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Question 10.1: Design of an Aerospace Component Because magnesium is a low-......

Design of an Aerospace Component
Because magnesium is a low-density material (\rho_{Mg} = 1.738 g/cm³), it has been suggested for use in an aerospace vehicle intended to enter outer space. Is this a good design?

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The pressure is very low in space. Even at relatively low temperatures, solid magnesium can begin to transform to a vapor, causing metal loss that could damage a space vehicle. In addition, solar radiation could cause the vehicle to heat, increasing the rate of magnesium loss.
A low-density material with a higher boiling point (and, therefore, lower vapor pressure at any given temperature) might be a better choice. At atmospheric pressure, aluminum boils at 2494 °C and beryllium boils at 2770 °C, compared with the boiling temperature of 1107 °C for magnesium. Although aluminum and beryllium are somewhat denser than magnesium, either might be a better choice. Given the toxic effects of Be and many of its compounds when in powder form, we may want to consider aluminum first.
There are other factors to consider. In load-bearing applications, we should not only look for density but also for relative strength. Therefore, the ratio of Young’s modulus to density or yield strength to density could be a better parameter to compare different materials. In this comparison, we will have to be aware that yield strength, for example, depends strongly on microstructure and that the strength of aluminum can be enhanced using aluminum alloys, while keeping the density about the same. Other factors such as oxidation during reentry into Earth’s atmosphere may be applicable and will also have to be considered.

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