MAY WE MAKE IT ANY LONGER?
Long-term aging as the key to mechanical excellence in AMAG CrossAlloy®.57
The AMAG CrossAlloy® initiative aims to develop pioneering aluminum alloys with high sustainability and performance standards. This article focuses on CrossAlloy®.57, which exhibits outstanding strength and ductility properties thanks to long-term aging (LTA) in the low temperature range. Particularly in comparison to conventional T6 heat treatment, it shows significant advantages in terms of formability combined with high yield strength - a stress-strain performance that could open the door to new applications.
AMAG CrossAlloy®.57 represents an innovative class within the commercially available aluminum alloys. The CrossAlloy® concept allows the combination of the properties and strengths of 5xxx and 7xxx series alloys.
The targeted microstructural optimization results in a striking combination of high strength, improved ductility, and unique functional properties, making this concept increasingly relevant [1]. The age-hardening potential with low Zn/Mg ratios (<1) has been studied using transmission electron microscopy (TEM) and atom probe tomography (APT). The precipitation sequence favors either the formation of equilibrium T-phase (Mg₃₂(Al, Zn)₄₉ or its metastable form known as Tη-phase), or a combination of the latter with the MgZn2 phase (known as η-phase) [2-4], which is decisively influenced by the specific heat treatments [5,6]. In addition to conventional precipitation hardening, cluster hardening as an alternative hardening mechanism has increasingly become the focus of research in recent times. Cluster hardening has proven to be a promising mechanism for improving the strength-ductility trade-off and offers a competitive alternative to conventional precipitation hardening treatments [7-10].
Despite numerous studies on cluster formation in aluminum alloys [7,8,11], a universally accepted definition of a "cluster" remains elusive. In [7], clusters are described as localized accumulations of alloying element atoms lacking recognizable structure or order. However, recent findings [8] reveal that most clusters, although disordered on the long-range scale, do exhibit local atomic order characterized by preffered column structures with near-neighbor configurations. Recent studies [10,12] have shown that cluster-hardened aluminum alloys can achieve strength levels comparable to or even exceeding those of conventional T6-treated alloys while maintaining excellent ductility.
This advantageous combination results from maintaining an exceptionally small particle spacing, ideally equal to or less than the critical annihilation distance of dislocations, which requires an extremely high number density of ultrafine clusters or precipitates [10,13].
Building on this concept, this study investigates the potential of cluster hardening during low-temperature long-term ageing in the novel AMAG CrossAlloy®.57. The objective is to improve understanding of how microstructural changes influence mechanical properties. In particular, the study seeks to determine the extent to which clusters contribute to strength enhancement without significantly compromising ductility. Given the scarcity of data for this class of alloys, the work provides a significant contribution to the targeted development of heat-treatable high-strength aluminum alloys with optimized property profiles.
Chemical composition and heat treatment strategies: The basis for optimized aging conditions
The two investigated alloys are based on an EN AW-5182 with tailored additions of Zn and, optionally, Cu. As shown in Table 1, the Zn/Mg ratio is maintained at ~ 0.8, with a combined Zn+Mg content of roughly 9 wt.-%. Mn (~ 0.4 wt.-%) and Fe (~ 0.2 wt.-%) are added to promote stable dispersoid formation. The Cu-free variant contains < 0.1 wt.-% Cu, while the Cu-containing alloy includes 0.7 wt.-% Cu. The composition is designed to facilitate both cluster hardening and good recyclability.
