Electron imaging reveals atomic structure of strengthening precipitates in high-silicon aluminum alloy

A cast aluminum alloy designed to carry heavy loads in automotive components owes its strength to structures only a few nanometers across, and a new study has now imaged those structures atom by atom. Researchers examining a high-silicon Al–Si–Mg alloy report that the dominant strengthening contribution in the peak-aged material comes from an exceptionally high number density of coherent β″ precipitates, and they describe a practical imaging route to resolve the atomic arrangement of these precipitates even when conventional contrast mechanisms fall short. The work, published in Applied Microscopy, combines hardness testing, electron microscopy and a carefully controlled specimen preparation strategy to make visible what is normally hidden inside the aluminum matrix.

The alloy in question belongs to a family of lightweight materials valued for high strength and ductility, excellent corrosion resistance and superior castability. These qualities make Al–Si–Mg alloys widespread in automotive applications, particularly for complex, intricate components produced by die-casting. The alloys are often used in the as-cast state, where the manufacturing process demands high castability, typically achieved by pushing the silicon content toward the upper end of the eutectic composition, around 12.6 weight percent. This elevated silicon level improves fluidity during casting. After casting, the material receives age-hardening treatments, either directly in the T5 route or after a high-temperature solution treatment in the T6 route, producing nano-sized metastable precipitates that significantly influence strength and final mechanical properties.

Within high-silicon Al–Si–Mg alloys, precipitation follows a well-documented sequence: a supersaturated solid solution first forms solute-rich clusters, then GP zones, then the β″ phase, followed by β′, U1, U2 and B′ phases, and finally equilibrium β and silicon precipitates. The principal strengthening phase is β″, a needle-like precipitate with a monoclinic unit cell defined by lattice parameters a = 1.516 nm, b = 0.405 nm, c = 0.674 nm and β = 105.3°, belonging to space group C2/m. Each β″ unit cell contains two structural units known as the β″-eye or low-density cylinders. The eye structure resembles a face-centered cubic aluminum unit cell, with magnesium atoms occupying the four corner positions, silicon atoms at the four face centers, and a central column occupied by magnesium or aluminum atoms.

Those central atoms are larger than their neighbors, and their presence distorts the surrounding lattice. To minimize this distortion, the larger Mg/Al atoms shift by half a unit cell along the b-axis, moving into an interstitial position between four smaller silicon atoms. The shift yields a more symmetric and spacious configuration, reducing elastic strain within the β″ structure. In two-dimensional transmission electron microscope images, the β″-eye appears as a central atom surrounded by four magnesium and silicon atoms arranged nearly in a circle, resembling an eye. The orientation relationship between β″ and the aluminum matrix is strict: [100]β″ is parallel to [302]Al, [010]β″ to [010]Al, and [001]β″ to [10-3]Al. The [010]β″ direction forms the long axis of the needles, with a periodicity of 4.05 nm matching face-centered cubic aluminum, and along this axis the precipitates are fully coherent with the matrix while remaining semi-coherent in the other two directions.

That strict orientation relationship is precisely what makes these precipitates so difficult to observe. When the microscope’s zone axis aligns with <001>Al, the (010)β″ plane sits nearly parallel to {010}Al planes, and the needle axis of β″ aligns with <10-3>Al. In this orientation, aluminum atoms project as a simple square grid while the β″ magnesium-silicon sublattice adds a periodic modulation that does not coincide with the aluminum columns, generating extra diffraction reflections and streaks clearly separated from the fundamental aluminum spots. But when the zone axis aligns with <110>Al or <111>Al, the β″ atomic columns overlap with aluminum columns, the extra reflections crowd close to the strong matrix reflections, and the nanoscale precipitates become difficult to distinguish or essentially invisible against the aluminum background.

A second obstacle is chemical. Aberration-corrected high-angle annular dark-field imaging, a mainstay of atomic-resolution microscopy, relies on differences in atomic number, yet magnesium, aluminum and silicon sit at atomic numbers 12, 13 and 14. The resulting Z-contrast is insufficient to clearly identify atomic columns in nanometer-sized clusters embedded within the thicker aluminum matrix. High-angle detectors also collect less signal, degrading the signal-to-noise ratio. For commercial high-silicon cast alloys, specimen preparation adds a further complication: electro-polishing readily produces pitting associated with silicon particles, so TEM specimens are usually prepared by focused ion beam lift-out. FIB milling, however, can introduce lamella bending and ion-beam-induced damage, both of which complicate imaging. Most earlier studies of wrought 6xxx-series alloys with lower silicon-to-magnesium ratios used electro-polishing and avoided these artefacts entirely.

