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The intergranular corrosion behaviour of Al-Cu-Li-Sc alloy with different microstructures was systematically investigated in this work. The SKPFM characterisation revealed that the AlCuSc (W) phase has more negative potential compared to the Al matrix and undergoes dealloying in the intergranular corrosion solution. The dealloying rate of the W phase in the corrosive solution is relatively slow, which does not lead to serious pitting characteristics. As denser anodic T1 precipitates formed on low-angle grain boundaries of the partially recrystallised sample will reduce the precipitates on the sub-grain interior, resulting in the low-angle boundary being prone to preferential corrosion and reduced intergranular corrosion resistance. The complete recrystallised sample exhibits corrosion of grain interior and propagates along corroded bands parallel to {111}Al habit planes of T1 precipitates, reducing the expansion rate of corrosion along the grain boundaries. Restraining the IGC along the low-angle grain boundary and inducing crystallographic corrosion to improve the intergranular corrosion resistance. [ABSTRACT FROM AUTHOR]

To meet the lightweight demands of automobiles, Al composite sheets require excellent mechanical properties under the condition of minimal thickness after high-temperature brazing processing. Generally, the standard used Al brazing sheets have a low strength before and after brazing. To overcome this issue, this work develops a novel 4343/3003/6111/3003 four-layer Al clad sheet, whose microstructure and mechanical properties are systematically investigated. The results show the observable fibrous microstructure with elongated grains parallel to the rolling direction in the developed four-layer Al clad sheet of the cold-rolled and annealed states. After brazing, this fibrous microstructure transforms into coarse equiaxed grains. In addition, the 4343 layer is the brazing layer. Si is mainly distributed in the 4343 layer of the cold-rolled Al clad sheets, whereas Si penetrates into the core layer along the grain boundaries after brazing. The cold-rolled samples present a certain brittleness from fracture morphology, whereas the final annealed ones show a typical ductile fracture. Meanwhile, the typical intergranular fracture is visible after brazing. The mechanical properties of the Al clad sheets are improved after brazing, with an increase of 76% in tensile strength and 62% in yield strength, compared with the final annealed ones. The elongation is increased by 29% compared with that of the cold-rolled ones. This study provides a theoretical basis for further improvement of the strength of aluminum honeycomb panels. [ABSTRACT FROM AUTHOR]

The intergranular corrosion (IGC) resistance of the Al–Mg–Si alloy with low Zn addition treated by the interrupted aging treatment (T6I6) was investigated. The IGC results of the Al–Mg–Si alloy show that IGC resistance is enhanced by T6I6 aging treatment. The EIS results reveal that the T6I6-treated Al–Mg–Si alloy has lower double layer capacitance and higher charge transfer resistance than the T6-treated alloy. The superior IGC performance of the T6I6-treated alloy may be attributed to the finer and more densely distributed precipitates within the grains, smaller and discretely distributed precipitates along the grain boundaries and narrower precipitate-free zones (PFZs) compared to the T6-treated alloy. [ABSTRACT FROM AUTHOR]

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs. [ABSTRACT FROM AUTHOR]

Multilayer brazeable aluminum alloy sheet is prone to collapse during high-temperature brazing process. The sagging resistance of the aluminum composite sheet needs to be further improved for quality control. Effects of annealing and rate of reduction on sagging resistance, microstructure, and Si diffusion of a hot-rolled, four-layered Al clad sheet (4343/3003/6111/3003) were investigated by means of a sagging device, OM, SEM, and TEM. Results showed that once annealed at 360°C, the sagging distance was increased from 3 to 15.7 mm as the reduction rate changed from 10% to 40%. By increasing annealing temperature to 410°C, those were changed from 3.1 to 20.8 mm accordingly. At 360°C/40% and 410°C/40%, specimens exhibited weak sagging resistance, whereas fine recrystallized grains were formed in the core promoting Si penetration along grain boundaries. While the specimens were treated at 360°C/10% and 410°C/10%, better sagging resistance was observed due to the formation of coarse recrystallized grains that can suppress erosion of Si. At the same reduction rate, the sagging resistance was higher for the sample annealed at a lower temperature as more precipitates appeared in the core (at 360°C), thus leading to an increase in strength. [ABSTRACT FROM AUTHOR]

