No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effect of recrystallization on plasticity, fracture toughness and stress corrosion cracking of a high-alloying Al-Zn-Mg-Cu alloy
Abstract
The plasticity, fracture toughness and stress corrosion cracking resistance of a high-alloying Al-Zn-Mg-Cu alloy with various recrystallization degrees introduced by solution treatment were investigated. The results exhibited that the elongation increased from 14.73% to 16.38% with the deepening of recrystallization degree from 8.80% to 20.20%, as well as the enhancement of fracture toughness (KIC) from 25.7 MPa·m1/2 to 28.9 MPa·m1/2. On the contrast, the critical stress intensity factor for stress corrosion cracking (KISCC) declined from 29.9 MPa·m1/2 to 25.0 MPa·m1/2. The recrystallization benefited plasticity and fracture toughness by not only breaking the tight connection between two deformed grains and eliminating the dislocations density but also increasing the “lubricating” thin grain boundary phases. Meanwhile, it degraded stress corrosion cracking resistance due to the increase of coarsening precipitated particles distributed on recrystallized grain boundaries. This work revealed the opposition trend among properties by modifying recrystallization in the high-alloying Al-Zn-Mg-Cu alloy.
... Al-Zn-Mg-Cu cast alloys have been widely used for aeronautical and automotive applications due to their excellent combination castability, mechanical properties, and formability [1][2][3][4][5]. These alloys are mostly fabricated using ingot metallurgy, during which casting defects, such as shrinkage and pores, may develop and deteriorate the mechanical properties of the final product [6][7][8]. ...
... Furthermore, the equiaxed cell size of the as-cast specimens also decreased after the heat treatment, as shown in Figure 1d. Figure 2 and Table 1 show the EDS elemental mapping analysis of the G-and E30-series specimens in as-cast and heat-treated conditions. Figure 2a,b shows that secondary phases present on the dendritic cell boundaries of the G-A and E30-A specimens are Al 7 Cu 2 Fe and Al 2 (ZnMgCu) 3 . In Al-Zn-Mg-Cu alloys, solute atoms tend to segregate on dendritic cell boundaries during solidification and form Zn-, Mg-and Cu-rich phases. ...
... In Al-Zn-Mg-Cu alloys, solute atoms tend to segregate on dendritic cell boundaries during solidification and form Zn-, Mg-and Cu-rich phases. However, Cu extends its solubility in the Zn-Mg-rich intermetallic phase, i.e., the Al 2 (ZnMg) 3 -T phase, changing its composition to Al 2 (ZnMgCu) 3 [35]. ...
The effects of nanoprecipitations on the mechanical properties of Al-Zn-Mg-Cu alloys after GBF (gas bubbling filtration) and EMS (electromagnetic stirring) casting were investigated. Dendritic cell structures were formed after GBF processing, while globular dendritic structures were nucleated after EMS processing. Equiaxed cell sizes were smaller in the EMS-processed specimens compared to the GBF-processed specimens, confirmed by EBSD (electron backscatter diffraction) analysis. Nanoprecipitations of η′ phases inside of dendrites were observed by TEM (transmission electron microscope), and other Fe-bearing compounds were located in the dendritic boundaries. The yield strength of the T4 and T6 heat-treated specimens was close to 400 MPa and 500 MPa, respectively. Fractographic analysis was performed to investigate the effect of precipitations on tensile fracture.
... According to the comparison of yield strength (σ y ) and J-inegralbased fracture toughness (J Ic ) for NL Zr-2.5Nb to other alloys in Fig. 2 (c), it is evident that the J Ic of NL Zr-2.5Nb exceeds that of most metallic materials, including Zr alloys (highlighted with a red background) and most other alloys (highlighted with a blue background) [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. In addition, when further compare their K-based fracture toughness (K C ), as shown in Fig. 2 (d), the introduction of a hierarchical nanolayered structure enables the fracture toughness of our NL Zr-2.5Nb to surpass that of almost all reported Zr alloys, while reaching the exceptional strength-fracture toughness synergy. ...
... The CL Zr-2.5Nb alloy also displays a similar semibrittle fracture mode, but with more dimples present (Fig. 3 (c, d)). In contrast, the NL Zr-2.5Nb alloy displays a fully ductile fracture mode, [25][26][27][28][29], austenitic steel [30], carbon steel [31], Cu alloys [32,33], Al alloys [34,35], Mg alloys [36][37][38], Ti alloys [39][40][41], and Ni alloys [42,43]. The J Ic values of other metallic materials in (c) are back-calculated from their K c values, J Ic = K 2 c /E ʹ . ...
