# Alloy 908 UNS N09908: Properties and Applications
## Introduction to Alloy 908 UNS N09908
Alloy 908, also known as UNS N09908, is a nickel-iron-based superalloy specifically designed for cryogenic applications. This material exhibits exceptional mechanical properties at extremely low temperatures while maintaining good strength and ductility at elevated temperatures.
## Chemical Composition
Keyword: Alloy 908 UNS N09908
The unique properties of Alloy 908 stem from its carefully balanced chemical composition:
– Nickel (Ni): 48-50%
– Iron (Fe): Balance
– Niobium (Nb): 4.0-4.5%
– Titanium (Ti): 1.3-1.8%
– Aluminum (Al): 0.75-1.25%
– Other elements (C, Mn, Si, S, P, Co, Cu, Mo): Present in smaller quantities
## Mechanical Properties
Alloy 908 offers an excellent combination of mechanical properties:
– Tensile Strength: 1,240 MPa (180 ksi) at room temperature
– Yield Strength: 1,000 MPa (145 ksi) at room temperature
– Elongation: 12% minimum at room temperature
– Excellent fracture toughness at cryogenic temperatures
– Good creep resistance at elevated temperatures
## Physical Properties
Key physical characteristics of Alloy 908 include:
– Density: 8.19 g/cm³ (0.296 lb/in³)
– Melting Range: 1,315-1,370°C (2,400-2,500°F)
– Thermal Expansion Coefficient: 11.5 μm/m·°C (6.4 μin/in·°F) from 20-100°C
– Electrical Resistivity: 0.85 μΩ·m at 20°C
## Key Advantages
Alloy 908 offers several distinct advantages for demanding applications:
– Exceptional cryogenic performance
– High strength-to-weight ratio
– Good weldability compared to other superalloys
– Excellent resistance to thermal fatigue
– Good oxidation resistance at moderate temperatures
## Primary Applications
Due to its unique properties, Alloy 908 finds use in several critical applications:
### Cryogenic Systems
– Superconducting magnet structures
– Cryogenic storage vessels
– Liquid helium containment systems
### Aerospace Components
– Rocket engine components
– Spacecraft structural elements
– Cryogenic fuel systems
### Energy Sector
– Nuclear fusion reactor components
– Superconducting power transmission systems
– Magnetic energy storage devices
## Fabrication and Processing
Alloy 908 can be processed using standard techniques with some special considerations:
– Hot working should be performed between 1,010-1,150°C (1,850-2,100°F)
– Cold working requires intermediate annealing
– Solution annealing is typically done at 980°C (1,800°F) followed by rapid cooling
– Aging treatment at 720°C (1,325°F) for 8 hours optimizes mechanical properties
## Welding Considerations
While Alloy 908 is weldable, special precautions are necessary:
– Gas tungsten arc welding (GTAW) is the preferred method
– Matching filler metals should be used
– Post-weld heat treatment is recommended for critical applications
– Proper shielding gas coverage is essential to prevent contamination
## Comparison with Other Superalloys
Alloy 908 offers distinct advantages over similar materials:
– Better cryogenic toughness than Inconel 718
– Superior weldability compared to many precipitation-hardened superalloys
– Lower thermal expansion than most nickel-based alloys
– More cost-effective than some specialized cryogenic alloys
## Future Developments
Ongoing research aims to further enhance Alloy 908’s capabilities:
– Improved processing techniques for better consistency
– Development of optimized heat treatment cycles
– Exploration of additive manufacturing applications
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