Magnesium (Mg), the lightest structural metal, offers significant advantages for lightweighting applications in transportation and electronics. Its low density, approximately 65% that of aluminum alloys and 25% that of steel, makes it highly desirable. However, its poor formability and susceptibility to rapid corrosion have hindered widespread industrial adoption. Adding lithium (Li) significantly enhances formability by inducing a ductile body-centered cubic (BCC) β-phase within the hexagonal close-packed (HCP) α-phase matrix. This results in Mg-Li alloys with densities as low as ~1.3 g/cm³, the lightest among all Mg alloys, and ductility exceeding 50% tensile elongation at room temperature. Despite these benefits, the strength of these alloys remains low (typically ~100 MPa) due to the presence of the soft β-phase. Adding aluminum (Al) can increase strength through quench strengthening, reaching yield strengths over 400 MPa. This strengthening is attributed to the formation of a supersaturated solid solution and precipitates. However, this quench strengthening approach often leads to brittleness. To maintain ductility, the Li and Al concentrations must be reduced, resulting in a dual-phase (α + β) structure. Unfortunately, this dual-phase structure suffers from accelerated corrosion due to microgalvanic coupling between the α and β phases, with the β-phase acting as the anode. Another significant drawback of quench-strengthened Mg-Li-Al alloys is their poor mechanical durability. Natural age-softening occurs as metastable precipitates coarsen and transform into a stable but soft AlLi phase, leading to a rapid decrease in strength. This instability severely limits their practical applications. This study addresses these longstanding challenges. The researchers employed a novel thermomechanical process combining friction stir processing (FSP) and liquid CO2 quenching to engineer the microstructure of a dual-phase Mg-9Li-4Al-1Zn (LAZ941) alloy. This approach aims to refine the grain size, control precipitate formation, and enhance both electrochemical and mechanical durability, solving the issues of rapid corrosion and age softening that have plagued Mg-Li alloys for decades. The purpose of this paper was to present these findings and explore the underlying mechanisms.

The literature review extensively covers the challenges and previous attempts to improve the properties of Mg-Li alloys. It highlights the use of Li for improved formability, but notes the resulting weakness and susceptibility to rapid corrosion. The use of Al as an alloying element to increase strength through quench strengthening is discussed, along with the inherent brittleness that often accompanies this method. Several studies on Mg-Li-Al alloys are cited, detailing the formation of precipitates and the age-softening phenomenon. The review emphasizes the ongoing struggle to achieve both high strength and corrosion resistance in Mg-Li-based alloys and the limited success of previous approaches. This contextualizes the innovation of this study's novel thermomechanical processing technique.

The study used hot-rolled Mg-9Li-4Al-1Zn (LAZ941) plates. Friction stir processing (FSP) was performed using a cemented tungsten carbide (WC) tool with specific parameters (rotation speed, tool geometry). A key innovation was the immediate quenching of the FSP-processed alloy using liquid CO2 (-78.5 °C), achieving a significantly faster cooling rate than traditional water quenching. This was compared to the as-rolled condition and water quenching (WQ) alone. Mechanical testing involved tensile tests on dog-bone-shaped samples to determine yield strength and ductility. Hardness measurements were conducted over a period of two years to assess mechanical durability. Electrochemical durability was evaluated through various techniques. Immersion tests in 0.1 M NaCl solution measured hydrogen evolution rate and mass loss. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) provided additional information on corrosion behavior. The corroded surfaces were characterized using profilometry, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDXS) mapping, and scanning transmission electron microscopy (STEM) to analyze microstructure and surface film composition. X-ray diffraction (XRD), including in situ heating and cooling experiments, was used to study phase transformations and the evolution of intermetallic phases (AlLi, θ phase). X-ray photoelectron spectroscopy (XPS) with depth profiling and grazing incident X-ray diffraction (GIXRD) provided further detail on the surface film composition. High-angle annular dark-field (HAADF) STEM imaging was utilized to characterize the nano-precipitates formed within the microstructure.

Friction stir processing (FSP) coupled with liquid CO2 quenching dramatically improved both the electrochemical and mechanical durability of the LAZ941 alloy compared to the as-rolled and water-quenched conditions. **Electrochemical Durability:** The FSP+CO2 quenched alloy showed a significantly lower corrosion rate. The hydrogen evolution rate was about 0.1 ml cm⁻² day⁻¹, and the mass-loss rate was 0.72 mg cm⁻² day⁻¹—an order of magnitude improvement compared to the as-rolled sample. Potentiodynamic polarization and EIS measurements confirmed the significantly reduced corrosion current density and enhanced corrosion resistance. This high corrosion resistance persisted even after two years of natural aging. **Mechanical Durability:** The FSP+CO2 quenched alloy exhibited a significantly higher yield strength (308 MPa) and ductility (16.5%) than the water-quenched samples. Crucially, the high strength was exceptionally stable, showing only a minor decrease in hardness (~15%) over nearly two years of natural aging. This is in sharp contrast to the water-quenched samples, which experienced a rapid and significant drop in hardness (40% in three months). Tensile testing confirmed the exceptional stability of the yield strength of the FSP-treated alloy. **Microstructural Analysis:** Microstructural characterization revealed a significant difference between the FSP+CO2 quenched and water-quenched samples. The FSP sample showed a substantially refined grain size (approximately 2 μm) and a high density of ~5 nm diameter coherent nanoprecipitates within the β-phase. These nanoprecipitates were identified as the θ phase. The water-quenched samples contained larger AlLi precipitates that coarsened and transformed during natural aging, leading to the observed age-softening. In contrast, the FSP-treated samples did not show the formation of AlLi particles, even after two years of aging, and a protective aluminum-rich surface layer was identified. XRD analysis confirmed the absence of the AlLi phase in the FSP samples even after prolonged aging.

The exceptional durability of the FSP+CO2 quenched alloy is directly linked to its unique microstructure: the combination of fine grain size and dense, coherent nanoprecipitates. The FSP process effectively refines the grains, increasing the density of grain boundaries that act as sinks for Al atoms, impeding the coarsening of θ precipitates and preventing the transformation to the detrimental AlLi phase. The rapid cooling provided by liquid CO2 quenching is crucial for the formation of these coherent nanoprecipitates, which contribute significantly to strengthening without sacrificing ductility. The aluminum-rich protective surface layer that forms further enhances corrosion resistance. The suppression of the AlLi phase is critical for reducing microgalvanic corrosion and improving electrochemical durability. The results challenge traditional approaches to strengthening Mg-Li alloys and suggest that controlling precipitate formation and grain size through tailored thermomechanical processing is key to achieving high strength and exceptional durability.

This study demonstrates a novel approach to producing high-strength and corrosion-resistant Mg-Li-Al alloys by combining friction stir processing (FSP) with rapid liquid CO2 quenching. The resulting microstructure, characterized by fine grains and a high density of coherent nanoprecipitates, suppresses the formation of the detrimental AlLi phase, leading to enhanced mechanical and electrochemical durability. The findings provide a promising strategy for designing durable, lightweight alloys for a wide range of industrial applications. Further studies could explore other alloy compositions and process parameters to further optimize properties and investigate the long-term stability under various environmental conditions.

The study primarily focused on a single alloy composition (LAZ941). While the results are promising, further investigations are needed to determine the generalizability of this approach to other Mg-Li-based alloys. The long-term stability (beyond two years) under different environmental conditions requires additional testing. The mechanism of formation and long-term evolution of the protective Al-containing surface film requires additional research to fully elucidate its composition and protective properties. The study's focus on specific mechanical and corrosion tests leaves other potential application-relevant properties to be investigated in future studies.