4.0 SUBSYSTEMS 4.1 MATERIALS Many of the material requirements of the SPS will be satisfied by the use of aluminum, magnesium and titanium alloys. However, some subsystems contain components which operate at elevated temperatures. Selection of alloys for these SPS applications is based on the temperature range involved, as shown in Figure 4-1. The tungsten/ rhenium and tantalum alloys are less well defined than the columbium and cobalt alloys. The materials identified will be used for heat exchanger tubing (e.g., within solar cavity absorbers) and for manifolds, etc., in the radiator systems. Note that the material strength shown in Figure 4-1 is the predicted 30-year creep rupture strength. Many SPS subsystems require long term confinement of pressurized gases or liquid at high temperatures, thus a fundamental problem is the long-term creep rupture at high temperatures. Table 4-1 shows additional considerations in material selection, and alloys considered as option. A trend of improvement of alloys for service above 1000K (1340°F) is shown in Figure 4-2. Iron, cobalt, columbium and nickel base systems were compared. A number of alloys having good strength properties were not considered due to their poor fabrication capabilities. While strength Fig. 4-1. Material Selection Approach rupture capabilities of the nickel and cobalt base alloys have shown only a modest advance in the past 25 years, significant improvements in thermal fatigue, oxidation resistance, and stability characteristics have been achieved. CONCLUSIONS: 1. Little or no improvement trend in the cobalt base alloys. 2. Nickel base alloys have been improving at the rate of approximately 3.4K (6.2°F) per year. Table 4-1. Material Considerations
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