Overview of the Four Materials

1. Overview of the Four Materials

Discover the science behind the four leading lightweight materials—magnesium, titanium, carbon fiber, and aluminum. Explore how each achieves the balance of strength, weight, and cost across aerospace, medical, and industrial applications, revealing the design logic that powers modern engineering innovation. 

1. Overview of the Four Materials

Explore the performance, structure, and application of the four leading lightweight materials—magnesium, titanium, carbon fiber, and aluminum. Learn how each achieves the balance of strength, weight, and cost in aerospace, medical, and industrial design. 

1.1 Magnesium Alloy: The Lightest Structural Metal on Earth

Walk into any modern aerospace factory, and you're likely to see engineers handling silvery-white magnesium alloy parts – surprisingly light, yet capable of withstanding the immense stresses of aircraft takeoff and landing. With a density of only 1.74-1.8 g/cm³, about 35% lighter than aluminum and 75% lighter than steel, magnesium alloy rightfully earns the title "King of the Lightest Structural Metals."

Core Characteristics Analysis
The tensile strength of magnesium alloys can reach 180-440 MPa, with an elastic modulus of about 42-45 GPa. While its absolute strength may not match that of titanium alloys or high-strength aluminum alloys, its specific strength (strength/density) is excellent. More importantly, magnesium alloys offer unparalleled machinability among commercial metals, allowing for easy casting, forging, and extrusion.

However, magnesium has its "temperamental flaws." It has an extremely high affinity for oxygen, requiring a protective atmosphere during melting and processing, which increases manufacturing costs and can produce greenhouse gas emissions. Additionally, magnesium alloys are prone to corrosion in environments containing carbon dioxide or sodium chloride and perform poorly in high-wear applications, making surface coating treatments necessary.

Alloy Systems and Application Prospects
Mainstream magnesium alloys typically contain 3-13% aluminum, along with elements like zinc and manganese. Magnesium-lithium alloys can have densities as low as 1.4 g/cm³, making them highly attractive for aerospace applications. Biomedical magnesium alloys have an elastic modulus (~45 GPa) close to that of human bone (10-40 GPa) and can degrade in vivo, emerging as a new choice for orthopedic implants, replacing stainless steel and titanium alloys.

From helicopter gearboxes to laptop casings, the application range of magnesium alloys is rapidly expanding. However, to truly unlock their potential, reducing costs and improving corrosion resistance remain two key challenges.

1.2 Titanium Alloy: The Gold Standard for Strength and Corrosion Resistance

If magnesium represents the pursuit of ultimate lightness, then titanium represents the perfect balance of performance. Titanium alloys have a density of about 4.42-4.51 g/cm³, approximately 60% lighter than steel, yet their strength can surpass that of steel. This "lightweight yet strong" characteristic makes titanium alloys the preferred choice for aerospace, high-end medical devices, and extreme sports equipment.

Industrial Workhorse: The Legend of Ti-6Al-4V
Ti-6Al-4V (also known as Ti64) is the most widely used titanium alloy, accounting for over 70% of all titanium alloy production. Its secret weapon lies in its alpha-beta dual-phase structure: containing 6% aluminum as an alpha phase stabilizer and 4% vanadium as a beta phase stabilizer. Its density is approximately 4420 kg/m³, Young's modulus is 120 GPa, and tensile strength can reach 1000 MPa.

Ti-6Al-4V can be used in environments up to 400°C, maintaining excellent mechanical properties. The titanium usage in the Boeing 787 reaches 15% of the airframe weight, and 14% in the Airbus A350 – behind these numbers lies the exceptional performance of titanium alloys under extreme temperatures, repeated stress, and corrosive environments.

Versatile Performer from Deep Sea to Outer Space
Titanium alloys perform excellently in corrosive environments like seawater and high humidity. The dense passive film formed on their surface resists oxidation and chemical attack. In aircraft engines, titanium alloy blades, discs, and rings operate stably under high temperature and pressure. In the biomedical field, titanium's biocompatibility makes it an ideal material for joint replacements, bone plates, and dental implants.

But perfection comes at a cost. Titanium alloys are difficult to machine, requiring specialized tools, and their toughness decreases at very low temperatures. The high material and processing costs limit the widespread adoption of titanium in mass markets – which is why your bicycle likely uses aluminum alloy, while an F1 car uses titanium alloy suspension.

1.3 Carbon Fiber: The Performance Beast Among Composites

Imagine a material: five times lighter than steel, yet twice as strong; with a coefficient of thermal expansion close to zero, maintaining dimensional stability like a rock under extreme temperatures. This is carbon fiber – the "Black Gold" of modern engineering materials.

The Power Code in Microstructure
Carbon fiber possesses high strength (3-7 GPa), high modulus (200-500 GPa), and low density (only 1.75-2.0 g/cm³). The average density of carbon fiber is 1.8 g/cm³, and when composited with epoxy resin, the density is about 1.6 g/cm³, resulting in a specific strength 50 times that of steel. But the real magic of carbon fiber lies in its anisotropy – strength is concentrated along the fiber axis. By carefully designing the density and orientation of the fibers, engineers can "customize" the mechanical properties in every direction of a part.