As a widely used nuclear structural material, zirconium (Zr) alloys are susceptible to brittle hydride-induced cracking. The intrinsic inability to arrest this cracking tendency in Zr poses a significant threat to the service safety of Zr cladding tubes. Here, we propose a microstructural design strategy, by introducing a hierarchical nanolayered duplex-phase structure in the Zr-2.5Nb alloy, to effectively inhibit the propagation of cracks. An unprecedented fracture toughness of K JIc ~ 165 MPa⋅m 1/2 , corresponding to J Ic ~ 256 kJ/m 2 , is achieved due to the uniform and dense plastic deformation that develops ahead of the crack tip, which is uncharacteristic of Zr-based materials. Our analysis reveals that the effective crack tip blunting occurs because the high density of multiply oriented α/β interfaces in the hierarchical Zr-2.5Nb alloy stimulates ample <c+a> dislocations and facilitates deformation twin nucleation, both of which are challenging to activate in conventional Zr-based materials at room temperature. The exceptional fracture toughness enabled by the hierarchical nanolayered structure provides a new pathway to designing damage-tolerant hexagonal metals for safety-critical applications.
... This is attributed to the significant potential difference between GBPs and PFZs at GBs and the matrix. In the corrosive medium, GBPs and PFZs with lower potentials are preferentially dissolved as the anode, forming preferential pathways for anodic dissolution along GBs [34], resulting in corrosion cracks propagating along GBs, as illustrated in Figure 11. Based on the experimental results, the schematic diagrams of EXCO propagation for EG and BG samples under different cooling rates are shown in Figure 13. ...
The influence of grain structure and quenching rates on the exfoliation corrosion (EXCO) susceptibility of 7085 alloy was studied using immersion tests, optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The results show that as the cooling rate decreases from 1048 °C/min to 129 °C/min; the size of grain boundary precipitates (GBPs); the width of precipitate-free zones (PFZ); and the content of Zn, Mg, and Cu in GBPs rise, leading to an increase in EXCO depth and consequently higher EXCO susceptibility. Meanwhile, there is a linear relationship between the average corrosion depth and the logarithm of the cooling rate. Corrosion cracks initiate at the grain boundaries (GBs) and primarily propagate along the HAGBs. In the bar grain (BG) sample at lower cooling rates, crack propagation along the sub-grain boundaries (SGBs) was observed. Compared to equiaxed grain (EG) samples, the elongated grain samples exhibit larger GBPs, a wider PFZ, and minor compositional differences in the GBPs, resulting in higher EXCO susceptibility.
... These seemed to back the positive effect of grain refinement. As for fracture toughness, Schwalbe [10] had asserted that moderate grain size with low aspect ratio led to higher fracture toughness values than large equiaxed grains, which was also furtherly proved by the research on recrystallization grains of Yu [11]. However, Ritchie [12] had summarized the conflict between strength and fracture toughness, which refuted the above discoveries. ...
In present work, a high Zn-containing Al-Zn-Mg-Cu alloy with different grain sizes was fabricated by extrusion and related precipitation characteristics and mechanical property were investigated after uniform heat treatments. The results showed that precipitation characteristics for the three alloys were almost the same. Matrix precipitates were GPII zone and η' phase and possessed small size and dense distribution while grain boundary precipitates exhibited discontinuous distribution. The rank of strength and fracture toughness for the three alloys are SG>MG>LG. Tearing ridges had been found on all the fracture surface while only LG alloy possess obvious dimple characteristics. The a-N curve showed that crack length list is MG>LG >SG under a same cycle number. The da/dN-ΔK curve also proved that fatigue crack propagation (FCP) rate of MG alloy is slightly larger than that of LG alloy, both were apparently larger than that of SG alloy. The width of fatigue striations on FCP fracture surface also backed it. Besides, obvious transgranular cracking characteristics and apparent secondary cracks were found on the FCP fracture surface.
... The development of high-strength lightweight body materials is crucial for vigorously promoting energy conservation and pollution reduction in the automobile industry. Given its relatively high specific strength and ductility, good toughness and corrosion resistance, the 7xxx-series aluminium alloy exhibits significant lightweight advantages [1,2]. However, forming a 7xxx-series aluminium alloy sheet at room temperature is practically impossible. ...
Application of HFQ® technology to 7xxx-series alloy sheet has attracted increased attention because of the rapidly growing demand for lightweight automobiles. The present research employed the AA7075-H18 cold-rolled sheet suitable for continuous HFQ® production as the investigation object for the first time. Tensile experiments were performed using a Gleeble-3800 thermo-mechanical simulator within the temperature range of 300 °C –450 °C and strain rate range (0.001 s−1–10 s−1) to characterise the plastic flow behaviour under the application of HFQ® conditions. The tensile samples were soaked at the solution temperature, after which they were quenched to the target temperature and immediately tested. Static recovery (SRV) and static recrystallization (SRX) occurred during the application of solution heat treatment (SHT) process. The modified Zerilli-Armstrong (ZA) constitutive model of AA7075 under conditions representative of HFQ® was developed based on the Gleeble test data. At elevated temperatures, the AA7075 alloy exhibited noticeable temperature and strain rate sensitivity. The plastic flow stress of the sheet decreased substantially with the increase in deformation temperature or the decrease in strain rate, and the elongation after fracture slightly increased in parallel with the deformation temperature or strain rate. Microstructural evolution and plastic flow curves confirmed that dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred at elevated temperatures. The proposed ZA equation and Yld2000-2d yield function were applied to model the HFQ® process of a cup-shaped part, and the numerical results were in good agreement with the experimental data.
