Kovar Alloy UNS K94610: Properties and Applications

# Kovar Alloy UNS K94610: Properties and Applications

## Introduction to Kovar Alloy UNS K94610

Kovar alloy, designated as UNS K94610, is a nickel-cobalt ferrous alloy specifically designed to have a thermal expansion coefficient that closely matches that of borosilicate glass and certain ceramics. This unique property makes it invaluable in applications where metal and glass or ceramic components must be joined together without causing stress or cracking due to thermal expansion mismatch.

## Chemical Composition

The typical composition of Kovar alloy includes:
– Iron (Fe): ~54%
– Nickel (Ni): ~29%
– Cobalt (Co): ~17%
– Manganese (Mn): ≤0.50%
– Silicon (Si): ≤0.30%
– Carbon (C): ≤0.10%

This carefully balanced composition gives Kovar its distinctive thermal expansion characteristics.

## Physical Properties

Kovar alloy exhibits several notable physical properties:
– Density: 8.36 g/cm³
– Melting Point: ~1450°C (2642°F)
– Thermal Conductivity: 17.3 W/m·K (at 20°C)
– Electrical Resistivity: 49 μΩ·cm
– Magnetic Properties: Ferromagnetic below Curie temperature (~435°C)

## Mechanical Properties

The mechanical properties of Kovar alloy include:
– Tensile Strength: 517 MPa (75 ksi)
– Yield Strength: 345 MPa (50 ksi)
– Elongation: 30% (in 50 mm)
– Hardness: 135 HV (annealed condition)
– Modulus of Elasticity: 138 GPa (20 x 10⁶ psi)

## Thermal Expansion Characteristics

Kovar’s most remarkable property is its thermal expansion behavior:
– Coefficient of Thermal Expansion (CTE):
– 20-300°C: ~5.1 x 10⁻⁶/°C
– 20-400°C: ~5.5 x 10⁻⁶/°C
– 20-450°C: ~5.8 x 10⁻⁶/°C

This closely matches the CTE of borosilicate glass (~3.25 x 10⁻⁶/°C) over a wide temperature range, making it ideal for glass-to-metal seals.

## Applications of Kovar Alloy

Kovar finds extensive use in various industries:

### Electronics and Telecommunications
– Hermetic seals for electronic packages
– Microwave tube components
– Transistor and diode housings
– Feedthrough pins in glass insulators

### Aerospace and Defense
– Satellite components
– Radar systems
– Missile guidance systems

### Medical Devices
– Implantable medical devices
– Diagnostic equipment components
– X-ray tube components

### Scientific Instruments
– Vacuum systems
– Particle accelerator components
– Laboratory equipment requiring glass-metal seals

## Fabrication and Processing

Kovar can be processed using standard metalworking techniques:
– Machining: Can be machined using standard tools, though work hardening may occur
– Welding: Can be welded using TIG or resistance welding methods
– Forming: Can be cold worked, though annealing may be required for extensive forming
– Heat Treatment: Typically annealed at 800-900°C followed by controlled cooling

## Advantages of Kovar Alloy

The primary advantages of Kovar include:
– Excellent thermal expansion match with glass and ceramics
– Good mechanical strength and ductility
– Corrosion resistance in many environments
– Relatively easy to machine and form
– Proven reliability in critical applications

## Limitations and Considerations

While Kovar is highly useful, some limitations exist:
– Higher cost compared to standard steels
– Requires careful processing to maintain properties
– Limited availability in some forms
– Potential for stress corrosion cracking in certain environments

## Conclusion

Kovar