A First-Principles Study of the Cu-Containing β″ Precipitates in Al-Mg-Si-Cu Alloy

Abstract

The nanostructured β″ precipitates are critical for the strength of Al-Mg-Si-(Cu) aluminum alloys. However, there are still controversial reports about the composition of Cu-containing β″ phases. In this work, first-principles calculations based on density functional theory were used to investigate the composition, mechanical properties, and electronic structure of Cu-containing β″ phases. The results predict that the Cu-containing β″ precipitates with a stoichiometry of Mg4+xAl2−xCuSi4 (x = 0, 1) are energetically favorable. As the concentration of Cu atoms increases, Cu-containing β″ phases with different compositions will appear, such as Mg4AlCu2Si4 and Mg4Cu3Si4. The replacement order of Cu atoms in β″ phases can be summarized as one Si3/Al site → two Si3/Al sites → two Si3/Al sites and one Mg1 site. The calculated elastic constants of the considered β″ phases suggest that they are all mechanically stable, and all β″ phases are ductile. When Cu atoms replace Al atoms at Si3/Al sites in β″ phases, the values of bulk modulus (B), shear modulus (G), and Young’s modulus (E) all increase. The calculation of the phonon spectrum shows that Mg4+xAl2−xCuSi4 (x = 0, 1) are also dynamically stable. The electronic structure analysis shows that the bond between the Si atom and the Cu atom has a covalent like property. The incorporation of the Cu atom enhances the electron interaction between the Mg2 and the Si3 atom so that the Mg2 atom also joins the Si network, which may be one of the reasons why Cu atoms increase the structure stability of the β″ phases.

Microstructure and Properties of 6 Series Aluminum Alloy Under Different Aging Treatment Systems

“…The strengthening effect of the alloy improved significantly [15]. With the extension of natural aging, the supersaturation of solute atoms diminishes gradually, the precipitation rate of atomic clusters decreases, and the hardness value of the alloy increases slowly and stabilizes [16]. After artificial aging treatment, there is a sharp rise in the hardness value of natural aging specimens.…”

First-Principles Study on Structure and Stability of GP Zones in Al-Mg-Si(-Cu) Alloy

“…The formula for studying the interaction between vacancies within the GP zones and point defects is similar to (1). The interaction energy is defined as follows:…”

“…The 6xxx series Al-Mg-Si(-Cu) alloy is widely used in the transportation and construction industries due to its good physical and chemical properties (such as excellent corrosion resistance, high specific strength, good formability, and weldability) [1][2][3]. Nanoscaled precipitation strengthening is a crucial method for enhancing the mechanical properties of Al-Mg-Si alloy [4][5][6].…”

“…Nanoscaled precipitation strengthening is a crucial method for enhancing the mechanical properties of Al-Mg-Si alloy [4][5][6]. The precipitation sequence of Al-Mg-Si alloy is as follows [1,7,8]: supersaturated solid solution (SSSS) → atomic clusters → Guinier-Preston (GP) zones → β , β , U1, U2, B → β, Si. Among these precipitates, the GP zones denote the nanoprecipitate phase, which forms in the initial stages of aging precipitation, possessing an ordered structure and perfect coherency with the matrix [9].…”