... The authors concluded that the incorporation of rare earth Ce improved the corrosion resistance remarkably, mechanical properties improvement and homogeneous microstructure. Yu et al. [9]investigated the plasticity behaviour, fracture toughness, and stress corrosion cracking performance of Al-Zn-Mg-Cu alloy recently. The hot rolled alloy of Al-Zn-Mg-Cu has taken for investigation, the prepared samples were solution treated in the furnace, quenched in water at room temperature, and then the samples were subjected to aging at 393k for 24 h. ...
The present research goal is to manufacture the novel Al-x wt. % Zn die-casting alloys through liquid metallurgy route. The amount of Zn content was varied from 0 to 30. % with a step of 10 percentages in the pure Al, The cast samples were squeezed under semi-solid condition (temperature ~500oC) and then all the samples were extruded at a temperature of 673k to achieve improved density of at least 95 % and above. The microstructures of developed Al-Zn die-casting alloys were analysed using advanced electron microscope. The microstructures results revealed the formation of primary -Al phase and (+η) phase with η-agglomerates for higher amount of Zn content (greater than 20 wt. %). The consolidated Al-Zn alloys were mechanically tested by three-point ending test to examine the flexural behaviour of developed alloys. The flexural results explained that Al-20Zn alloy exhibited improved mechanical performance compared to other alloys. Maximum flexural strength of 196 MPa with the flexural strain of 7.48 was achieved in Al-20Zn alloy. Hence, Al-20Zn alloy is recommended to use for structural and automotive parts in real time application.
... Al-Zn-Mg-Cu alloy have been extensively used in aerospace applications due to its excellent specific strength [1,2]. With the increasing demands for superior strength, development of these alloys tends to high-alloying for the enrichment of precipitations [3]. However, increased Zn and Mg content would greatly reduce the corrosion resistance of these alloys [4,5]. ...
The corrosion resistance (exfoliation corrosion and inter-granular corrosion) and mechanical properties (strength and hardness) of a high-alloying Al-Zn-Mg-Cu-Zr alloy were improved by the synergistic effect of Ce addition and aging treatment. Ce addition promotes the morphology change of grain boundary precipitates from continuous form to discontinuous form at T6 temper. But the Cu content in grain boundary precipitates became much lower than that in Ce-free alloy, owing to a large amount of Cu being trapped in AlCuCe phase. Hence retrogression and re-aging (RRA) treatment were then adopted. The Cu content in grain boundary precipitates was improved, which can be attributed to the removal of Cu from solid solution during high temperature aging and its subsequent incorporation into grain boundary precipitates. In addition, the size and the distribution discontinuity of the grain boundary precipitates can be further increased and the main intra-grain phases of RRA alloy are still fine η′ phase similar to T6 temper. Therefore, the alloy at RRA temper obtains the optimal corrosion resistance without loss of high strength.
... The authors concluded that the incorporation of rare earth Ce improved the corrosion resistance remarkably, mechanical properties improvement and homogeneous microstructure. Yu et al. [9]investigated the plasticity behaviour, fracture toughness, and stress corrosion cracking performance of Al-Zn-Mg-Cu alloy recently. The hot rolled alloy of Al-Zn-Mg-Cu has taken for investigation, the prepared samples were solution treated in the furnace, quenched in water at room temperature, and then the samples were subjected to aging at 393k for 24 h. ...
The present research goal is to manufacture the novel Al-x wt. % Zn die-casting alloys through liquid metallurgy route. The amount of Zn content was varied from 0 to 30. % with a step of 10 percentages in the pure Al, The cast samples were squeezed under semi-solid condition (temperature ~500 o C) and then all the samples were extruded at a temperature of 673k to achieve improved density of at least 95 % and above. The microstructures of developed Al-Zn die-casting alloys were analysed using advanced electron microscope. The microstructures results revealed the formation of primary-Al phase and (+η) phase with η-agglomerates for higher amount of Zn content (greater than 20 wt. %). The consolidated Al-Zn alloys were mechanically tested by three-point ending test to examine the flexural behaviour of developed alloys. The flexural results explained that Al-20Zn alloy exhibited improved mechanical performance compared to other alloys. Maximum flexural strength of 196 MPa with the flexural strain of 7.48 was achieved in Al-20Zn alloy. Hence, Al-20Zn alloy is recommended to use for structural and automotive parts in real time application.
