Mechanical Properties of Alnico Magnets and Comparison with Other Permanent Magnets

1. Introduction

Alnico (aluminum-nickel-cobalt) magnets are a class of permanent magnets developed in the 1930s, known for their excellent thermal stability, high remanence (Br), and moderate coercivity (Hc). While their magnetic properties are well-documented, their mechanical performance—including hardness, tensile strength, flexural strength, and toughness—is equally critical for engineering applications. This article provides detailed mechanical property data for Alnico magnets and compares them with other permanent magnets, such as rare-earth (NdFeB, SmCo) and ferrite magnets.

2. Mechanical Properties of Alnico Magnets

(1) Hardness

• Typical Value: Alnico magnets typically exhibit a Rockwell C hardness (HRC) of 40–50 or a Vickers hardness (HV) of 500–700.
• Factors Affecting Hardness:
  • Alloy Composition: Higher cobalt (Co) and nickel (Ni) content increases hardness but reduces ductility.
  • Heat Treatment: Proper aging and solution annealing improve hardness by optimizing the precipitate distribution (e.g., Ni-Al or Co-Ti phases).
  • Microstructure: Fine-grained structures with uniform precipitates enhance hardness through domain wall pinning.

(2) Tensile Strength

• Typical Value: The ultimate tensile strength (UTS) of Alnico ranges from 150–300 MPa, depending on the alloy grade and processing method.
• Comparison with Other Magnets:
  • Ferrite Magnets: Lower UTS (~50–100 MPa) due to their ceramic nature.
  • NdFeB Magnets: Higher UTS (~300–500 MPa) but brittle, limiting machinability.
  • SmCo Magnets: Moderate UTS (~200–400 MPa) with better toughness than NdFeB.

(3) Flexural Strength

• Typical Value: Alnico magnets have a flexural strength of 100–200 MPa, making them more resistant to bending than ferrite magnets but less robust than rare-earth magnets.
• Key Factor: The anisotropic structure of Alnico (due to directional solidification) improves flexural strength along preferred crystallographic axes.

(4) Toughness (Impact Resistance)

• Typical Value: Alnico magnets are brittle, with a Charpy impact energy of <5 J/cm², similar to ferrite magnets but lower than rare-earth magnets.
• Limitation: Low toughness restricts Alnico to applications where mechanical shocks are minimal (e.g., sensors, aerospace).

(5) Density

• Typical Value: Alnico has a density of 7.2–7.6 g/cm³, higher than ferrite magnets (4.8–5.2 g/cm³) but lower than NdFeB (7.4–7.6 g/cm³) and SmCo (8.3–8.5 g/cm³).

3. Comparison with Other Permanent Magnets

Table 1: Mechanical Properties of Permanent Magnets

(1) Alnico vs. NdFeB Magnets

• Advantages of Alnico:
  • Better Thermal Stability: Alnico retains magnetism up to 600°C, while NdFeB degrades above 200°C.
  • Higher Machinability: Alnico can be cut, drilled, or ground, whereas NdFeB requires diamond tools due to its hardness.
• Disadvantages of Alnico:
  • Lower Magnetic Energy Product: Alnico’s BHmax (~5–10 MGOe) is much lower than NdFeB’s (~40–50 MGOe).
  • Lower Coercivity: Alnico is prone to demagnetization under reverse fields.

(2) Alnico vs. SmCo Magnets

• Advantages of Alnico:
  • Lower Cost: SmCo contains expensive rare-earth elements (e.g., samarium, dysprosium), making Alnico more economical.
  • Better Corrosion Resistance: Alnico does not require coatings, while SmCo may oxidize in humid environments.
• Disadvantages of Alnico:
  • Lower Magnetic Performance: SmCo has a higher BHmax (~25–32 MGOe) and coercivity (~600–800 kA/m).
  • Lower Temperature Coefficient: SmCo’s remanence changes less with temperature (-0.03%/°C vs. Alnico’s -0.02%/°C).

(3) Alnico vs. Ferrite Magnets

• Advantages of Alnico:
  • Higher Remanence: Alnico’s Br (~1.0–1.4 T) is much higher than ferrite’s (~0.2–0.4 T).
  • Better Mechanical Strength: Alnico’s tensile and flexural strengths exceed those of ferrite magnets.
• Disadvantages of Alnico:
  • Higher Cost: Ferrite magnets are significantly cheaper due to abundant raw materials (e.g., iron oxide, strontium carbonate).
  • Lower Magnetic Stability: Ferrite magnets have a higher temperature coefficient of remanence (~-0.2%/°C).

4. Applications Based on Mechanical Properties

(1) Alnico Magnets

• High-Temperature Sensors: Used in aerospace and automotive sensors due to thermal stability.
• Electric Guitar Pickups: Machinable shapes and stable magnetic fields enhance sound quality.
• Military Equipment: Resistant to demagnetization under shock and vibration.

(2) NdFeB Magnets

• High-Performance Motors: Used in electric vehicles (EVs) and wind turbines for maximum torque.
• MRI Machines: Require strong magnetic fields for imaging.

(3) SmCo Magnets

• Aerospace Actuators: Operate reliably at extreme temperatures.
• Medical Devices: Used in pacemakers and hearing aids due to biocompatibility.

(4) Ferrite Magnets

• Low-Cost Motors: Found in household appliances (e.g., refrigerators, fans).
• Loudspeakers: Provide adequate magnetic fields at minimal cost.

5. Conclusion

Alnico magnets exhibit moderate hardness, tensile strength, and flexural strength but suffer from low toughness, limiting their use in high-impact applications. Compared to rare-earth magnets (NdFeB, SmCo), Alnico offers better thermal stability and machinability but lower magnetic performance. Ferrite magnets, while cheaper, are inferior in both magnetic and mechanical properties. The choice of magnet depends on the application’s requirements for temperature resistance, magnetic strength, cost, and mechanical durability.

For high-temperature, precision applications, Alnico remains irreplaceable, while high-performance, cost-sensitive applications favor NdFeB or SmCo. Understanding these trade-offs is essential for optimal magnet selection in engineering designs.