The modulus of carbon fiber is typically 234 GPa, with an ultimate tensile strength of 4-4.8 GPa. In comparison, 2024-T3 aluminum alloy has a modulus of only 10 MSI and an ultimate tensile strength of 65 KSI; 4130 steel has a modulus of 30 MSI and an ultimate tensile strength of 125 KSI. A basic carbon fiber plain weave plate has twice the specific stiffness of aluminum or steel, and over five times the specific strength of aluminum and over four times that of steel. When combined with sandwich structures and lightweight cores, the performance advantages grow exponentially.

From Polyacrylonitrile to High-Performance Composites
Modern carbon fiber is primarily made from polyacrylonitrile (PAN) precursor. Pitch-based carbon fiber can achieve a modulus of up to 900 GPa, thermal conductivity of 1000 W/mK, and electrical conductivity of 10⁶ S/m. For carbon fiber-epoxy matrix composites with a fiber volume fraction of 60%, the longitudinal Young's modulus can reach 220 GPa, longitudinal ultimate tensile strength 1.4 GPa, elongation at break 0.8%, and coefficient of thermal expansion -0.2×10⁻⁶/°C.

But carbon fiber isn't perfect. Its biggest weakness is oxidation starting at 350-450°C, with the oxidation temperature decreasing as fiber impurities increase. Recycling is also a challenge – unlike metals that can be melted and reformed, the recycling process for carbon fiber composites is complex and expensive, becoming an increasingly prominent issue in today's world of tightening environmental regulations.

A Revolution in Design Philosophy
Carbon fiber parts are neither homogeneous nor isotropic; one cannot simply replace steel or aluminum designs with carbon fiber. Due to carbon fiber's high strength, thin-walled shell structures are typically used, whereas metal parts can be machined from solid blocks. This means that adopting carbon fiber is not just a material substitution, but a shift in design thinking – from "subtractive manufacturing" to "additive construction," from standardized parts to topology-optimized structures.

1.4 Aluminum Alloy: The King of Cost and Processability Balance

Among the "Big Four" of lightweight materials, aluminum alloy may not be the lightest nor the strongest, but it is undoubtedly the most "down-to-earth." With a density between 2.6-2.8 g/cm³, excellent processability, moderate cost, and a high recyclability rate of up to 95% – these qualities make aluminum alloy the versatile material for everything from soda cans to Boeing 747 wings.

The Trio of Aviation Aluminum
7075 aluminum alloy was secretly developed in 1936 by Sumitomo Metal Industries of Japan, first used in the Mitsubishi A6M Zero fighter in 1940, and from 1943 onwards, widely used in aviation manufacturing in the US and allied nations. 7075 has a density of 2.81 g/cm³, with a tensile strength of 570 MPa and yield strength of 500 MPa in the T651 state, making it one of the strongest aluminum alloys, but it also has relatively high stress corrosion cracking sensitivity.

2024 aluminum alloy is the most well-known aircraft aluminum, using copper as the main alloying element, boasting exceptional fatigue strength and strength-to-weight ratio. 2024-T3 has a density of 2.78 g/cm³ and a tensile strength of about 470 MPa, primarily used in wings and fuselages and other structures. Its weakness is relatively poor corrosion resistance, usually requiring alclad or coating protection.

6061 aluminum alloy belongs to the Al-Mg-Si series, is a medium-strength alloy with excellent weldability, good corrosion resistance, and outstanding processability. 6061 density is about 2.70 g/cm³, with a tensile strength of 310 MPa in the T6 state. Although its strength is lower than 2024 and 7075, its versatility makes it widely used in aircraft skins, fuselage frames, and civilian products.

Material Evolution from War to Peace
Following the introduction of 7075-T6 in 1943, a series of improved alloys were developed, including 7178-T6 (higher strength), 7079-T6 (higher transverse ductility), and 7050-T74 (for thick-section applications, resistant to stress corrosion). Third-generation aluminum-lithium alloys like 2099 and 2199 have slightly lower lithium content but offer excellent corrosion resistance and fatigue crack growth resistance.

The real advantage of aluminum alloy lies in its "democratic nature" – cost is only a fraction of titanium, recyclability far exceeds carbon fiber, and processing equipment is highly accessible. This allows aluminum alloys to dominate not only aerospace but also penetrate into various fields like automotive, construction, and consumer electronics.

 References

• Polmear, I. J., Saindrenan, G., StJohn, D., & Nie, J.-F. (2017). Light Alloys: Metallurgy of the Light Metals (5th ed.). Elsevier Academic Press.

• Leyens, C., & Peters, M. (Eds.). (2003). Titanium and Titanium Alloys: Fundamentals and Applications. Wiley-VCH.

• Harris, B. (Ed.). (1999). Engineering Composite Materials (2nd ed.). Institute of Materials.

  More chapters coming soon.