In this study, Al-7Zn-2.5 Mg-2Cu-0.1Zr-0.2Sc alloys with different aging states were selected as experimental objects to carry out slow strain rate tensile stress corrosion experiments, and characterized and analyzed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that when the aging time of the alloy is 10 h, 22 h and 34 h, respectively, the intragranular precipitates appear as GP zone, η’ phase, η’ phase + η phase in turn. At the same time, the grain boundary precipitates gradually grow with the extension of aging time, and there is no precipitation zone at the grain boundary when aging for 22 h and 34 h. When the tensile test was carried out at a slow strain rate of 1 × 10-6 s-1 in 3.5 wt. % NaCl solution, the alloy treated by solution + aging for 22 h showed the highest stress corrosion cracking (SCC) sensitivity. The fracture surface was mainly composed of cleavage facets and a small amount of small dimples. The SCC sensitivity of the alloy after aging for 10 h is second, and 34 h is third. The fracture surface is composed of a large number of dimples and small cleavage planes, showing ductile fracture characteristics.
The present work investigated an Al-4.5Zn-2.9Mg-0.25Cu (wt.%) alloy with superhigh yield ratio and favorable plasticity prepared by an improved powder metallurgy (PM) technique, including elemental powder mixing compared with ball milling, cold isostatic pressing, vacuum sintering and hot extrusion followed by T6 heat treatment. Microstructure evolution in different processes was characterized by optical microscopy, scanning electron microscopy, energy dispersive spectrometry and electron backscattered diffraction. Tensile properties were measured at room temperature. The results show that the sample fabricated by mixed elemental powder has comparatively excellent mechanical properties. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the corresponding sample after T6 heat treatment come up to 527 MPa, 512 MPa and 11.5%, respectively, with high yield ratio of 0.97, very close to the sample using ball-milled powder (547 MPa UTS, 514 MPa YS, 11% EL, 0.94 yield ratio). Fine grains and Al2O3 nanoparticles in PM aluminum alloys in this study are distinguished from other aluminum alloys, which contribute to the enhancement of mechanical properties. This provides a novel strategy for the simple and efficient preparation of high-performance powder metallurgy aluminum alloy.
The 7046 aluminum alloy possesses a favorable fatigue property, corrosion resistance and weldability, but its moderate strength and plasticity limit its wider application and development. In the present study, severe plastic deformation (SPD) was applied prior to T6 treatment to significantly enhance the strength and toughness of the 7046 aluminum alloy. The results show that the alloy processed by four passes of equal channel angular pressing (ECAP) at 300 °C prior to T6 treatment exhibits an excellent mechanical performance, achieving an ultimate tensile strength (UTS) and elongation (EL) of 485 MPa and 19%, respectively, which are 18.6% and 375% higher than that of the T6 alloy. The mechanical properties of the alloy are further improved by an additional room temperature (RT) rolling process, resulting in a UTS of 508 MPa and EL of 23.4%, respectively. The increased presence of η′ and Al6Mn phases in the 300°C4P-R80%-T6 and 300°C4P-T6 alloys contributes to a strengthening and toughening enhancement in the SPD-processed T6 alloy. The findings from this work may shed new insights into enhancing the 7046 aluminum alloy.
Dislocation tangle and coarse precipitate segregation cause incalculable damage to the strength and ductility of high-strength Al-Zn-Mg-Cu aluminum alloys. These drawbacks are inevitable for the alloys undergoing deformation and artificial aging. In this work, electroshock treatment (EST) was successfully introduced between deformation and artificial aging of the alloys to improve their strength and ductility. The uniaxial tensile test results show that the elongation of the samples with EST could increase by 28.8%, and the ultimate strength simultaneously increases by 20 MPa relative to that without EST samples. Through microstructure characterization, it was found that EST eliminates dislocation tangle resulting from pre-stretching in the material. Furthermore, the distribution of coarse precipitates in EST samples is more uniform, and the size of precipitates is smaller in comparison with non-EST samples. The dislocation evolution induced by EST could be attributed to the non-equilibrium scattering of electron-dislocation, and a kinetic model for nucleation and growth of precipitates has been proposed to reveal the mechanism of precipitate evolution.
The duration of the parking time between quenching and artificial aging can affect the final mechanical and corrosion properties of the material after artificial aging, etc. At present, there are fewer studies on the parking time before artificial aging, and this paper investigates the influence of the parking time before artificial aging on the microstructure and properties of 7050 aluminum alloy. The results show that: with the prolongation of the parking time, the proportion of HAGBs of the alloy is gradually increased through EBSD analysis, the dislocations are mainly concentrated in HAGBs and areas with dense GBs, the precipitated phase of the alloy continuously precipitates and grows at the GBs, the alloy’s hardness shows a trend of stabilization and then decreasing, and the corrosion resistance is stabilized and then deteriorated quickly with the prolongation of the parking time. The alloy has stabilized comprehensive performance when the storage time does not exceed 6 h, with microhardness of 132.2 HV, the Ecorr of −0.991 V and −1.007 V, the Icorr of 6.14×10 ⁻⁸ A·cm ⁻² and 4.16×10 ⁻⁷ A·cm ⁻² .
