment, the decreased schmid factor value was the main factor for higher strength of Alloy-3 (4.5 wt% Cu). The number density and diameter of T1 precipitates simultaneously increased with rising the Cu content during T8 aging, leading to the highest strength (655.4 MPa) of T8-aged Alloy-3. However, the high Cu content level resulted in the preferential precipitation of Guinier-Preston (G.P) zones during T6 aging at 165 °C. The θ' precipitates were therefore significantly increased and the nucleation of T1 precipitates was suppressed in T6-aged Alloy-3, causing the reduced strength compared to that of T6-aged Alloy-2 (4.2 wt% Cu). Furthermore, the experimental yield strength was successfully predicted by invoking the modified precipitate strengthening model, in which the θ' precipitate-dislocation interaction was by-passing mechanism, whereas the shearable and by-passed T1 precipitates coexisted for all alloys. The strengthening transition from by-passing to shearing occurred in T1 precipitates with a thickness <1.9 nm or diameter <65 nm, and was particularly associated with the decrease in thickness of T1 precipitates.

Grain structure and co-precipitation behavior of high-Zn containing Al–Zn–Mg–Cu aluminium alloys during deformation via high-temperature upsetting-extrusion

A study on the deformation behavior with increase in strain during upsetting-extrusion (UE) was studied (i.e., No-UE, Small-UE, and Large-UE strains) at 350 °C in Al-8.9Zn-2.2Mg-2.1Cu-0.14Zr-0.02Sc (wt%) alloys to evaluate the strengthening effects of grain size/texture and co-precipitation for different ageing (i.e., as-received, T4, and T6 aging) conditions. In as-received samples, electron back-scattered diffraction observations revealed that increasing UE strain enhances the formation of continuous and discontinuous dynamic recrystallization grains. This reduces the size difference between fine and coarse grains in bimodal structure and produces stronger copper/cubic texture. In small angle X-ray scattering (SAXS), a large UE strain brings about more fraction of Y-phase and GP zones. In T4 Large-UE samples, SAXS indicated higher size stability of disk-like Y-phase. High-angle annular dark field scanning transmission electron microscopy images revealed five-layered atomic arrangement of a unit cell of Y-phase, where the center layer was composed of Al and sandwiched layers, 2Cu/2Zn/2Cu and Al/ZnCu/ZnCu/Al. In T6 Large-UE samples, it was observed that the growth of Y-phase is accompanied by intrinsic atomic shift along the [101̅0]Y-phase direction. Furthermore, SAXS quantitatively showed size evolution of η'/η precipitates and formation of a higher aspect ratio of Y-phase. The collective effects of finer bimodal grain structure, stronger fiber texture, and co-precipitation of Y-phase and η'/η precipitates provide evidence higher ultimate tensile strength (∼766 ± 5 MPa) and superior strain (∼10.9 ± 0.4) of large-UE strain samples under T6 ageing.

The precipitates and properties evolution behaviors of AlZnMgCu alloy during the retrogression process with slow heating

In order to develop the retrogression and re-aging (RRA) effect on the performance of AlZnMgCu alloy products with large scale, the hardness, electrical conductivity, tensile properties, and exfoliation corrosion resistance of the Al7.7Zn2.1Mg2.0Cu alloy were investigated at two pre-aging temperatures and a 3 °C/min retrogression heating rate. Transmission electron microscopy (TEM) was used to investigate the evolutions of precipitates at different RRA routes. Results revealed that the standard compliant performances could also achieved by setting the pre-aging as the under-aged state when the slow retrogression heating rate existed. Continuous precipitation occurred at the early stage of heating retrogression. The dissolution behavior of the GP zone with the radius lower than the critical value had finished in heating retrogression. The η′ phase re-precipitated firstly in the isothermal retrogression, then grew and coarsened with the retrogression time increase. During the whole retrogression including heating and isothermal stages, the hardness undergone an “up-down-up-down” transversion. After RRA-treated at (105 °C, 24 h + (heating to 190 °C at 3 °C/min)190 °C, 50 min +120 °C, 24 h), excellent corrosion resistance achieved with only a 5.5% sacrifice of tensile strength when compared to peak aged state. Due to the slow temperature-rising process in retrogression for the large section products, the optimized RRA schedule is more practical for industrial applications.

Accelerating the precipitation kinetics of nano-sized T1 and S’ phases in Al-Cu-Li alloys by hot-deformation and creep-aging

ABSTRACT It is well known that a Cu/Li ratio over 4.0 leads to significant precipitation hardening in the third generation of Al–Li alloys, which have been widely used in aerospace and aeronautic industries. However, increasing Cu levels not only cancels some gains in weight reduction but also results in reductions in ductility when the traditional age-hardening process is adopted. In this study, we show that higher strength and ductility can be achieved simultaneously by replacing traditional artificial aging with new creep aging and tailoring the entire process from casting to age hardening without changing any nominal chemical compositions. Surprisingly, the precipitation kinetics of AA2060 alloy is significantly enhanced through the combination of hot extrusion, creep aging, and pre-strain compared to traditional artificial aging. For example, at 160°C, it follows the transformation path of supersaturated solid solution (SSSS)→GP zones + δ′+T1 precursors which will further transform into T1 +S’; at 200°C, SSS→ GP zones, and they transform into T1 + S’ immediately. The precipitation of θ′ and δ′ phases is significantly inhibited at elevated temperatures due to the competing relationship with T1 and S’ phases at a peak-aged state. Contrary to traditional knowledge in Al–Li alloys, we have found that the greater extrusion ratio at 33.0 and higher aging temperature at 200°C can give a much higher peak aging strength with average ultimate tensile strength 659 MPa.