The intergranular corrosion (IGC) behavior of an extruded 6005A alloy profile with the coexisting of peripheral coarse grain (PCG) structure and partial recrystallized grain (PRG) structure was investigated by using an accelerated corrosion test, electrochemical impedance spectroscopy and a quasi in situ examination of IGC process. PCG structure was found to have a unique IGC behavior that pitting corrosion and subsequent IGC are less severe and will not be transformed into intragranular corrosion as they were found in PRG structure. Microstructure characterization reveals that the microstructural differences in grain boundary precipitates, primary α-AlFeMnSi intermetallic particles and grain characteristic between PCG structure and PRG structure are the reason for these phenomena. Further analysis indicates that the grain boundaries decorated with more AlFeMnSi particles and Q phase precipitates are more sensitive to corrosion, where Q phase precipitates are the primary cathodes and the most important factor affecting IGC; AlFeMnSi particles are supposed to initiate pitting corrosion since they are dissolved as anodes in the early stage of corrosion. With the development of corrosion, they are transformed into cathodes and become the bridges of IGC propagation by connecting the Q phase precipitates at grain boundary. In addition, grain characteristic was also found to have great effects on IGC. With the decrease in grain size and the increase in the frequency of high-angle grain boundaries and the dislocation density, corrosion becomes more severe and more likely to be transformed into intragranular corrosion. [ABSTRACT FROM AUTHOR]

The Al-Zn-Mg-Cu alloy with different sub-grains and recrystallized grains was isothermal forged at 400 °C with strain rate of 10−4 s−1, and the grain structures were examined by electron backscatter diffraction (EBSD). The results showed that the coarsening of sub-grains and the formation of recrystallized grains at grain boundaries were the primarily changes of grain structures during isothermal forging. Besides, it found that the coarsening of sub-grains occurred first, and then recrystallization started at grain boundaries when quite a few sub-grains were found within grain interiors. Coarsening of sub-grains was found during isothermal forging of the one containing many sub-grains, and the recrystallization was hindered as strain energy was consumed by such coarsening of sub-grains. But more recrystallized grains were found after isothermal forging of the one containing many fine grains. [ABSTRACT FROM AUTHOR]

Abstract Some specific corrosion environments require Al-Mg-Si aluminum alloy extrusions to have superior corrosion resistance. In order to clarify its corrosion mechanism and enhance its corrosion resistance, the corrosion occurrence, propagation path and corrosion morphology of the alloy was characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It was found that the corrosion of the alloy first occurred at the grain boundary. Then the grain boundary becomes the origin of the crystallographic pitting after being attacked, and Mg in the Mg 5 Si 6 phase inside the grain is preferentially corroded by precipitate free zone (PFZ), so that the corrosion propagates into the interior of the grain. The morphology of crystallographic pitting presents a regular step-like planes and is always propagated parallel to the direction of {100} Al in the grain. Further, the corroded Mg 5 Si 6 precipitates becomes cathodes in the local area at a later stage, leading to the regular step-like planes has been transformed into dimple-like appearances. Highlights • The grain boundary is the origin of the crystallographic pitting after being attacked. • The morphology of crystallographic pitting presents a regular step-like planes. • The crystallographic pitting is always propagated parallel to the direction of {100} Al in the grain. [ABSTRACT FROM AUTHOR]

While it is clear that precipitates near grain boundaries in aluminum alloys nucleate more easily and grow more quickly, the mechanism of how the grain boundaries influence the anisotropic distribution of S precipitates is still unknown. In this study, a bicrystal Al-Cu-Mg alloy specimen was prepared to obtain a single grain boundary. The precipitation distribution and hardness of the Al-Cu-Mg alloy bicrystal were analyzed after stress aging at 30 MPa. Transmission electron microscopy was used to analyze the precipitation behavior of S precipitates, and the hardness was tested along the location on the bicrystal specimen. The hardness of the right area around the grain boundary is lower than that of the left area around the grain boundary after stress aging. The precipitation behavior around the grain boundary is different from that in the matrix. For the left area near the grain boundary with the (1, −1, 8) Al plane orientation, the anisotropic distribution of the S precipitates is inhibited. For the right area near the grain boundary with the (3, 5, 6) Al plane orientation, the anisotropic distribution of the S precipitates is even more pronounced than that in the right matrix. This difference in the influence depends on the crystallographic orientations of the matrix around the two sides of the grain boundary. [ABSTRACT FROM AUTHOR]