To provide a new idea to reduce anisotropy for sheets in the thickness direction by microstructure modification and, meanwhile, maintain or even enhance tensile performance, the in-situ ZrB2 particle/AA7085 composite sheets with thicknesses of 1, 2, 3, and 6 mm were investigated. The as-cast grain size was significantly refined by the heterogeneous nucleus of the ZrB2 particle. The microstructure results show that severe hot deformation converts as-cast disordered microstructure into a sequential microstructure by crushing the remanent phases and matrix into a fiber structure. After the solid solution and aging heat treatment, the composite sheets exhibit largely recrystallized grains compared with partially recrystallized grains in the matrix sheets, and weak or even free texture exists in the composite. The grain size in the composite sheets decreases with the increase in thickness reductions. For the thickness range of 1–3 mm, the composite sheets maintain a similar tensile performance as that in matrix sheets, while the strength and ductility in the 6-mm-thick composite sheet are improved.
The influence of low-frequency electromagnetic casting (LFEC) and Sc addition as variables on the microstructure, properties, and deformation behavior of 7A36 aluminum alloy has been systematically examined. The results have predicted that the LFEC could make the grains of the as-cast alloy finer by electromagnetic stirring and forced convection. By the addition of Sc, the average diameters of the equiaxed grains have been reduced and the intensity of the fiber texture has become weakened. The fine and continuous η′ precipitates on the grain boundaries (GBs) instead of the coarse large-sized discontinuous η′ precipitates, which results in a more uniform distribution of plastic deformation and which is beneficial for improving plasticity. By adding Sc and applying LFEC, the large size η′ internal grain precipitated phase decreased slightly. The fine nano-sized Al3(Sc, Zr) phase particles were dispersed in the matrix. As the dislocation movement had to travel less distance, that's why this process consumed less energy. These features also reduced the strain difference within the grain and near the grain boundary, which result in the increase of EL. The excellent mechanical properties of 7A36 aluminum as extruded plates were obtained when it was aged at 120 °C for 8 h by Sc micro-alloying and applying LFEC. The yield strength (YS) and ultimate tensile strength (UTS) remained at 738.3 MPa and 767.0 MPa, respectively, while the elongation (EL) to failure 6.5% is increased by 160% than the value of direct chill casting.
The microstructures, mechanical properties and corrosion behaviors of the ultra-high strength Al–Zn–Mg–Cu-Sc aluminum alloy fabricated by wire-arc additive manufacturing process using a self-prepared 7B55-Sc filler wire were systematically investigated under different heat treatments. The results showed that the microstructures of the as-deposited, T6, T73, and retrogression and re-aging (RRA) heat treatments were all composed of fine equiaxed grains with a size of about 6.0 μm. The grain boundary precipitates (GBPs) of the as-deposited sample were very coarse and continuously distributed along the grain boundaries, and the intragranular precipitates (IGPs) mainly consisted of a small amount of ηMg(Zn,Cu,Al)2 and η′ phases. After T6 heat treatment, the GBPs became much finer, but still continuously distributed along the grain boundaries. The IGPs mainly consisted of fine GP zones and η′ phases. After RRA heat treatment, the GBPs became coarser and discontinuously distributed along the grain boundaries. The IGPs were composed of η′ phases and ηMg(Zn,Cu,Al)2 phases. After T73 heat treatment, the GBPs became much coarser and sparsely distributed along the grain boundaries. The IGPs mainly consisted of coarser ηMg(Zn,Cu,Al)2 phases. The T6 heat-treated sample achieved the highest tensile strength of 618 MPa. While the strength of T73 heat-treated sample was sacrificed by up to 17% compared with the T6 heat-treated sample, but the corrosion resistance was the best of all heat-treated conditions. After RRA heat treatment, the strength was about 10% lower than the strength of the T6 condition, but the corrosion resistance was better than that of the T6 heat-treated sample.
