Research Progress on Precipitation Behavior and Tailoring of Ω Phase in Al-Cu-Mg-Ag Alloys AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY25010016

Chinese Library Classification Number:TG146

Citation Information:

Aluminum alloys are widely used in fields such as aerospace due to their excellent mechanical properties, corrosion resistance, and low density. The high strength of aged aluminum alloys is attributed to the formation of nanoscale metastable precipitates during the aging process. To address the challenge that conventional aged aluminum alloys suffer from severe coarsening of nano-precipitates at elevated temperatures, leading to a sharp decline in high-temperature performance, researchers have discovered that the addition of Mg and Ag to Al-Cu binary alloys leads to the formation of Ω phase, which exhibits excellent thermal stability. This not only enhances the room-temperature mechanical properties but also significantly improves the high-temperature mechanical performance. The orderly distribution of Ω phase on the {111}_α plane has garnered significant attention in materials science. Its formation mechanism, structural characteristics, and influence on alloy properties remain hot topics of ongoing research. In recent years, with the development of advanced characterization techniques such as spherical aberration corrected transmission electron microscope （AC-TEM）, three-dimensional atom probe tomography （APT）, and in-situ electron microscopy, significant progress has been made in the study of the atomic-scale structure and precipitation behavior of Ω phase, further deepening the understanding of its formation, evolution, and strengthening mechanisms. This paper reviewed the research progress of Ω phase in Al-Cu-Mg-Ag alloys, focusing on discussions in the following five aspects. Firstly, the precipitation behavior of Ω phase was analyzed. There were two main mechanisms for the precipitation of Ω phase. It was currently widely accepted that Ω phase nucleated directly from the aluminum matrix. Specifically, Mg and Ag atoms formed Mg-Ag atomic clusters in the early stages of aging, which subsequently combined with Cu atoms to precipitate and form Ω phase. However, an intermediate phase in the formation process of Ω phase had been observed in recent studies. This suggested that Ω phase might evolve from a precursor phase, Ω', providing strong support for an alternative viewpoint. Although the formation mechanism of Ω phase was still a matter of debate, the crucial role of Mg and Ag elements in the formation process of Ω phase was beyond doubt. The interactions of these elements not only affected the type and distribution of precipitates but also determined the microstructure and macroscopic properties of the alloy. Secondly, the composition and structure of Ω phase were investigated. The chemical composition of Ω phase was Al₂Cu, with a face-centered orthorhombic structure and lattice constants of a=0.496 nm, b=0.859 nm, and c=0.848 nm. The main controversy regarding the crystal structure of Ω phase focused on whether it was a variant of θ phase. Initially, researchers believed that the structure of Ω phase was different from that of θphase. However, with the development of aberration-corrected electron microscopy, it was found that the structure of Ω phase was a variant of θ phase. Unlike the interface structure of θ phase, Ω phase had a double atomic layer of Mg and Ag atoms at the phase interface, which significantly enhanced the overall stability of the phase structure. Thirdly, the thermal stability of Ω phase was studied. Ω phase exhibited excellent thermal stability, maintaining its size stability over extended periods at high temperatures. Its resistance to coarsening was superior to that of the conventional θ phase. However, as the temperature increased, the thickening rate of Ω phase accelerated, leading to a decrease in its stability. The thickening of Ω phase was mainly related to the formation of growth steps. It had been found that the formation of growth steps generated a coherent strain field, which made it difficult for coarsening steps to nucleate on Ω phase. When the service temperature was raised above 250 ℃, the increased number of coarsening steps accelerated the coarsening rate of Ω phase. Fourth, the interaction mechanism between Ω phase and dislocations was analyzed. The interaction mechanisms between precipitates and glide dislocations were mainly divided into two types：bypassing and shearing. The deformation behavior of Ω phase under room temperature tensile and creep conditions differed, but the primary deformation mechanism was shearing mechanism. In addition, this paper also explored the influence mechanism of the interaction between Ω phase and dislocations on the thickening behavior of Ω phase. Finally, this paper discussed the effects of microalloying elements on Ω phase. Microalloying elements could regulate the microstructure of the alloy by influencing the precipitation and coarsening behavior of Ω phase. By altering the number density and size of Ω phase precipitates, microalloying elements could significantly affect the properties of the alloy. Currently, the mechanisms by which microalloying elements influenced the precipitation behavior of Ω phase can be broadly categorized into the following three types：1） forming intermetallic compounds that inhibited the precipitation of Ω phase； 2） suppressing the precipitation of Ω phase while promoting the precipitation of θ' phase； 3） promoting the homogeneous precipitation of Ω phase. Moreover, relevant studies had found that the addition of Sc could induce an in-situ phase transformation of Ω phase during thermal exposure, resulting in a new phase with both high volume fraction and excellent thermal stability. Other microalloying elements also had the potential for similar interstitial ordering. Based on the current research progress, this paper provided an in-depth discussion and outlook on the potential future research priorities and development directions. The aim was to offer valuable references and insights for subsequent studies in this field, thereby promoting the further development and application of Al-Cu-Mg-Ag alloys.