Effect of Precipitate Embryo Induced by Strain on Natural Aging and Corrosion Behavior of 2024 Al Alloy

Abstract

Pre-aging precipitates in 2024 Al alloy re-dissolved during rolling, and Mg and Cu atoms were enriched in the dislocation structure to form "precipitate embryo", which is beneficial for the formation of second phase during the subsequent natural aging process. Due to the solubility of the large precipitate particles during deformation, the precipitates in the natural aging 2024 Al alloy were consist of two parts: The undissolved pre-precipitates and the natural aging precipitates derived from the embryos. The electrochemical corrosion behavior and passive film of the samples with different pre-aging time and strain were investigated by electrochemical tests and SEM. The results showed that the corrosion resistance and the passive film performance were deteriorated with the increase of pre-aging time due to the simultaneous increase of the quantity of the two-parts precipitates. The corrosion resistance can be improved by the increase of strain, because there will be smaller grain size and the existence of large undissolved precipitates in smaller strain samples.

Manipulating Precipitation Through Thermomechanical Treatment to Control Corrosion Behavior of an Al–Cu–Mg Alloy

“…The other part was peakaged and then retrogression-treated, instant by applying cold rolling deformation with a reduction of 0, 30, 50, or 70% and then also naturally aged at room temperature for 7 days. The purpose of natural aging is to stabilize the aging state and avoid generating other types of precipitates [13,16,17]. The process parameters are listed in Table 1.…”

“…Studies have shown that when cold rolling deformation was applied directly after solution treatment, the precipitates (> 10 nm) cannot be generated [12,25]. Even pre-aging after solution treatment was treated, the metastable precipitates within the grain interiors can dissolve when the subsequent cold rolling deformation was applied [13].…”

“…In addition, the blocked dislocations at the gap between the adjacent S-phases can also promote the diffusion of solutes, resulting in the formation of new S-phases at the gap and increasing the continuous trend of S-phase on grain boundaries. By XRD tests, Zhang et al [13] found that the sub-grain size of 2024 aluminum alloy was refined with the increase of the cold rolling deformation. When the cold rolling deformation exceeded 40%, the sub-grain size was significantly refined, indicating that the grain boundaries of the original equiaxial grain were destroyed in the rolling process.…”

“…Because the thermomechanical treatment relies on existing processing conditions and is feasible for industrial applications [12]. For example, when the as-aged 2024 alloys were subjected to rolling deformation, the Sʹ phase (Al 2 CuMg) that was formed by solution treatment and aging was dissolved, resulting in the improved corrosion resistance [13]. Specifically, the IGC resistance of 2024 Al alloy was improved after solution treatment, 83% cold rolling deformation, and peak re-aging or over-re-aging; the improvement was attributed to the dislocation pile-ups on grain boundaries, which prevented the diffusion of the solutes and the continuous precipitation of S-phase (Al 2 CuMg) on grain boundaries [12].…”