The solution heat treatment (SHT) is an important step in hot forming and in-die quenching (HFQ) process. Herein, the SHT of cold-rolled AA7075-H18 alloy sheets is carried out in the temperature range of 462-518 °C for 0.9-29 min, and a response surface model based on strength and toughness is established. The results reveal that the maximal strength and toughness are achieved after SHT at 490.3 °C for 19.1 min and 475.1 °C for 21.1 min, respectively. One should note that the mechanical properties are closely related to the microstructural evolution during the SHT process. In particular, the dissolution amount of MgZn2 particles increases with the increase of SHT temperature or time, which further increases the strength of AA7075 alloy after age-hardening treatment. However, the alloy is overheated when the solution temperature is close to 510 °C, which deteriorates its strength and toughness. On the other hand, the cold-rolled texture is disappeared after the SHT process and a random recrystallization orientation is formed. Moreover, an excellent consistency between the variation trend of recrystallized grains and the response surface model is obtained, confirming that the response surface model is reliable. The experimental results show that the mechanical properties under different SHT process conditions depend on the coupling effect of precipitation strengthening and grain size strengthening. Finally, the process windows for optimizing the SHT process to balance the strength and toughness of sheets are defined. The current research provides crucial theoretical support and systematic guidance for the manufacturing of hot-formed AA7075 alloy components.
In this study, 7A85 aluminum alloy was subjected to 20% three-dimensional (3D) deformation at different temperatures (-196, 25, 250, and 480 °C), followed by solid solution, water quenching, 3% cold deformation, and retrogression and re-aging (RRA) treatment. The microstructural evolution and 3D mechanical properties, fracture toughness, electrical conductivity, and corrosion resistance of the alloy under different deformation processes were analyzed for comparison. The results indicated that conducting the 3D deformation process prior to solution treatment significantly improved the strength and fracture toughness of the alloy as compared to the undeformed (UD) specimen, and maintained its good corrosion resistance under the RRA treatment. In addition, with the decrease of the deformation temperature, the greater flow stress strongly sheared and crushed the coarse second-phase particles into fine particles, thus facilitating their full dissolution in the aluminum matrix during subsequent solution treatment and increasing the driving force for aging precipitation. Furthermore, more dislocations were introduced at low-temperature deformation, thus providing more nucleation sites for static recrystallization during solution treatment and refining grains. Ultimately, the densest precipitates and the finest equiaxed grains with the narrowest grain boundary precipitate-free zones (PFZs) were acquired for the cryogenic deformation (CD) sample, resulting in the highest ultimate tensile strength (UTS), elongation (EL) with the least anisotropy, and fracture toughness of the CD specimen. The optimal comprehensive performance was obtained for the CD specimen with the average 3D UTS, average 3D yield strength (YS), average 3D EL, and fracture toughness of 545 MPa, 476 MPa, 16%, and 42.49 MPa·m1/2, respectively, which were respectively 2.3%, 0.2%, 18.5%, and 21.4% higher than those of the UD specimen.
The effects of the addition of Cr, Mn and Cu on the microstructure and mechanical properties of extruded Al-Mg-Si alloys were investigated. Compared with base alloy (alloy A free of Cr, Mn and Cu), modified alloys (alloy B added with Cr + Mn and alloy C added with Cr + Mn + Cu) show improved strength and toughness due to the increased recrystallization resistance and refined precipitates. Furthermore, alloy C added with Cu shows reduced coarse grain ring thickness, enhanced strength, and slightly decreased toughness in comparison with alloy B free of Cu.
In this study, the effect of trace addition of Ag on the microstructure, tensile strength, and fracture toughness of novel Al-Mg-Zn alloys was evaluated using transmission electron microscopy, scanning electron microscopy, electron back-scattered diffraction analysis, tensile test, and Khan tear test. The age-hardening response was improved by Ag owing to the stimulation of matrix precipitation. The type of MPts was unchanged by Ag, and the density and size of MPts were improved and refined. But the ability of matrix precipitates (MPts) to obstruct dislocation motion was weakened by Ag, thus, the Al-Mg-Zn alloys strength was not improved by Ag as expected. The addition of 0.2 wt% Ag improved the fracture toughness, while the addition of 0.4 wt% and 0.6 wt% Ag decreased the fracture toughness of Al-Mg-Zn alloys. The fracture toughness results are based on many factors such as MPts, misorientation of grain boundaries, fraction of deformed grains, and stress-strain concentration. The mechanism of the effect of microstructure on properties was also elucidated.
The precipitation behavior and the microstructural evolution of a dual-phase Cu-15 wt%Ag alloy during annealing were investigated. We discussed the effects of dislocation density, grain size, solute atom concentration, and precipitation morphology on strength and electrical conductivity. The results showed that continuous precipitation and discontinuous precipitation simultaneously occurred during the annealing of the Cu-15 wt%Ag alloy. It was accompanied by pronounced recovery and recrystallization, significantly decreasing the strength and electrical resistivity of the cold-rolled Cu-15 wt%Ag alloy. After 48 h of annealing at 350 °C, the fully recrystallized Cu-15 wt%Ag alloy exhibited a higher strength (299 MPa) and a lower electrical resistivity (1.92 μΩ cm) compared to the samples annealed at higher temperatures. The higher strength was attributed to smaller grain size and higher volume fraction of the continuous precipitation zone. Nanoscale Ag precipitates formed by continuous precipitation provided more extensive precipitation strengthening than sub-micrometer Ag precipitates formed by discontinuous precipitation. In addition, reducing the dislocation density and the concentration of solid atoms by controlling the annealing conditions was an effective strategy to weaken the dislocation scattering and impurity scattering effect in the Cu-15 wt%Ag alloy. The electrical resistivity of the fully crystallized Cu-15 wt%Ag alloy was linearly correlated to the concentration of Ag solute atoms. The electrical resistivity of the annealed Cu-15 wt%Ag alloy increased by 0.117 μΩ cm when 1 at.% Ag was dissolved in the Cu solid solution.
