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• Increasing dislocation density decreases the creep Ⅱ-stage's duration time. • It is revealed that T 1 precipitates nucleate and grow during the creep II stage. • Different densities-induced dislocation configurations affect the creep Ⅱ-stage. • New insights are proposed for the creep mechanism of an Al-Li alloy at each stage. Creep aging is well-known to be a time-dependent, coupled process of deformation and precipitation strengthening for age-hardening alloys. Its existing mechanisms are mainly attributed to those interactions between atomic diffusion and dislocation motion. However, an understanding of the relationship between dislocation density and a special multistage creep behavior, i.e., double steady creep feature, is still far limited. Here we investigate the effect of various dislocation density levels on such an abnormal multistage creep of an Al-Cu-Li alloy. We find that the increased dislocation densities enable an apparent time decrease (from 6.2 h to 0.8 h) of their first steady Ⅱ-stage. The yield strength of post-aged samples increases from 425.0 MPa to 580.0 MPa while the corresponding elongation decreases from 12.3% to 7.3% for the creep-aged samples #1 to #4. Microstructural results also reveal that a great difference in dislocation configuration, tailored by various density levels, results in varying creep processes of the Ⅱ-stage. This stage is closely related to the nucleation and early growth of T 1 precipitates. Their number densities (maximum: 2.9 × 1019 m–3) and the average length (maximum: 21.3 nm) of T 1 precipitates are much smaller than those of the stable peak-aged T 1 phases, suggesting that creep Ⅱ-stage of all three creep-aged samples is dominant by the nucleation and initial growth of those T 1 precipitates. This study provides valuable insights into the dislocation density-mediated creep deformation of an Al-Cu-Li alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

• The microstructure evolutions and fatigue crack propagation behaviors of Al-Cu-Li alloys with hot-rolled and cold-rolled were firstly investigated by FCP tests. • The mechanisms of subgrain boundaries within grains on the crack initiation and propagation have been discussed. • The relationship between crystal orientation and crack path deflection and its influences on the fatigue crack propagation has been discussed in detail. • Direct experiments were carried out to prove the retardation of crack propagation by grain boundaries and deflection. Recrystallization behavior during optimized heat treatments provides a potential to obtain desirable grain structure, which significantly improves the mechanical properties of aluminum alloys. The influence of grain structures on fatigue crack propagation (FCP) behaviors of Al-Cu-Li alloy with hot-rolled (HR) and cold-rolled (CR) was investigated. Subgrain boundaries have a significant impact on small crack growth rates, which is reflected in the pronounced fluctuation of fatigue crack growth of HR specimens after solution treatment. Moreover, the specific cellular structure within grains can improve the deformation capacity of alloys due to their accommodation of plastic deformation, which contributes to the lower fatigue crack growth rates and higher threshold values in HR specimens. The intragranular deflection also decelerates the FCP rate and occurs in these regions of large grain without subgrain boundaries. Recrystallization occurs in the CR specimens, resulting in small anisotropy on the fatigue resistance for the different orientations in the Paris stage due to the recrystallization texture. Fatigue cracks can be deflected and tend to propagate along the grain boundaries when it goes into the grain with a relatively low Schmidt factor value. [Display omitted] [ABSTRACT FROM AUTHOR]

• Our findings proved another dislocation reconfiguration responsible for the electroplastic behaviour. • Synergy of the climb force F c and electron wind force F ew on the reconfiguration of helical dislocations. • Athermal component of electropulsing is proven to be more important during the dislocation reconfiguration. Creep mechanism was well-known to be mainly dominated by the dislocation sliding and climbing during creep deformation. Here we study the creep deformation of an Al-Cu-Li alloy with the assistance of electropulsing and subsequent microstructural observations. We find that creep strain increased drastically under electropulsing and was almost twelve times as much as that of the non-pulsed sample. Microstructural observations confirmed that dislocation reconfiguration happens via electropulsing, namely helical dislocations being opened rapidly. This opened dislocation structure can possess a much higher mobility than the initial helical dislocation, which mostly responsible for the greatly increased creep strain. Our results revealed a new mechanism accountable for the distinctly electroplastic creep deformation. [Display omitted] [ABSTRACT FROM AUTHOR]

Uniaxial compression tests were carried out on an Al-Cu-Li alloy at a temperature range of 300–500 °C and a strain rate range of 0.001–10 s−1. Four representative instability maps based on Gegel, Alexander-Malas (A-M), Kumar-Prasad (K-P) and Murty-Rao (M-R) criteria were constructed. Through formula deduction and microstructural observation, it can be concluded that M-R criterion is more accurate than K-P criterion, and the first two criteria are better than Gegel and A-M criteria. From a power dissipation map and a M-R instability map, the optimized processing parameters are 480–500 °C/0.001–0.1 s−1 and 420–480 °C/0.1-1 s−1. The corresponding microstructural analysis shows that dynamic recovery and partial dynamic recrystallization are main dynamic softening mechanisms. Transmission electron microscopy observation indicated that a large number of primary coarse T 1 (Al 2 CuLi) particles precipitated in the homogenized specimen. After deformation at 500 °C, most of the primary T 1 particles dissolved back into the matrix, and secondary fine T 1 particles precipitated at deformation-induced dislocations, high angle grain boundaries and other dispersed particles. [ABSTRACT FROM AUTHOR]

The effects of different aging processes on the microstructure and mechanical properties of a novel Al–Cu–Li alloy have been investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the tensile properties of a novel Al–Cu–Li alloy are sensitive to aging processes, which correspond to different microstructures. σ (Al 5 Cu 6 Mg 2 ) and T 1 (Al 2 CuLi) phases are the major precipitates for the alloy in T6 aging condition (165 °C/60 h). After duplex aging condition (150 °C/24 h + 180 °C/12 h), σ, θ' (Al 2 Cu) and T 1 phases are detected. Only the T 1 phases can be found in the T8 state alloy (6% pre-strain+135 °C/60 h). The failure modes of alloy in T6 and duplex aging conditions are dimple-intergranular fracture, while typical quasi-cleavage fracture in T8 condition. [ABSTRACT FROM AUTHOR]