| Alloy | Mg | Zn | Cu | Fe | Mn | Si | Zn/Mg | Zn+Mg |
|---|---|---|---|---|---|---|---|---|
| Mg+Zn [wt.-%] | 5,1 | 3,8 | >0,1 | 0,1 | 0,4 | 0,1 | ~ 0.8 | 8,9 |
| Mg+ZnCu [wt.-%] | 5,0 | 3,8 | 0,7 | 0,2 | 0,4 | 0,1 | ~ 0.8 | 8,8 |
To evaluate the cluster behavior of the alloys, various heat treatments were performed on the material in cold rolled condition (see Table 2).Solution annealing is applied at 465 °C/35 min, followed by immediate water quenching. The PA (Pre-Aging) condition, which involves pre-aging at 60 °C/5 h, promotes the initial formation of clusters, which are precursors for subsequent precipitates. Condition LTA (Long-Term-Aging) represents long-term aging at 60 °C for 42 days.The parameters for conditions PB and PB-2 % (paint bake without and with 2 % plastic predeformation) are based on standard test parameters for cathodic dip coating (“paint baking”, 185 °C/20 min) commonly used in the automotive industry. Finally, condition T6 describes a two-stage heat treatment, starting with pre-aging (100 °C/5 h), followed by aging at 185 °C/1-7 h until peak hardness is reached.
| Temper | Solution heat treatment + WQ | Pre-Aging | Natural Aging | Pre- deformation | Paint-Bake | T6 |
|---|---|---|---|---|---|---|
| Pre-Aging | 465 °C 35 min | 60 °C 5 h | - | - | - | - |
| Long-Term-Aging | 465 °C 35 min | 60 °C 42 d | - | - | - | - |
| Paint Bake / predeformation | 465 °C 35 min | 60 °C 5 h | 25 °C 14 d | 2 % | 185 °C 20 min | - |
| Paint Bake | 465 °C 35 min | 60 °C 5 h | 25 °C 14 d | - | 185 °C 20 min | - |
| T6 | 465 °C 35 min | 60 °C 5 h | - | - | - | 185 °C 1, 2 h |
Mechanical properties: LTA compared to conventional heat treatment strategies
Figure 1 illustrates the favorable position of LTA in the yield strength-elongation diagram. LTA offers a balanced combination of strength and elongation, exceeding other heat treatments with 334 MPa / 19.2 % (Cu-poor) and 398 MPa / 17.1 % (Cu-rich). Compared to T6 condition, the yield strength drops only moderately (-89 MPa or -66 MPa), while elongation behavior is far superior. Paint-bake treatments (PB, PB 2 %) further improve strength (up to 455 MPa / 493 MPa), but lead to a distinct reduction in elongation (down to 9-12 %).
PA condition demonstrate the highest elongation (21.8 % / 19.4 %), but the lowest strength (204 MPa / 284 MPa). Consequently, LTA serves as an optimal basis for comparison in order to exploit the full mechanical potential of AMAG CrossAlloy®.57. These results provide the mechanical framework for subsequent APT analysis in LTA to determine the underlying microstructural reasons.
APT in focus: Details of cluster/precipitate structure and the impact of Cu
Atom probe tomography (APT) was employed to characterize the nanoscale solute distributions in LTA condition, enabling three-dimensional spatial resolution of individual atomic species. The APT analysis displayed in Figure 2 reveals a dense network of very fine clusters alongside medium-sized precursor phases or early-stage precipitates, with an average radius of approximately 2 nm. Pronounced differences in cluster and precipitate evolution between the Cu-free and Cu-containing variants were observed. Alloy Mg+ZnCu exhibits a significantly higher number density of cluster/precipitates compared to the Cu-free reference.
These cluster/precipitates are generally smaller in size but chemically more defined, with Cu preferentially incorporated over Zn. Compositional analysis (Table 3) reveals that the Cu-containing cluster/precipitates reach local concentrations of up to 1 at.-% Cu, whereas the Cu-free alloy shows Cu levels below 0.1 at.-% within equivalent features. This pronounced Cu enrichment gives rise to the formation of Cu-rich T-phase precursors, which are considered to contribute substantially to the mechanical strengthening in the aged state.