The Al 3 Zr particles formed in the Al–Cu–Li–Zr alloy during single and double-step homogenization treatments contributed to various microstructures and mechanical properties during the subsequent thermomechanical treatment. Compared with the single-step homogenization treatment (ST), the double-step homogenization treatment (DT) generated higher density and more uniform distribution of β′ (Al 3 Zr) phases, which makes the alloy retain more substructures and improve the initial strength of solid solution. During the ageing process, Al 3 Zr particles induced the T 1 precipitation in grain interior and the width of PFZ get narrowed. Therefore, the strength and toughness of the DT sample improved in peak aged state compared with that of the ST sample. The peak hardness value of DT sample is 167.8 HV, which is higher than that of ST sample with 153.1 HV. Furthermore, the tensile strength and elongation of the DT sample at peak aged state (521 MPa, 556 MPa and 8.8 %, respectively) are higher than those of the STA sample (492 MPa, 535 MPa and 6.7 %, respectively), and the impact toughness of the DTA sample (13.2 J cm−2) is also higher than those of STA sample (9.1 J cm−2). Strength model showed that more uniform distribution of Al 3 Zr phases and substructures improve the initial strength of solid solution state, which increased by 16 MPa in the DT sample compared with the ST sample. The promotion of T 1 precipitation inducing the strength increased by 18 MPa in the DT sample at the peak aged state compared with that of the ST sample. The increase of elongation and toughness contributed from the oriented grains with finer distribution and higher Schmid factor, less grain boundaries precipitates and narrower PFZ width. [ABSTRACT FROM AUTHOR]

The EXCO and IGC corrosion behavior of an extruded rod of Al–Zn–Mg–Cu alloy with gradient granular grain structure were investigated. The corrosion resistance increased gradually from the center to the surface of the extruded rod due to the increase of distance between η precipitates and the decrease of the precipitate length along grain/sub-grain boundary, which is assumed as the result of the gradually decrease of grain/sub-grain size from the center to the surface. • The precipitate distance along grain boundaries increases with the decrease of grain size of the Al–Zn–Mg–Cu alloy. • The precipitate length along grain boundaries decreases with the decrease of grain size. • The corrosion resistance could be improved with the increase of the distance between η precipitates. [ABSTRACT FROM AUTHOR]

The influences of aging treatments on microstructures and the corrosion properties of an Al–Cu–Li alloy were investigated through an immersion test in intergranular corrosion (IGC) solutions, a potentiodynamic polarization test, and electrochemical impedance spectra (EIS), combined with scanning and transmission electron microscopy. The results demonstrated that the Al–Cu–Li alloy displayed outstanding comprehensive mechanical properties and IGC resistance after treating with pre-strain deformation and a double aging process (PDA). Both the PDA and pre-strain followed by creep aging (PCA) treatments significantly increased the number densities of T1 and θ' precipitates in the grain interior. The increase in precipitates in the grain interior greatly reduced the Cu-rich precipitates on the grain boundaries and inhibited the formation of a precipitate-free zone (PFZ). The electrochemical characteristics of the Al–Cu–Li alloy were influenced by the precipitates in the grain interior and grain boundaries. The studied alloy gained high IGC resistance due to the refinement of its microstructure, and the main corrosion mode was intra-granular pitting corrosion; thus, the corrosion diffusion rate was slowed down. [ABSTRACT FROM AUTHOR]

The mechanical properties of Al-Zn-Mg alloys via multi-stage aging treatments were studied to explore the strength-toughness trade-off. The microstructures of aged alloys were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that large numbers of GP zones and some part of η′ phases were formed during the low-temperature (90 °C) aging at the third-stage aging, which consumed considerable solid solubility. This prevents fast growth and coarsening of precipitates at the fourth-stage aging (150 °C). Finally, the mixed precipitation state of mainly η′ phases and some η phases formed. The grain boundary phase grew insignificantly, which is beneficial to decrease the strength gap between intragranular and grain-boundary. The intragranular equilibrium η phase is beneficial to pin the movement of dislocation, which avoids fracture caused by accumulation of dislocations around the large second-phase and on the grain boundaries. Although the strength of materials decreased slightly, the impact toughness of materials significantly increased due to the positive effect of the intragranular equilibrium η phase. Compared with the conventional retrogression and re-aging (RRA) treatment (120 °C/24 h + 185 °C/105min + 120 °C/12 h), the designed four-stage ageing treatment (FSA) (120 °C/24 h + 185 °C/105min + 90 °C/12 h + 150 °C/12 h) resulted in the decrease of tensile strength and yield strength by only 3% and 4%, respectively, but contributed to the increases of elongation and impact toughness by 15% and 17%, respectively. [ABSTRACT FROM AUTHOR]