Influence of Quenching and Subsequent Artificial Aging on Tensile Strength of Laser-Welded Joints of Al–Cu–Li Alloy

The research aim was to optimize post-weld heat-treatment (PWHT) modes for a laser-welded joint of the Al–Cu–Li alloy and improve their respective strength properties. As a result, the ultimate tensile strength, yield point, and elongation of the joint were enhanced up to 95%, 94%, and 38%, respectively, of those inherent in the base metal. Before and after PWHT, both microstructures and phase compositions have been examined by optical and scanning electron microscopy, as well as synchrotron X-ray diffractometry. In the as-welded metal, the α-Al and T1(Al2CuLi) phases were found, along with the θ′(Al2Cu) and S′(Al2CuMg) phases localized at the grain boundaries, significantly reducing the mechanical properties of the joint. Upon quenching, the agglomerates dissolved at the grain boundaries, the solid solution was homogenized, and both Guinier–Preston zones and precipitates of the intermediate metastable θ″ phase were formed. After subsequent optimal artificial aging, the (predominant) hardening θ′ and (partial) T1(Al2CuLi) phases were observed in the weld metal, which contributed to the improvement of the strength properties of the joint.

Effects of Sn addition on precipitation of Al–Zn–Mg alloy at early stage of natural aging

The effect of Sn addition on the natural aging behavior of Al–Zn–Mg alloy was investigated using atom probe tomography (APT) experiments and density functional theory (DFT) calculations. The results demonstrate that Sn retards the natural aging behavior by facilitating the preferential formation of Mg–Sn clusters, which suppress the formation of Guinier-Preston zones (I) during the earliest aging stage. Quantitative solute analysis conducted by APT has suggested increasing amount of Mg–Sn clusters in Sn-contained alloy due to their high binding energy, supported by DFT calculations on the Sn.

A Novel Al-Cu Composite with Ultra-High Strength at 350°C via Dual-Phase Particle Reinforced Submicron-Structure.

Thermal stability determines a material's ability to maintain its performance at desired service temperatures. This is especially important for aluminum (Al) alloys, which are widely used in the commercial sector. Herein, an ultra-strong and heat-resistant Al-Cu composite is fabricated with a structure of nano-AlN and submicron-Al2 O3 particles uniformly distributed in the matrix. At 350°C, the (8.2AlN+1Al2 O3 )p /Al-0.9Cu composite achieves a high strength of 187MPa along with a 4.6% ductility under tension. The high strength and good ductility benefit from strong pinning effect on dislocation motion and grain boundary sliding by uniform dispersion of nano-AlN particles, in conjunction with the precipitation of Guinier-Preston (GP) zones, enhancing strain hardening capacity during plastic deformation. This work can expand the selection of Al-Cu composites for potential applications at service temperatures as high as ≈350°C.

Atomic-scale investigation of Si/Al interface in multilayer aluminum sheets using HRTEM and first-principle calculation

The atomic-scale structure and electronic structure of the interface between Si precipitates and the Al matrix in multi-layered aluminum sheets are investigated by combining high-resolution transmission electron microscopy (HRTEM) with first-principles calculation. The results show that Si needle precipitates and Guinier Preston (GP) zones occur during the natural aging process of the core material in automotive heat exchangers. In addition, Al(Fe,Mn)Si precipitate can serve as effective heterogeneous nucleation sites for the formation of high-density Si precipitates during aging. The charge density differences analysis shows that the formation of covalent bonds between interfacial Si and Al atoms is crucial to the interfacial bonding strength. These results provide important insights for future research efforts focused on the production of multilayer aluminum sheets with high-performance.

Observation of in-plane oriented Guinier-Preston zones in Al-Cu

Known for a long time, the first metastable precipitate which forms in Al-Cu alloys during natural ageing are the so-called Guinier-Preston zones (GPZ), platelets of Cu on {100} planes of only one atomic layer thickness. Only with the development of aberration corrected transmission electron microscopes (TEM), direct observation and imaging of these platelets was possible, but with the restriction, that only edge view was possible. Here we show that under appropriate conditions an observation in a plan-view is possible and allows further insight into the shape and arrangement of the GP-zones. Furthermore, this demonstrates that single atom detection of one Cu atom in a column of Al atoms is possible.

Enhanced age-hardening of an Al–5Mg–2Zn–1Cu alloy by pre-aging combined with pre-straining

In this work, the synergistic effect of pre-aging and pre-straining on the enhanced age-hardening response of an Al–5Mg–2Zn–1Cu alloy has been systematically investigated using high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). After pre-aging, only Guinier Preston (GP) zones uniformly distribute in the α-Al matrix. Moreover, dislocations induced by pre-straining can promote the nucleation and growth of T″ and T′ precipitates. With the synergy of pre-aging (80 °C/5 h) and pre-straining (∼4%) treatments, the alloy after second-aging at 130 °C for 12 h exhibits a giant increase of yield strength (∼182 MPa) in comparison to the as-solutionized alloy, reaching ∼348 MPa. The above results show that pre-aging and pre-straining are effective in accelerating the hardening response of Al–Mg–Zn–Cu alloys. Especially, this extraordinary age-hardening response depends strongly on the high number density and uniform distribution of the GP zones induced by pre-straining, acting as preferential nucleation sites for T″ phase and T′ phases during subsequent aging treatment. In addition, it proves that pre-aging and pre-straining can effectively control the precipitation free zones (PFZs) size, relieving the strain concentration in PFZs and improving the comprehensive mechanical properties.