Of particular interest is the remarkable microstructural stability observed after 5 % plastic deformation. Neither the number density nor the average size of the cluster/precipitates changes significantly. This initially appears to contradict the chemical analysis in Table 3, which shows that relevant amounts of Mg, Zn and Cu are still dissolved in the matrix after LTA.
| LTA condition | Strain (%) | Composition | Mg | Zn | Cu | Zn/Mg |
|---|---|---|---|---|---|---|
| Mg+Zn | 0 % | Overall (at.-%) Matrix (at.%) Cluster / Precipitate Solute fraction (%) | 5.58 ± 0.02 5.13 ± 0.02 8.75 ± 0.07 | 1.67 ± 0.03 1.01 ± 0.02 6.29 ± 0.07 6.75 ± 0.03 | 0.03 ± 0.03 0.03 ± 0.02 0.05 ± 0.07 6.75 ± 0.03 | 0.30 ± 0.005 0.20 ± 0.004 0.72 ± 0.01 6.75 ± 0.03 |
| Mg+ZnCu | 0 % | Volume fraction (%) Overall (at.-%) Matrix (at.%) Cluster / Precipitate (at.%) | 5.68 ± 0.04 5.10 ± 0.03 9.56 ± 0.10 | 1.65 ± 0.04 0.86 ± 0.03 6.89 ± 0.10 7.21 ± 0.04 | 0.23 ± 0.04 0.21 ± 0.03 0.39 ± 0.10 7.21 ± 0.04 | 0.29 ± 0.008 0.17 ± 0.01 0.72 ± 0.01 7.21 ± 0.04 |
| Mg+ZnCu | 5 % | Solute fraction (%) Volume fraction (%) Overall (at.-%) Matrix (at.%) | 5.43 ± 0.02 4.95 ± 0.02 8.71 ± 0.06 | 1.69 ± 0.02 1.01 ± 0.02 6.28 ± 0.06 7.01 ± 0.02 | 0.26 ± 0.02 0.24 ± 0.02 0.41 ± 0.06 7.01 ± 0.02 | 0.31 ± 0.004 0.20 ± 0.003 0.72 ± 0.01 7.01 ± 0.02 |
However, a detailed examination suggests that the system has reached a kinetic equilibrium after 42 days at 60 °C, in which the remaining supersaturation no longer has any significant driving force for the formation of new clusters. Although the plastic deformation increases the vacancy concentration by several orders of magnitude and thus favors the diffusion of dissolved Mg and Zn (see Figure 3), neither the remaining supersaturation nor the deformation energy is sufficient to initiate strain-induced clustering. Instead, there is a moderate re-dissolution of Zn from existing cluster/precipitates and thus a slight decrease in the number density of larger cluster/precipitates. Cu, on the other hand, shows a shorter diffusion length and additionally stabilizes the remaining cluster/precipitates. This behavior indicates that the system remains in a metastable, kinetically blocked state after LTA, in which dissolved atoms are present, but their reassembly into new clusters is suppressed by energetic and kinetic barriers.
The finely dispersed clusters and precipitates act as effective obstacles to dislocation motion, maintaining their size and number density even under moderate plastic deformation. This not only increases the yield strength, but also significantly inhibits dynamic recovery. The small distance between the cluster/precipitates favors the accumulation of dislocations, enabling high strain hardening and thus exceptionally high elongations.
In short, the APT data in LTA condition show that Cu not only increases the number density of cluster/precipitates and chemical stability, but also produces a microscopic structure that persists even under moderate plastic deformation. This microstructural robustness lays the foundations for the remarkable strength-ductility synergy of the investigated CrossAlloy®.57.
Summary:
The results of this investigation underscore the efficacy of low-temperature, long-term aging as a key instrument for making fine adjustments to mechanical properties in modern aluminium alloys. Particularly when combined with the targeted addition of Cu, it is possible to produce a microscopic cluster structure that simultaneously optimizes strength and formability. In addition to their scientific relevance, these insights also offer practical value for industrial applications in a wide variety of areas.
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Sources:
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