Properties of Aluminum Alloys at Cryogenic and Elevated Temperatures

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Temperature Effects on Mechanical Properties

Mechanical and physical properties of aluminum and aluminum alloys undergo changes when operating temperatures range from cryogenic (-195°C) to elevated temperatures (400°C). These changes are less pronounced compared to other materials such as steel. The property changes depend primarily on chemical composition and temper condition of the alloy.

Elevated Temperature Performance

The 7xxx series of age-hardenable alloys, based on the Al-Zn-Mg-Cu system, exhibit the highest room-temperature tensile properties among conventional aluminum alloys. However, their strength decreases significantly at elevated temperatures due to precipitate coarsening. While 2xxx series alloys (such as 2014 and 2024) show better performance at higher temperatures, they are typically not specified for elevated-temperature applications.

Strength Enhancement at High Temperatures

Performance at temperatures above 100-200°C can be enhanced through solid-solution strengthening or second-phase hardening. Advanced manufacturing techniques, such as rapid solidification technology, have enabled the production of aluminum alloys with high supersaturations of elements like iron or chromium. These elements, which diffuse slowly in solid aluminum, have resulted in experimental materials with promising creep properties up to 350°C. Recent developments include experimental Al-Cu-Mg alloys with silver additions, showing improved creep resistance.

Cryogenic Temperature Properties

Aluminum alloys represent a crucial category of structural metals for subzero applications, functioning effectively at temperatures as low as -270°C. Most aluminum alloys demonstrate favorable property retention at subfreezing temperatures, with some characteristics improving:

• Yield and tensile strengths typically increase
• Elongation shows minimal decrease
• Impact strength remains largely constant

Fracture Toughness Characteristics

Alloy 5083-O, the most widely used aluminum alloy for cryogenic applications, exhibits exceptional property improvements when cooled from room temperature to -195°C (nitrogen boiling point):

• 40% increase in ultimate tensile strength
• 10% increase in yield strength

Comparative Alloy Performance

Among various aluminum alloys, 5083-O demonstrates substantially greater fracture toughness compared to other options. Notably, its fracture toughness increases as exposure temperature decreases. Of the weldable aluminum alloys, 2219-T87 offers the best combination of strength and fracture toughness, both at room temperature and at -196°C.

Specific Alloy Characteristics:

• Alloy 6061-T651: Exhibits good fracture toughness at both room temperature and -196°C, though its yield strength is lower than 2219-T87
• Alloy 7039: Demonstrates good weldability and maintains an excellent combination of strength and fracture toughness at both room temperature and -196°C
• Alloy 2124: Similar to 2024 but manufactured with a higher-purity base and special processing for improved fracture toughness. The tensile properties of 2124-T851 at subzero temperatures are expected to match those of 2024-T851

Advanced Alloy Developments

Several newer aluminum alloys have been developed specifically for enhanced room-temperature fracture toughness:

• 2214
• 2419
• 7050
• 7475

These alloys were developed to achieve superior room-temperature fracture toughness compared to traditional 2000 and 7000 series alloys. While comprehensive data on their subzero properties remains limited, they are expected to maintain their improved fracture toughness characteristics at both room and subzero temperatures.

Notch Sensitivity and Testing

Unlike many materials, aluminum alloys show no ductile-to-brittle transition, eliminating the need for low-temperature Charpy or Izod testing in ASTM and ASME specifications. Alternative testing methods, including notch-tensile and tear tests, are used to evaluate the notch-tensile and tear toughness characteristics of aluminum alloys and their welds at low temperatures.

Fatigue Performance

Axial and flexural fatigue testing at 106 cycles demonstrates improved fatigue strength at subzero temperatures compared to room temperature performance. While this trend may not hold true for higher stress levels and shorter fatigue lives, the results at 106 cycles consistently align with the observed improvements in tensile strength at cryogenic temperatures.

Conclusion

Aluminum alloys demonstrate remarkable versatility across extreme temperature ranges, with many variants showing improved mechanical properties at cryogenic temperatures. Their unique combination of strength, toughness, and fatigue resistance makes them invaluable for specialized applications in both high and low-temperature environments.