The microstructures and mechanical properties of friction stir welded (FSW) Al–Zn–Mg alloy plate under different travel speeds were investigated. Both the average grain sizes (AGSs) of the shoulder affected zone (SAZ), nugget zone (NZ), and the widths of thermo-mechanically affected zone (TMAZ) decreased with the increase of travel speed. Moreover, the AGSs of NZ are always about 60% of that of SAZ at different travel speeds. The fractions of high-angle grain boundaries (HAGBs) in the FSW joints reduce with the distance away from the stir zone (SZ). Furthermore, the initial η' strengthening precipitates in NZ and TMAZ dissolve and GP zones form during subsequent natural aging, so that the hardness is similar in the two zones. The precipitate evolution in the heat-affected zone (HAZ) at hardness minima are affected by travel speeds, which induce the hardness minima and ultimate tensile strength (UTS) of FSW joints and increase with the increase of travel speed, and a fracture tends to occur at hardness minima location of HAZ during tensile testing. [ABSTRACT FROM AUTHOR]

An Al-Zn-Mg-Cu alloy was isothermal compressed at temperatures ranging from 300 °C to 450 °C with strain rates ranging from 10−5 s−1 to 10 s−1 to study its deformation and recrystallization behaviors. The evolution of grain structure during the isothermal compression demonstrated that sub-grains found in grain interiors of those deformed with high strain rates from 10−2 s−1 to 10 s−1, were mainly the results of continuous dynamic recrystallization (CDRX), while many recrystallized grains formed at grain boundaries of those deformed with low strain rates from 10−5s−1 to 10−3s−1, were largely believed to be the result of discontinuous dynamic recrystallization (DDRX). Furthermore, two totally different constitutive equations as the result of different dynamic recrystallization mechanisms were proposed. • Two different dynamic recrystallization mechanisms (CDRX and DDRX) occur at different strain rates. • The formation of recrystallized grains at the lower strain rates leads to a rapider decreasing of flow stress. • The activation energy increases when the dynamic recrystallization mechanism changes from CDRX to DDRX. • Two different constitutive equations are constructed at the two different strain rate ranges. [ABSTRACT FROM AUTHOR]

An Al–Zn–Mg–Cu support bracket forging was isothermal precision forged at 400 °C with speeds of 0.1 mm/s, 0.01 mm/s and transient speed changing from 1 mm/s to 0.01 mm/s by finite element simulation and experiment. The stress concentration at high ribs of the simulated forging corresponded to the defects of under-filling and folding of the experimental forging at the speed of 0.1 mm/s while none defects appeared at the speeds of 0.01 mm/s and transient speed. Besides, not only less time was used to finish forging but also much more fine grains and sub-grains were formed in the forging that precision forged at the transient speed as compared with that at the speed of 0.01 mm/s, which leaded to the higher mechanical properties. Furthermore, the resistance to IGC was much better in the forging that precision forged at the transient speed for the particles η distributed along the grain boundaries was much wider. [ABSTRACT FROM AUTHOR]