The team addressed these difficulties with a low-angle annular dark-field approach, which derives contrast from small atomic displacements rather than from atomic-number differences. The alloy, with a composition of Al-10.2Si-0.35Mg, was produced by conventional gravity casting and analyzed by inductively coupled plasma optical emission spectroscopy. It received a solution treatment at 480 °C for 6 hours followed by eutectic silicon spheroidization at 535 °C for 1 hour, and was then aged directly at 155 °C without natural aging at room temperature. TEM lamellae were cut with a focused ion beam, with ion milling performed at low currents of roughly 50 picoamperes and voltages between 5 and 3 kilovolts after pre-thinning, to limit damage, bringing the final sample thickness to approximately 80 nanometers.

Because the FIB process leaves only a small analysis area of about 3 by 6.5 micrometers, compared with the 3-millimeter discs obtained by electro-polishing, lamellae often require substantial tilting to reach a desired zone axis, and tilting changes the sample’s effective thickness and degrades diffraction conditions. To avoid this, the researchers used electron backscatter diffraction before FIB thinning to select a single grain oriented close to <001>Al along all three sample axes, ensuring a nearly perfect cube-cube relationship between sample and crystal and minimizing the tilt needed during imaging. Imaging was performed on a Thermo Fisher Themis Z operated at 300 kilovolts, with simultaneous collection of LAADF signals at inner and outer angles of 20 and 78 milliradians and HAADF signals at 83 and 203 milliradians; the raw images were presented without filtration.

The mechanical measurements traced the alloy’s response to aging. After solution treatment the alloy showed a Vickers hardness of 72.94 Hv, which climbed steadily during artificial aging at 155 °C and peaked at 110.18 Hv after 12 hours, the peak-aged condition attributed to the high number density of coherent β″ precipitates. Beyond 12 hours the hardness declined as the coherent precipitates transitioned into semi-coherent ones. Bright-field TEM of the peak-aged alloy revealed dot-like precipitates corresponding to cross-sections of the β″ phase, with an average diameter of 1.30 nanometers and a number density of 3.09 × 10⁻⁴ nm⁻³. High-resolution TEM along the [010]Al zone axis showed distinct β″ reflections in the fast Fourier transform, while along the [110]Al zone axis only streaks appeared, corresponding to precipitates nearly invisible against the aluminum background.

The decisive images came from comparing the two dark-field modes. In both HAADF and LAADF images the atomic planes of the precipitates differed clearly from the surrounding matrix, but the high-angle scattering in HAADF made it difficult to distinguish silicon, magnesium and aluminum atoms within the precipitates. The low-angle scattering in LAADF provided improved atomic contrast, resolving the ordered eye-like structure: four larger magnesium atoms and smaller silicon atoms forming the rim, with a single larger atom at the center. A high-magnification LAADF image and schematic showed that each precipitate contains six such eyes, the minimum reported for a long-range ordered monoclinic β″ structure in three dimensions, and the observed precipitates maintained fully consistent ordering without the stacking irregularities typical of disordered β″. The authors conclude that the high number density of coherent β″ precipitates is the primary strengthening phase in the peak-aged, high-silicon alloy, and that LAADF imaging along the <010>Al zone axis offers an efficient route to visualize β″ atomic structure where conventional Z-contrast methods struggle.

Subject of Research: Atomic-resolution electron microscopy of β″ strengthening precipitates in a high-silicon Al–Si–Mg cast alloy

Article Title: Atomic structure of β″ precipitates in high-Si containing Al–Si–Mg alloy

Article References: Kayani, S. H., Lee, S.-I., Lee, Y.-H., Lee, J.-M., Euh, K., & Cho, Y.-H. (2026). Atomic structure of β″ precipitates in high-Si containing Al–Si–Mg alloy. Applied Microscopy, 56(1), Article 8. https://doi.org/10.1186/s42649-026-00128-8

Image Credits: AI Generated

DOI: 10.1186/s42649-026-00128-8

Keywords: Al–Si–Mg alloy, β″ precipitates, LAADF imaging, HAADF, transmission electron microscopy, precipitation hardening, peak aging, atomic structure, focused ion beam, aluminum alloys, age-hardening, nanoscale precipitates