Key Takeaways & Executive Findings
- •• Optimal single-stage aging at 120°C for 24h yields UTS of 523 MPa, YS of 482 MPa, and elongation of 1.75%. • Double-stage aging (120°C/24h + 150°C/52h) significantly improves ductility to 4.17% elongation but reduces strength. • Increasing solution time dissolves strengthening phases, reducing hardness and coarsening grains. • TEM reveals that longer second-stage aging enlarges grain size, precipitate spacing, and precipitate-free zone width.
Abstract
Effects of solution and aging treatment on the microstructure and mechanical properties of a novel Al-Zn-Mg-Cu alloy by microalloying rare elements Sc and Er were studied. The results show that solution time has a visible influence on the microstructure and mechanical properties of the alloy. Specifically, as the solution time increases, the area fraction of the residual phase in the alloy decreases, and the shape of the grain becomes more spheroidal and coarser, leading to a decrease in hardness. This is attributed to the dissolution of strengthening phases during the solution treatment, which weakens the solid solution strengthening effect. The single-stage aging treatment shows an initial increase in strength and hardness of the alloy, followed by a decrease as the aging time is extended, until a steady state is achieved. The optimal single-aging conditions are found to be at 120 °C for 24 h, where the alloy exhibits an excellent combination of high strength and good ductility, with an ultimate tensile strength (UTS) of 523 MPa, yield strength (YS) of 482 MPa, and elongation (El) of 1.75%, respectively. Compared to single-stage aging, double-stage aging (120 °C for 24 h and then 150 °C for 52 h) significantly increases the elongation of the alloy (4.17%), but the UTS reduces to 465.29 MPa, and YS reduces to 410.64 MPa. Transmission electron microscopy (TEM) observations disclose that the grain size, the distribution spacing of precipitates along the grain boundary, and the width of the precipitation-free zone (PFZ) all undergo augmentation as the duration of the second stage aging process elongates.
1. Introduction
High-strength, precipitation-hardened 7xxx series Al-Zn-Mg-Cu alloys, characterized by their advanced age-hardening response, are widely utilized in the fabrication of critical aircraft structural components, where they are exposed to stringent conditions. Strength, ductility, modulus, corrosion resistance, and damage tolerance (such as fatigue resistance and fracture toughness) are crucial characteristics that need to be taken into account for these applications. With suitable elemental alloying, processing and heat treatment, Al-Zn-Mg-Cu alloys could be applied more widely in the aerospace industry due to their remarkable properties [1,2].
In order to meet the high requirements of modern industry, the micro-alloying technology, as an effective method, has been widely applied to improve the comprehensive performance of many aluminum alloys, including Al-Zn-Mg-Cu alloys. Sc and Zr transition metals, as well as Er could react with liquid aluminum and form thermodynamically stable dispersed phases known as Al3M trialuminides. These phases have a similar crystal structure and lower lattice parameter mismatch in relation to the α-Al matrix [3,4].
The addition of Sc has a strong refining effect on the grain structure of Al-matrix and forms a super saturated solid solution during solidification. Fine, coherent precipitates with an L12 structure, specifically Al3Sc, can form during the decomposition of an Al-Sc solid solution [5-7]. Scanium (Sc) enhances the recrystallization resistance of the alloys. It is reported that recrystallization of aluminum alloys containing Sc occurs at temperatures above 375 °C, which is about 100 °C higher than that of Sc free alloys [8]. Li et al. [9] investigated the effects of minor Sc on the microstructure and mechanical properties of Al-Zn-Mg-Cu-Zr based alloys. With the addition of 0.21% Sc, both the ultimate tensile strength and yield strength were improved greatly. Additionally, the as-cast crystal grains were refined and the recrystallization of the studied alloys was strongly retarded. Similar to Sc, the heaviest rare earth element erbium (Er) can partially or fully substitute for Sc in the L12 structured Al3(Sc, Er) phase [10]. This substitution is found to enhance the strength and refine the grains by forming the Al3Er phase. The dispersed nanosized Al3Er phase serves as the sites for heterogeneous nucleation of the η(MgZn2) phase, which results in grain refinement and an improvement in mechanical properties, and at the same time reduce the cost of alloys [11]. Li et al. [12] studied the effect of Er addition on microstructure and mechanical properties of Al-20% Si alloy. It has been found that Er can significantly refine the primary Si crystals and modify eutectic Si structure, fin
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Yan-mei Yang, Xin-chen Tang, Ye-fei Li, Xue-wei Fang, Da-wei Yi, Zhi-yun Zhang, Qiao-ling Zheng, Zhi-xi Wang, Guo-yu Zhang, Su Zhang, Hao Cheng, Yang He, Yi-bo Liu, Zi-han Wu, Ya-nan Chen, Yi-min Gao (2025). Effect of heat treatment on microstructure and mechanical properties of a novel Al-Zn-Mg-Cu alloy. China Foundry. https://doi.org/10.1007/s41230-025-4017-1
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Frequently Asked Questions
What is the optimal single-stage aging condition for the novel Al-Zn-Mg-Cu alloy?
The optimal single-stage aging condition is at 120 °C for 24 hours, which yields an ultimate tensile strength of 523 MPa, yield strength of 482 MPa, and elongation of 1.75%.
How does double-stage aging affect the mechanical properties compared to single-stage aging?
Double-stage aging (120 °C for 24 h followed by 150 °C for 52 h) significantly increases elongation to 4.17%, but reduces ultimate tensile strength to 465.29 MPa and yield strength to 410.64 MPa.
What is the effect of increasing solution time on the alloy's microstructure and hardness?
Increasing solution time decreases the area fraction of residual phases, makes grains more spheroidal and coarser, and reduces hardness due to the dissolution of strengthening phases.
What microstructural changes are observed with prolonged second-stage aging?
Transmission electron microscopy reveals that longer second-stage aging increases grain size, the distribution spacing of precipitates along grain boundaries, and the width of the precipitate-free zone.
What role do rare earth elements Sc and Er play in the alloy?
Sc and Er form thermodynamically stable Al3M phases that refine grains, enhance recrystallization resistance, and improve mechanical properties by serving as nucleation sites for strengthening precipitates.
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