Magnesium-based metal-matrix nanocomposites (MMNCs) have emerged as a promising class of materials due to their high strength-to-weight ratio, excellent damping capacity, and biocompatibility. The incorporation of nanoparticles such as silicon carbide (SiC), alumina (Al2O3), or graphene into magnesium matrices enhances mechanical properties, thermal stability, and corrosion resistance, making them suitable for demanding applications in biomedical implants and transportation industries. The development of these materials involves advanced processing techniques, including disintegrated melt deposition and friction stir processing, which address challenges like nanoparticle dispersion and interfacial bonding.

The production of magnesium MMNCs begins with the selection of an appropriate matrix and reinforcement. Magnesium alloys, such as AZ31 or ZK60, are commonly used due to their lightweight nature and moderate strength. Reinforcements like SiC, Al2O3, or graphene are chosen for their high hardness, thermal conductivity, or exceptional mechanical properties. The uniform distribution of these nanoparticles within the magnesium matrix is critical to achieving optimal performance. Traditional methods like stir casting often face limitations in achieving homogeneous dispersion, leading to the adoption of more advanced techniques.

Disintegrated melt deposition (DMD) is a semi-solid processing method that combines the benefits of stir casting and spray forming. In this process, magnesium alloy ingots are melted and mechanically stirred while nanoparticles are introduced into the molten metal. The slurry is then disintegrated by a high-speed gas stream, resulting in fine droplets that solidify rapidly upon deposition. This method minimizes particle settling and agglomeration, ensuring a more uniform distribution. Studies have shown that DMD-processed magnesium MMNCs reinforced with 1-2 vol.% SiC exhibit a 20-30% increase in yield strength compared to unreinforced magnesium alloys, while maintaining ductility.

Friction stir processing (FSP) is another effective technique for fabricating magnesium MMNCs. This solid-state process uses a rotating tool to generate frictional heat and plastic deformation, enabling the incorporation of nanoparticles into the metal matrix without melting. FSP refines the grain structure and improves particle-matrix bonding, leading to enhanced mechanical properties. For instance, magnesium composites reinforced with graphene via FSP demonstrate a 40-50% improvement in tensile strength and a significant reduction in wear rates. The absence of melting in FSP also prevents undesirable interfacial reactions between magnesium and reactive reinforcements like Al2O3.

The mechanical properties of magnesium MMNCs are significantly influenced by the type and concentration of nanoparticles. SiC-reinforced composites exhibit high specific strength, with yield strengths reaching up to 250 MPa at room temperature. Creep resistance is another critical advantage, particularly for high-temperature applications. The addition of 1-3 vol.% Al2O3 nanoparticles has been shown to reduce creep rates by an order of magnitude at temperatures up to 200°C, attributed to particle pinning of dislocations and grain boundaries. Graphene-reinforced composites, though less studied, offer exceptional stiffness and damping characteristics, making them ideal for vibration-sensitive applications.

Corrosion behavior is a major concern for magnesium-based materials, especially in biomedical and marine environments. While pure magnesium is highly susceptible to corrosion, the incorporation of nanoparticles can mitigate this issue. Al2O3-reinforced MMNCs exhibit improved corrosion resistance due to the formation of a stable oxide layer, reducing corrosion rates by up to 50% in simulated body fluid. However, challenges remain with galvanic corrosion when conductive reinforcements like graphene are used, necessitating surface treatments or alloying modifications.

In biomedical applications, magnesium MMNCs are explored for biodegradable implants, such as bone fixation devices and cardiovascular stents. Their biocompatibility, combined with adjustable degradation rates through nanoparticle addition, addresses the limitations of permanent metallic implants. For instance, SiC-reinforced magnesium composites show controlled degradation and enhanced bone cell adhesion, making them suitable for orthopedic applications. In the transportation sector, these composites are used in lightweight automotive and aerospace components, where their high specific strength contributes to fuel efficiency and reduced emissions.

Despite their advantages, magnesium MMNCs face several challenges. Interfacial reactions between the matrix and reinforcements can form brittle intermetallic compounds, degrading mechanical properties. For example, reactions between magnesium and SiC at high temperatures may form Mg2Si, which can act as a stress concentrator. Nanoparticle agglomeration is another persistent issue, particularly at higher reinforcement loadings, leading to localized stress and reduced ductility. Advanced processing techniques and surface modifications of nanoparticles are being investigated to overcome these limitations.

Comparatively, polymer-matrix nanocomposites offer easier processing and lower densities but fall short in mechanical performance and thermal stability. For instance, polymer composites reinforced with similar nanoparticles exhibit tensile strengths typically below 100 MPa, limiting their use in structural applications. Magnesium MMNCs, with their superior strength and stiffness, are better suited for load-bearing components. However, polymer composites excel in corrosion resistance and electrical insulation, making them preferable for certain applications.

In summary, magnesium-based metal-matrix nanocomposites represent a significant advancement in lightweight materials engineering. Through techniques like disintegrated melt deposition and friction stir processing, these materials achieve superior specific strength, creep resistance, and corrosion behavior. While challenges like interfacial reactions and particle agglomeration persist, ongoing research continues to refine their performance. Their applications in biomedical implants and transportation highlight their versatility, offering a compelling alternative to traditional materials. Compared to polymer-matrix nanocomposites, magnesium MMNCs provide unmatched mechanical properties, positioning them as a key material for future technological demands.