The effect of different retrogression and re-aging (RRA) treatment on the microstructure, strength and stress corrosion cracking (SCC) resistance of a high Zn content Al alloy was investigated. The peak microhardness values after retrogression and re-aging at 180ºC/30 min and 200ºC/10 min were 214 HV and 215 HV, respectively, which are higher than that after T6 and T7 aging. The retrogression re-aged samples have large numbers of η′ precipitate (MgZn2), resulting in high mechanical properties of the alloy. The SCC resistance order of the alloys was found to be: T6 < RRA (120 ºC/24 h + 180 ºC/30 min + 120 ºC/24 h) < RRA (120 ºC/24 h + 200 ºC/10 min + 120 ºC/24 h) < T7, and the optimal retrogression re-aging process is 120 ºC/24 h + 180 ºC/30 min + quenching + 120 ºC/24 h. The improvement in the stress corrosion cracking resistance of the retrogression re-aged samples can be attributed to the increased interspaces of grain boundary precipitates and uniform distribution of the elements (Cu and Mg) in the grain boundary precipitates.
The effects of different ultrasonic treatment time on microstructure evolution and mechanical property of Cu/Sn-3.0Ag-0.5Cu (SAC305) solder joints were investigated. Experiments were set to obtain Cu/SAC305 solder joints, which were compared with solder joints without ultrasonic treatment. The application of short-term ultrasonic treatment on the solder joints would alter the morphology of interfacial intermetallic compound (IMC) layer, IMC thickness and grains size were reduced. Besides, ultrasonic treatment could significantly improve the shear strength of solder joints. In addition, the liquid solder would further flow along the direction of ultrasound, resulting in the reduction of contact angle. Due to the increase of ultrasonic time, the optimal growth direction of grains was changed after the ultrasonic treatment. However, excessive ultrasonic time would reduce the shear strength of solder joints and increase the IMC thickness, which would reduce the stability of solder joints.
The thin layers of a second phase (also called complexions) in grain boundaries (GB) and triple junctions (TJs) are more and more frequently observed in polycrystals. The prewetting (or premelting) phase transitions were the first phenomena proposed to explain their existence. The deficit of the wetting phase in case of complete wetting can also lead to the formation of thin GB and TJ phases. However, only the phenomenon of pseudopartial (or pseudoincomplete, or constrained complete) wetting permitted to explain, how the thin GB film can exist in the equilibrium with GB lenses of a second phase with non-zero contact angle.
Contact solid solution treatment (CST) could make the second phase dissolve into aluminum matrix in a short time. Compared to traditional furnace solid solution treatment（FST） the fine precipitated phase after CST ageing could make the strength slightly higher than that of T6.
The relationship among microstructures, mechanical properties and corrosion behaviors of spray formed 7055 aluminium alloy has been investigated upon peak-aging (T6), double-aging (DA) and retrogression and re-aging (RRA). The research is estimated by hardness test, tensile test, immersion test, detailed microstructure observation and potentiodynamic polarization under different aging treatments. Results demonstrate that the corrosion resistance rank of aging treatments is as follow: DA > RRA > T6. The continuous grains boundary precipitations (GBPs) and coarse Al7Cu2Fe particles increase the corrosion susceptibility. The appearance of the pit cavity attributes to coarse grains and the considerable potential difference between the matrix and GBPs or precipitate-free zones (PFZs) during intergranular corrosion (IGC) process. The wedging stress from hydrogen cracks and the accumulation of the corrosion products help induce the exfoliation corrosion (EXCO). Anodic dissolution and hydrogen embrittlement cause stress corrosion cracking (SCC) of the alloy under different aging treatments. The strengthening mechanism attributes to the distribution of matrix phases (MPs), η′ phase and Al3Zr particles.
High temperature pre-precipitation (HTPP) took place in 7005 alloy at various temperatures after solution treatment and its influence on mechanical properties, corrosion behaviors and microstructure of the alloy was investigated using tensile test, intergranular corrosion (IGC) test, slow strain rate testing (SSRT), together with microstructural examinations. It is found that Vickers hardness of the aged alloy decreases gradually with decreasing the HTPP temperature, and almost a reverse trend of electrical conductivity is found compared to the hardness changes. Depending on the changes, two HTPP temperatures of 440 and 420 °C were chosen for comparative study. Results reveal that HTPP alloy tempers exhibit higher resistance to stress corrosion cracking (SCC) and IGC than none pre-precipitate one with an acceptable strength loss due to the substantial enhancement of distribution discontinuity of the coarse grain boundary precipitates (GBPs), and the coarsening and interspacing effect on GBPs becomes more obvious with decreasing the pre-precipitation temperature.
