Effect of a prior stretch on the aging response of an Al-Cu-Li-Ag-Mg-Zr alloy

The objective of this study was to investigate the cyclic deformation mechanisms at the microstructural level and their correlation with the initiation and early propagation of small fatigue cracks in super austenitic stainless steel 654SMO. The findings revealed that localized microstructural variations in grain size and orientation led to strain concentration, which subsequently triggered crack initiation. The behavior of surrounding microstructural evolution influences the crack propagation mode, further affecting the crack propagation rate. The retarding effect of small grain cluster regions on grain growth is weaker compared to high-angle grain boundaries and pinning by small grains, and multiple slip will trigger cross slip, causing uneven local strain within the grains, thus altering the crack propagation path.

This work investigates the impact of Sc and Zr on the precipitation kinetics in an Al-Cu-Li alloy in the absence of pre-stretching. Modern Al-Cu-Li alloys usually require pre-stretching to accelerate the formation kinetics of the T₁ precipitates. A combination of in-situ Small-Angle X-ray Scattering (SAXS) and hardness are used to monitor the precipitation kinetics in an Al-Cu-Li alloy with and without Sc and Zr. The presence of Al₃(Sc, Zr) dispersoids is found to significantly accelerate the precipitation kinetics. Surprisingly, transmission electron microscopy (TEM) reveals the absence of T₁ precipitates in the peak aged condition and instead reveals that ${\theta }^{\prime }$ precipitates dominate the microstructure. Atom probe tomography (APT) and density functional theory (DFT) calculations reveals that the Al₃Zr-shell of the dispersoids is a preferred heterogeneous nucleation sites of ${\theta }^{\prime }$ precipitates. Additions of Sc and Zr can hence be a suitable solution to promote precipitation in Al-Cu-Li alloys where the pre-stretching is not applicable.

The fatigue crack propagation (FCP) behaviors of Al-Cu-Li cold-rolled sheets with different Cu contents were assessed systematically. The increase of Cu contents significantly reduces the threshold value of fatigue crack growth as secondary particles induce the initiation of microcracks, and provides more opportunities for crack deflection due to finer recrystallized grains. Moreover, Brass-orientated grains with low Schmid factor force the crack to change the direction of propagation, which prevents further crack propagation. Compared to Goss-orientated grains with small tilt and twist angle, cracks prone to deflect at its boundaries with large tilt angles difference between the adjacent grains.

The effects of re-solution, pre-stretching and double re-ageing (RPD) treatment on the microstructure and mechanical performance of 2195 Al–Cu–Li alloy were investigated. The results indicate that RPD treatment enhances the strength from ∼567 MPa with a standard T8 temper to ∼584 MPa. A simultaneous improvement in strength and ductility is attained in a PA-RPD sample processed by 510 °C × 1 h re-solution, 7% pre-stretching and 120 °C × 12 h + 155 °C × 24 h double re-ageing. The improved mechanical properties are ascribed to the re-tailor of microstructures during RPD treatment involving grain morphology, dislocation density, and precipitation feature. The PA-RPD samples with varied pre-strains exhibit no evident difference in grain size, but slightly larger than that at T8 state and much smaller than that at re-solution state. The dislocation density is mainly determined by the pre-stretching levels and suffered minor annihilation in double re-ageing. T₁ precursor phases are nucleated from first-step re-ageing (120 °C), most of which are connected to pre-stretched dislocations, and then grow into well-dispersed T₁ plates in second-step re-ageing (155 °C). The increased strength is dominantly attributed to the reversible finer precipitation of nano-sized T₁ plates, yielding considerable precipitation strengthening, as well as higher dislocation density, producing dislocation strengthening. The improved ductility in the PA-RPD sample is mainly on account of the finer and more uniform distribution of T₁ plates which enhance the plastic stability during tensile testing.

The tensile properties and cycle fatigue behaviors of 2A55 Al-Cu-Li alloy plates were assessed along the rolling direction (RD) and transverse direction (TD). The stronger anisotropy in elongation attributes to the grain boundary and crystallographic texture, and decreases as the formation of precipitation-free zones. The cyclic hardening is caused by the interaction between dislocations and dislocations or non-shearable precipitates. The non-shearable T₁ precipitates could be bypassed via dislocation cross-slip and then more multiple slip planes are activated, which causes more shearable precipitates could be cut, leading to the cyclic softening at higher strain amplitude.

The addition of minor elements is effective in designing high-strength aluminum alloys by modifying precipitation, such that clarifying the strengthening origin is highly significant. Here, 0.5 wt% Li is added in an Al–Cu–Mg alloy (AA2024 alloy), where, obvious increases in strength without reducing the ductility are achieved in both the unstretched and stretched alloys, as compared to those in the counterpart Li free alloys. The enhanced strength arising from Li addition is mainly related to the modified precipitation behavior (rather than the solute strengthening due to Li elements), in terms of Guinier–Preston–Bagaryatsky (GPB) zones and S-phase; the strengthening sources in the unstretched and stretched alloys, however, are fundamentally different. Correlative characterizations reveal that, for the unstretched alloy in the peak-aging (PA) state, the Li-addition favors a high density of uniformly distributed GPB zones while suppressing the nucleation of S-phase precipitates. Whereas, for the stretched alloys in the PA state, the Li-addition, in combination with the possible strengthening due to the T₁ phase, results in refined S-phase precipitates. These modified precipitates (GPB zones and S-phase) are decorated with Li enrichments, which are believed to be correlated with the proposed mechanism of the Li-Vacancy complex. Thus, elucidating the origin of strengthening in Li-added Al–Cu–Mg alloys under both unstretched and stretched conditions, is expected to provide basic insights into the strengthening mechanisms in other high-strength Al alloys when Li is added as the minor element.