Nanoparticle Composition and Fraction Evolution in Two‑Stage Aging of Al–Zn–Mg Alloys Revealed by APT and HRTEM

Abstract

In this study, we employed atom‑probe tomography (APT) in tandem with high‑resolution transmission electron microscopy (HRTEM) to quantify the size, fraction, and chemical makeup of precipitates that form during a two‑stage, double‑peak aging protocol for an Al‑Zn‑Mg alloy. We found that nanoparticle aluminum content is strongly size‑dependent: particles with equivalent radii below ~3 nm contain >50 at.% Al, whereas those above ~5 nm contain <40 at.% Al. The transformation from Guinier–Preston (G.P.) zones to the equilibrium η phase proceeds via an η′ intermediate that incorporates more Mg and Zn, displacing Al. In the first aging peak, G.P. zones comprise ~85 % of all nanoparticles, while in the second peak η′ phases dominate (~55 %). Even after extended over‑aging (T73), G.P. zones persist at ~20 % and η phases constitute ~63 % of the precipitate population, underscoring their influence on mechanical performance.

Background

Al–Zn–Mg (and Al–Zn–Mg–Cu) alloys rely on precipitation hardening to achieve high strength. Classical precipitation sequences for these systems progress from a supersaturated solid solution through coherent G.P. zones, semi‑coherent η′ intermediates, to incoherent equilibrium η (MgZn₂) phases. Earlier work identified two distinct hardness peaks in a two‑stage aging regime, attributing the first peak to G.P. zones and the second to η′ phases. However, conventional transmission electron microscopy (TEM) offers only two‑dimensional insight, limiting the accurate determination of precipitate fractions and compositions. Atom‑probe tomography (APT), with its 3‑D, sub‑nanometer resolution, provides precise elemental mapping and is uniquely suited to address these gaps. While prior APT studies have reported varying Zn/Mg ratios and Al contents in Al–Zn–Mg precipitates, none have comprehensively tracked the evolving fractions of each precipitate type across the entire aging sequence. This work bridges that gap by combining APT and HRTEM to deliver a full‑scale view of nanoparticle evolution, thereby guiding the optimization of aging schedules for superior alloy performance.

Methods

Material

The alloy investigated (7N01) contains 4.06 wt.% Zn, 1.30 wt.% Mg, 0.30 wt.% Mn, 0.18 wt.% Cr, 0.13 wt.% Zr, 0.05 wt.% Ti, and the remainder Al. Specimens were extrusion‑processed, water‑spray quenched, naturally aged for 72 h, then subjected to a two‑stage artificial aging sequence: 100 °C for 12 h followed by 170 °C for varying durations.

Characterization

Hardness was measured with a micro‑hardness tester. HRTEM imaging was performed on a FEI Tecnai F20 to identify precipitate morphology and orientation relationships. APT data were collected on a CAMECA LEAP 5000 XR, operating at 50 K with a 200 kHz voltage pulsing scheme. Specimen preparation involved a two‑step electro‑polishing protocol (10 % perchloric acid in acetic acid, then 4 % perchloric acid in 2‑butoxyethanol). 3‑D reconstructions and composition analyses were carried out with Imago Visualization and Analysis Software (IVAS) v3.8.0, using a 12.0 at.% (Mg+Zn) isoconcentration surface to delineate precipitates.

Results and Discussion

The two‑stage aging schedule produced four distinct states: under‑aged (UA), first peak (PAI), second peak (PAII), and over‑aged (T73). Hardness data (Fig. 1) confirm a ~15 % drop from the first to the second peak. HRTEM images (Fig. 2) reveal fully coherent G.P. zones in UA, slightly coarsened but still coherent zones in PAI, ellipsoidal η′ phases with lattice distortion in PAII, and fully incoherent hexagonal η phases in T73.

Aging‑hardening curve of the experimental alloy in the second‑stage aging process

BF HRTEM images of typical nanoparticles in different states of the second‑stage aging process: a UA, b PAI, c PAII, and d OA. Selected area electron diffraction (SAED) patterns near [110], [011], [011], and [001] zone axis are shown as insets in a–d, respectively

Three‑dimensional APT reconstructions (Fig. 3) complement the TEM observations and provide quantitative composition profiles. G.P. zones in UA have an average thickness of ~2 nm and contain ~13.8 at.% Zn, ~9.4 at.% Mg, and ~75.8 at.% Al (Zn/Mg ≈ 1.5). In PAI, the same precipitates expand to ~2.5 nm and become enriched in Zn (≈ 23.6 at.%) and Mg (≈ 17.2 at.%), reducing Al to ~57.5 at.%. PAII precipitates are ellipsoidal η′ phases with ~30.3 at.% Zn, ~25.7 at.% Mg, and ~43.4 at.% Al (Zn/Mg ≈ 1.2). In T73, the equilibrium η phase (~6 nm thick) contains ~50.2 at.% Zn, ~30.1 at.% Mg, and ~17.7 at.% Al (Zn/Mg ≈ 1.7). A clear size‑dependent Al trend emerges: particles larger than 5 nm possess <40 at.% Al, whereas those below 3 nm are >50 at.% Al.

Three‑dimensional reconstruction of specimens in different second‑stage aging states: a UA, c PAI, e PAII, and g OA. The composition profiles through marked typical nanoparticles in a, c, e, and g were measured using a selected cylinder (diameter, 3 nm) with a moving step of 0.5 nm and shown in b, d, f, and h, respectively

Distribution of equivalent radius (R_eq) and the corresponding Al content (in at. %) of the nanoparticles in different second‑stage aging states: a UA, b PAI, c PAII, and d OA

Statistical fraction of nanoparticles in different second‑stage aging states

Fraction analysis (Fig. 5) shows that G.P. zones dominate in the first peak (≈85 %) but decline sharply as aging proceeds. η′ phases rise to a peak fraction (~55 %) in PAII before decreasing in T73, where η phases become the majority (~63 %) while G.P. zones retain a modest ~20 %. These trends confirm the dual‑peak hardening mechanism: G.P. zones for the first peak, η′ for the second, and the weakening of hardness in T73 due to the less‑effective η phase.

Typical 1‑nm‑thick atom map (50 × 30 nm) showing the distribution of Mg, Zn, and Al atoms in OA state. The corresponding Al content within nanoparticles were shown as inset

Conclusions

1. In the first peak‑aged alloy, G.P. zones comprise ~92.5 % of all precipitates and contain >50 at.% Al, delivering the maximum hardness.
2. The second hardness peak arises from a mix of η′ phases (~54.5 %) and residual G.P. zones (~22.7 %). η′ phases exhibit intermediate Al content and size between G.P. zones and η phases.
3. η phases emerge only after the second aging stage, with Al content <40 at.% and equivalent radii >5 nm. In T73, η phases occupy ~63 % of the precipitate population, while their Al content gradually declines with further aging.
4. G.P. zones persist during over‑aging because Mg and Zn atoms preferentially feed the larger η phases, limiting the growth of the smaller G.P. zones and allowing them to remain Al‑rich.