Lightweight, strong single-phase Al-based complex concentrated alloy

Lightweight and strong aluminum-based alloys are crucial structural materials for achieving energy savings and emission reductions. However, due to the limited solubility of other lightweight elements in aluminum, developing high-performance single-phase aluminum alloys (which are lighter yet maintain high mechanical strength) through conventional methods faces significant theoretical challenges. Recently, a research team led by Dr. Qiaoshi Zeng from Center for High Pressure Science and Technology Advanced Research Center (HPSTAR) and Prof. Zhaoping Lü from the University of Science and Technology Beijing successfully synthesized a lightweight (with a density of 2.4 g/cm³), but high-strength (with a yield strength of 1.7 times of that of conventional aluminum alloys) face-centered cubic (fcc) single-phase aluminum-based complex concentrated alloy (CCA), Al55Mg35Li5Zn5, by employing high-pressure and high-temperature methods. This research provides a feasible approach to overcoming traditional theoretical limitations and exploring ultra-lightweight single-phase high-strength aluminum alloys with excellent mechanical properties. The results were recently published in Nature Communications.      

Aluminum-based alloys are widely valued for their low density (just one-third that of steel or brass), weak magnetism, and high specific strength, making them one of the most popular lightweight engineering materials. Developing even lighter aluminum-based alloys offers significant engineering and economic benefits, which often requires the incorporation of other, lighter elements to partially replace aluminum. However, due to limited solubility, the addition of too many lightweight elements can lead to the formation of brittle intermetallic compounds, which severely degrade the alloy's mechanical properties. Thus, synthesizing aluminum-based alloys that are both lightweight and extremely strong presents inherent theoretical chellenges.

The research team selected magnesium (with a density of 1.74 g/cm³) and lithium (with a density of 0.54 g/cm³), which are much lighter than aluminum (with a density of 2.70 g/cm³), as well as zinc, which often enhances the alloy's corrosion resistance and thermal stability, as alloying elements. They explored a range of Al-based alloys at a pressure of 8 to 30 GPa and a temperature from room temperature to 2000 K conditions to identify the optimal alloy compositions and synthesis conditions under conditions of ~10 GPa and 950 K. They discovered an alloy with significantly lighter density and higher specific yield strength compared with most previously reported alloy materials. Synchrotron X-ray diffraction and high-resolution transmission electron microscopy characterization revealed that the synthesized lightweight, high-strength alloy has a fcc single-phase structure.

The research team also investigated the mechanisms of pressure, temperature, and compositional effects on the synthesis of lightweight, high-strength aluminum alloys. Through experimental exploration and theoretical simulations of different compositional alloys, they proposed a criteria for guiding the design of lightweight complex concentrated single-phase aluminum-based alloys, noting that atomic size and electronegativity data of elements under pressure are crucial for designing such alloys. The team had previously accumulated substantial experience and research foundation in this area (e.g., the discovery of pressure-induced transformation of Ce-Al binary intermetallic compounds to disordered solid solutions in 2009 [Zeng et al., PNAS, 106, 2515 (2009)]). In this work, the introduction of high temperatures is significant for complex alloy systems. Firstly, pressure increases the excess entropy of the system, but only at sufficiently high temperatures high entropy can stabilize the solid solution energetically and prevent the formation of intermetallic compounds. Additionally, forming a single phase from an initial multiphase mixture requires long-range atomic diffusion, so a single-phase solid solution can only form under high pressure and temperature. Once synthesized, the high energy barrier of complex concentrated alloys helps retain the single-phase solid solution under normal conditions.

Caption: Mechanical properties. a: Stress-strain curves of the as-cast and single-phase FCC Al55Mg35Li5Zn5 CCAs under compression at room temperature. The insets a1 and a2 are the optical images of the sample before and after compression, respectively. b: Specific fracture strength and density of the single-phase FCC Al55Mg35Li5Zn5.

"The formation of the face-centered cubic single-phase lattice is attributed to the ability of high pressure to effectively reduce the overall differences in atomic size and electronegativity between the solute elements and aluminum, as well as the high-entropy effect from the complex compositional disorder at high temperatures and pressures,” explained Dr. Qiaoshi Zeng. “The face-centered cubic single-phase structure provides a solid basis for plastic deformation. High concentrations of multiple alloying elements lead to significant solid-solution strengthening, and nanoscale chemical fluctuations can pin dislocations effectively. All these contribute to an alloy that is both lightweight and ultra-strong.”

轻而强的铝基合金是帮助实现节能减排的重要结构材料。然而，由于其他轻质成分在铝中有限的固溶度，通过常规路线开发高性能（更轻质又保持高机械强度）的单相铝合金面临难以克服的理论瓶颈。近日，北京高压科学研究中心的曾桥石研究团队和北京科技大学的吕昭平教授团队合作，通过引入高压-高温处理的方法，成功合成了一种轻质（密度为2.4 g/cm³），高强度（屈服强度是传统铝合金的1.7倍）的面心立方单相铝基复杂成分合金，Al55Mg35Li5Zn5。该研究为克服传统理论限制探索具有优异机械性能的超轻质单相高强度铝合金提供了一条可行途径。相关结果以“Lightweight single-phase Al-based complex concentrated alloy with high specific strength”为题近日发表于《Nature Communications》。 

文章链接：https://doi.org/10.1038/s41467-024-51387-6