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The influence of ultrasonic treatment (UT) on the microstructure morphology and hardness of Al–Cu–Mg–Ag alloy is systematically investigated by a combination of electron backscatter diffraction (EBSD), x-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. Our results show that the hardness of as-cast and T6-tempered alloys becomes higher by UT. TEM analysis indicates that UT greatly accelerates the precipitation of θ precipitates and prohibits its rapid coarsening during exposing at 225 °C. This positive effect is ascribed to the combined results of the increasing Cu content in matrix, reduced dendritic segregation and more uniform distribution of Cu solutes by UT. Besides, UT is found to affect the precipitation kinetics of Ω phase because the higher number density of Ω plates is detected in UT alloys. This feature ensures a greater precipitation strengthening contribution to hardening in T6-termperd alloy with UT. Although the grain sizes of as-cast and T6-tempered alloys are reduced by UT, the grain boundary strengthening effect is very limited in UT alloys. This work also reveals that the solute content within the matrix increases by UT and therefore favors a stronger solid solution strengthening effect, leading to the higher hardness of as-cast UT alloys. After exposing at 225 °C for 100 h, all the T6-tempered alloys with and without UT exhibit a similar hardness despite the exposed microstructures exhibit clear differences. The absence of the enhanced hardening by UT in exposed alloys is arising from the fact that Ω and θ precipitates significantly coarsen during exposing.

Pre-stretching prior to aging is known to degrade the performance of Al–Cu–Mg–Ag alloys at elevated temperatures. In this work, small pre-stretching by 0.5% and 1.8% is selected to explore its potential influences on the microstructure and strengthening capacity of an Al–Cu–Mg–Ag alloy. The strength and microstructure of Al–Cu–Mg–Ag alloy are found to be sensitive to the pre-stretching. Even 0.5% pre-stretching can reduce the alloy strength by more than 10 MPa. Quantitative transmission electron microscopy (TEM) and atom probe tomography (APT) analysis indicate that this detrimental effect of pre-stretching on strength is directly associated with the suppressed Mg–Ag co-clustering, which greatly delays the Ω precipitation and induces a weaker strengthening effect. Quantitative calculation reveals that Mg–Ag co-clusters exhibit the greatest strengthening contribution to the yield strength of the underaged material because of its strongest cluster hardening, which is mainly originated from its order hardening capacity. As the pre-stretching increases from 0.5% to 1.8%, the θ coarsening is progressively intensified during exposing at 200 °C, accompanied by a gradual drop in the number density of Ω phase. Besides, the formation of a cubic σ phase is found to be considerably enhanced after exposing at 200 °C for 500h as the pre-stretching increases.

In the present work, different aging treatments were performed to investigate the pre-aging effect on the formation of Ω phase and mechanical properties in Al-Cu-Mg-Ag alloy. The results showed that pre-strain could inhibit the formation of Ω phases, which was detrimental to the alloy strength. Due to the introduction of pre-aging treatment before pre-strain, the adverse effect of pre-strain on the precipitation of the Ω phase was reduced, and the alloy strength was increased by at least 15 MPa. Besides this, increasing the pre-aging temperature promoted the precipitation of Ω phases, inhibited the formation of θ′ phases, and improved the alloy strength. This was because the higher pre-aging temperature promoted more pre-precipitated Ω phases in the pre-aging process, and most of the pre-precipitated Ω phases could be retained and grew in the subsequent aging process. As a result, the tensile strength of the alloy increased from 523 MPa to 540 MPa. In addition, pre-aging with a higher temperature consumed more solute atoms, leading to less residual solute atoms in the matrix. Thus, the adverse effects of pre-strain, which inhibit the formation of clusters by the segregation of solute atoms, were reduced.