7N01 aluminum (Al) alloys are treated by five heat treatment methods as peak aging (T6), over aging (T74), high temperature and subsequently low temperature aging (HLA), retrogression and reaging (RRA) and double retrogression and reaging (DRRA). The strength and fracture toughness of the five samples are tested, and the microstructures are investigated by optical microscopy (OM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results show that 7N01 Al-alloy treated at T6 condition has high strength but low fracture toughness. Compared with T6 treatment, T74 and HLA treatments increase the fracture toughness by 67% and 90% respectively, while the strength decrease by 9% and 17%. RRA process is a proper treatment method for 7N01 which improves the fracture toughness without sacrificing strength. The fracture toughness of DRRA treated alloy is much lower than that of RRA. Quantitative analysis through TEM images shows that the heat treatment affects the mechanical properties of 7N01 Al-alloy highly through changing the precipitates in grains and on grain boundaries, which can be explained by the coherency strengthening mechanism and Orowan mechanism.
The effect of natural aging on quench-induced inhomogeneity of microstructure and hardness in high strength 7055 aluminum alloy was investigated by means of end quenching technique, transmission electron microscopy and differential scanning calorimetry thermal analysis. The hardness inhomogeneity in the end-quenched specimens after artificial aging decreases with the increase of natural aging time prior to artificial aging. The quench-induced differences in the amount and size of η′ phase are large in the end-quenched specimen after artificial aging at 120 °C for 24 h, leading to high hardness inhomogeneity. Natural aging for a long time results in a larger amount of stable GPI zones in the slowly-quenched sample, and thus decreases such differences in the end-quenched specimens after subsequent artificial aging, leading to lower hardness inhomogeneity. The hardness inhomogeneity can be reduced from 14% to be 4% by natural aging for 17,280 h prior to artificial aging.
The influences of heat treatment on stress corrosion cracking (SCC), fracture toughness and strength of 7085 aluminum alloy were investigated by slow strain rate testing, Kahn tear testing combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the fracture toughness of T74 overaging is increased by 22.9% at the expense of 13.6% strength, and retrogression and reaging (RRA) enhances fracture toughness 14.2% without reducing the strength compared with T6 temper. The fracture toughness of dual-retrogression and reaging (DRRA) is equivalent to that of T74 with an increased strength of 14.6%. The SCC resistance increases in the order: T6<RRA<DRRA≈T74. The differences of fracture toughness and SCC were explained on the basis of the role of matrix precipitates and grain boundary precipitates.
Abstracts
The microstructural and mechanical properties of an ultrafine‐grained (UFG) Al–Zn alloy processed by high‐pressure torsion (HPT) are investigated using depth‐sensing indentations, focused ion beam, scanning electron microscopy and scanning transmission electron microscopy. Emphasis is placed on the microstructure and the effects of grain boundaries at room temperature. The experiments show the formation of Zn‐rich layers at the Al/Al grain boundaries that enhance the role of grain boundary sliding leading to unique plastic behavior in this UFG material. The occurrence of significant grain boundary sliding at room temperature is demonstrated by deforming micro‐pillars. Our results illustrate a potential for using UFG materials as advanced functional materials in electronic micro‐devices.
The effect of elastic tensile stress on the microstructure of 7075 Al alloy aged at 393–453 K for 1 h has been investigated in this paper. It was found that during the low temperature aging, the external stress lessens the size of the GPI zones. GPII zones are just identified in the stress-free aged condition. A new edge-on η′ platelet is observed in the stress-aged specimen. During the high temperature aging, the external stress promotes the formation of MgZn2 precipitates without changing the type of it. The peak microhardness value of the stress-aged specimens reaches 180 HV. The aging period of this alloy is shortened using the stress aging treatment.
The effect of grain size upon the stress corrosion cracking of 7475 Al-alloy plates has been investigated. Grain refinement resulted in a more homogeneous slip mode and a smaller size of grain boundary precipitates (GBPs) to influence the stress corrosion cracking (SCC) resistance. The more homogeneous slip mode is always beneficial for improving the SCC resistance. However, if the GBPs size was smaller than a critical precipitate size for nucleating hydrogen bubbles, the improvement of SCC resistance due to grain refinement, resulting from a more homogeneous slip mode, could not be obtained. The correlation of SCC susceptibility and hydrogen embrittlement susceptibility has been evaluated. The SCC susceptibility of the 7475 aluminum alloys is mainly controlled by hydrogen induced cracking mechanism.