Properties of Aluminum Alloys at Cryogenic and Elevated Temperatures

Mechanical and physical properties of aluminum and aluminum alloys change when working temperature change from cryogenic (-195^oC) to elevated temperatures (max. 400^oC). These changes are not so intensive compared to another materials such as steel and others. Changes of properties of aluminum alloys with temperature depend on chemical composition and temper.

The 7xxx series of age-hardenable alloys that are based on the Al-Zn-Mg-Cu system develop the highest room-temperature tensile properties of any aluminum alloys produced from conventionally cast ingots. However, the strength of these alloys declines rapidly if they are exposed to elevated temperatures due mainly to coarsening of the fine precipitates on which the alloys depend for their strength. Alloys of the 2xxx series such as 2014 and 2024 perform better above these temperatures but are not normally used for elevated-temperature applications.

Strength at temperatures above about 100 to 200 °C is improved mainly by solid-solution strengthening or second phase hardening. Another approach to improve the elevated-temperature performance of aluminum alloys has been the use of rapid solidification technology to produce powders or foils containing high supersaturations of elements such as iron or chromium that diffuse slowly in solid aluminum. Several experimental materials are now available that have promising creep properties up to 350oC. An experimental Al-Cu-Mg alloy with silver additions has also resulted in improved creep properties. Iron is also being used to improve creep properties.

Low-Temperature Properties. Aluminum alloys represent a very important class of structural metals for subzero-temperature applications and are used for structural parts for operation at temperatures as low as -270^oC.

Below zero, most aluminum alloys show little change in properties; yield and tensile strengths may increase; elongation may decrease slightly; impact strength remains approximately constant. Consequently, aluminum is useful material for many low-temperature applications.

The chief deterrent is its relatively low elongation compared with certain austenitic ferrous alloys. This inhibiting factor affects principally industries that must work with public safety codes. A notable exception to this has been the approval, in the ASME unfired pressure vessel code, to use alloys 5083 and 5456 for pressure vessels within the range from -195 to 65^oC. With these alloys tensile strength increases 30 to 40%, yield strength 5 to 10% and elongation 60 to 100% between room temperature and -195^oC.

The wrought alloys most often considered for low-temperature service are alloys 1100, 2014, 2024, 2219, 3003, 5083, 5456, 6061, 7005, 7039 and 7075. Alloy 5083-O which is the most widely used aluminum alloy for cryogenic applications, exhibits the following cooled from room temperature to the boiling point of nitrogen (-195^oC):

• About 40% in ultimate tensile strength
• About 10% in yield strength.

Retention of toughness also is of major importance for equipment operating at low temperature. Aluminum alloys have no ductile-to-brittle transition; consequently; neither ASTM nor ASME specifications require low-temperature Charpy or Izod tests of aluminum alloys. Other tests, including notch-tensile and tear tests, assess the notch-tensile and tear toughness of aluminum alloys at low temperature characteristics of welds in the weldable aluminum alloys.

Compared with other alloys, alloy 5083-O has substantially greater fracture toughness than the others. The fracture toughness of this alloy increases as exposure temperature decreases. Of the other alloys, evaluated in various heat-treated conditions, 2219-T87 has the best combination of strength and fracture toughness, both at room temperature and at -196^oC, of all the alloys that can be readily welded.

Alloy 6061-T651 has good fracture toughness at room temperature and at -196^oC, but its yield strength is lower than that of alloy 2219-T87. Alloy 7039 also is weldable and has a good combination of strength and fracture toughness at room temperature and at -196^oC. Alloy 2124 is similar to 2024 but with a higher-purity base and special processing for improved fracture toughness. Tensile properties of 2124-T851 at subzero temperatures can be expected to be similar to those for 2024-T851.

Several other aluminum alloys, including 2214, 2419, 7050 and 7475, have been developed in order to obtain room-temperature fracture toughness superior to that of the other 2000 and 7000 series alloys. Information on subzero properties of these alloys is limited, but it is expected that these alloys also would have improved fracture toughness at subzero temperatures as well as at room temperature.

Fatigue Strength. Results of axial and flexural fatigue tests at 106 cycles on aluminum alloy specimens at room temperature and at subzero temperatures indicate that, for a fatigue life of 106 cycles, fatigue strength is higher at subzero temperatures than at room temperature for each alloy. This trend is not necessarily valid for the tests at higher stress levels and shorter fatigue lives, but at 106 cycles results are consistent with the effect of subzero temperatures on tensile strength.