Grain boundaries in metals would introduce extra nucleation energy to promote the growth of θ′ precipitate during ageing process. The influence of grain boundaries on the distribution behavior of two θ′ precipitates variants (010) and (001) were studied in Al–2Cu alloys bicrystals. Ageing conditions without external stress and with 40 MPa and 60 MPa stresses were conducted to study the effect of grain boundaries on the oriented distribution of two θ′ precipitates variants. The hardness differences between the near grain boundaries area and the single crystal of the bicrystal samples were tested to understand the relationship between microstructures and mechanical properties. Results of transmission electron microscopy (TEM) images showed that the stress-orienting effect on precipitates was inhibited by the grain boundary. TEM images and quantitatively measurement of two θ′ precipitates variants indicated that grain boundaries inhibited the orienting effect of external stresses on the distribution of precipitates. Especially in 60 MPa stress-aged sample, θ′ precipitates near the grain boundary distribute homogeneously and have the same density as in single crystal areas. Hardness difference in stress-aged sample was smaller compared to the stress-free-aged sample. External stresses during ageing also narrowed the PFZ width near grain boundaries. The anisotropy of precipitation strengthening contribution of two θ′ variants were reduced due to the extra energy provided from grain boundaries. The compressive yield strength of Al–2Cu alloys bicrystal increased with the applied stresses during ageing. This work provides new insight into solving the anisotropy problem during stress ageing based on the effect of grain boundaries. Image 1 • Al–2Cu bicrystals were used to study the effect of stress ageing on precipitation. • The orienting degree of (010) and (001) θ′ variants was described quantitatively. • The grain boundaries inhibited the orienting effect of stress ageing. • Hardness lines along the bicrystal were tested to study the hardness differences. • Hardness difference becomes smaller in 60 MPa stress-aged bicrystals. [ABSTRACT FROM AUTHOR]

Abstract: Superplastic mechanism and microstructure of Mg–Gd–Y–Zr alloy extruded rod with an initial grain size of 10μm were investigated. For the purposes, tensile tests at various temperatures and strain rates were conducted. The tests revealed that the material showed a maximum elongation of 410% at 450°C and 2×10−4 s−1. The high ductility was attributed to grain boundary sliding accommodated by dislocation motion assisted by lattice diffusion. It is suggested from microstructural analysis results that second phases, cuboidal Mg3Gd and irregular-shaped β, exhibited significant effect of pinning grain boundaries, and that the irregular-shaped β was deformable and the strain was partly transferred from the matrix to β phases. [Copyright &y& Elsevier]

In the present study, the influence of Zn content on the microstructure and mechanical properties of the Al-Mg-Si alloy is systematically investigated. The results show that the precipitation sequence within the grains of the alloy does not change when Zn content does not exceed 2.0 wt%. The intragranular precipitates are mainly composed of β" precipitates with a small amount of η' precipitates when Zn content reaches 4.0 wt.%. Zn addition does not affect the precipitation sequence along the grain boundaries of the alloy when Zn content does not exceed 1.0 wt.%. But the grain boundary precipitates (GBPs) transform into Zn-rich phases when Zn content increases to 2.0 wt.%. The strength of the alloy firstly gradually improves and then reduces when Zn content increases to 2.0 wt.%. This is mainly due to the effect of Zn content on the distribution of the intragranular precipitates. The strength of the alloy significantly enhances again as a result of the increase of the number density of β" precipitates and the formation of η' precipitates when Zn content reaches 4.0 wt%. [Display omitted] • Zn addition promotes the precipitation behavior of the intragranular precipitates. • The size and type of the GBPs change with the variation of Zn content in the alloy. • Zn content has great effect on the PFZ width. • Zn addition improves the strength mainly due to the high number density of the intragranular precipitates. [ABSTRACT FROM AUTHOR]

Abstract: The effect of microstructure on the quench sensitivity of 7055 type aluminum alloys was investigated by hardness testing, optical microscopy and transmission electron microscopy. The results showed that the homogenized and hot-rolled alloys without Zr and the homogenized alloy with Zr were not quench sensitive, while the hot-rolled alloy with Zr was quite quench sensitive. The reason was analyzed according to the difference in the microstructure in these alloys, i.e. type of nucleation sites including dispersoids, (sub)grain boundaries and constituent particles for heterogeneous precipitation during slow quenching. Quench-induced η phase was observed mainly on large dispersoids and (sub)grain boundaries. In the homogenized and hot-rolled alloys without Zr, grain boundaries, which were the main heterogeneous precipitation nucleation sites, resulted in low quench sensitivity. In the homogenized alloy with Zr, grain boundaries and fine and coherent Al3Zr dispersoids led to low quench sensitivity. In the hot-rolled alloy with Zr, the large number of coarse Al3Zr dispersoids and subgrain boundaries were mainly responsible for high quench sensitivity. Constituent particles seemed to have little effect on quench sensitivity. [Copyright &y& Elsevier]