The microstructures and mechanical properties of novel cast Al-Cu-Mg-Ag alloys with and without minor additions of Er (0.09 and 0.2 wt %) are investigated by Vickers hardness tests, tensile tests, optical metallographic examination, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results reveal that the Er addition decreases the hardness value of peak-aged Al-Cu-Mg-Ag alloy but has little influence on the time required for achieving the peak aging condition. Meanwhile, the Ω phase is suppressed in Er-added alloys, leading to a lower tensile strength at room temperature, which causes the (Mg, Ag, Er, V, Ti)-rich phase in the matrix in Er-added alloys. This blocky phase consumes available Mg and Ag atoms for Ω nucleation, leading to the low number density of Ω plates. The strength properties of Er-added alloys at 300 °C are found to be enhanced, which benefits from the pinning effect of the Al8Cu4Er phase on grain boundaries. Meanwhile, the brittle fracture of Er-added alloys at room temperature is directly associated with the Al8Cu4Er phase and the blocky (Mg, Ag, Er, V, Ti)-rich phase, which acts as the source of microcracks during deformation. In addition, no obvious grain refinement effect can be observed in Er-added alloys.

P-texture effect on the fatigue crack propagation (FCP) resistance in an Al-Cu-Mg alloy containing a small amount of Ag, is investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD). Results shows that the high intensity P-texture sheet has lower σ 0.2 / σ b , lower FCP rate and higher damage tolerance than random texture sheet. Fracture analysis indicates that the striations spacing of high intensity P-texture sheet is much smaller than that of random texture sheet and it has a rougher fatigue fracture surface, which causes a significant roughness induced crack closure (RICC) effect. The calculation results manifest that high intensity P-texture sheet possesses a higher crack closure level reaching 0.73 as compared to random texture sheet (only 0.25). The statistical analysis results reveal the P-grains have large twist angle of 105⁻170° and tilt angle of 5⁻60° with neighboring grains, which is similar to Goss-grains. This is the fundamental reason that P-texture sheet has the same FCP resistance and induces fatigue crack deflection as Goss-texture sheet. Additionally, the most {111} slipping planes of P-grains are distributed in the range of 30⁻50° deviating from transverse direction of the sheet. This results in more {111} slipping planes to participate in cyclic plastic deformation, which is beneficial to reduce fatigue damage accumulation and improve the damage tolerance of Al-Cu-Mg-Ag alloy.

The microstructural evolution in an Al-Cu-Mg-Ag alloy with trace Zr addition during homogenization treatment was characterized by Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray Spectroscopy (EDS). It was shown that the low-melting-point phase segregating toward grain boundaries is Al2Cu, with a melting point of 523.52°C. A two-step homogenization process was employed to optimize the microstructure of the as-cast alloy, during which the alloy was first homogenized at a lower temperature, then at a higher temperature. After homogenized at 420°C for 6 h, Al3Zr particles were finely formed in the matrix. After that, when the alloy was homogenized at an elevated temperature for a longer time, i.e., 515°C for 24 h, most of the precipates at the grain boundaries were removed. Furthermore, the dispersive Al3Zr precipitates were retained, without coarsening greatly in the final homogenization step. A kinetics model is employed to predict the optimal homogenization time at a given temperature theoretically, and it confirms the result in present study, which is 420°C/6h+515°C/24h. [ABSTRACT FROM AUTHOR]

Microstructures and mechanical properties of Al-Cu-Mg alloys containing various Ag additions were investigated by a combination of the tensile testing and quantitative transmission electron microscopy (TEM) analysis. Our results indicated that an increase of Ag from 0.46 to 0.88wt% promoted the precipitation of Ω phase, thereby leading to the noticeable improvement on the strength properties of the underaged Al-Cu-Mg alloys at various temperatures. This enhanced precipitation of Ω phase was also accompanied by the suppressed formation of θ' phase. Despite further exposing at 300°C lead to a pronounced decrease in the number density of Ω phase, the higher plate number density was still revealed in 0.88Ag alloy rather than 0.46Ag alloy. Moreover, quantitative TEM results of the studied alloys highlighted the differences in the coarsening kinetics of Ω phase during further exposing at 300°C. Ω phase tended to experience a transition from the plate thickening to the plate lengthening by increasing Ag at 300°C. Our results also present the positive effect of pre-aging at 165°C for 2h on retarding the thickening kinetics of Ω phase at 300°C. [ABSTRACT FROM AUTHOR]