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Chapter 3

Magnesium and Beryllium

Considering their location on the periodic table of elements, one would think that both magnesium and beryllium would play major roles as aerospace structural materials. However, both of these extremely lightweight metals have serious limitations that make them only minor players. Magnesium alloys usually compete with aluminum alloys for structural applications. Compared to high strength aluminum alloys, magnesium alloys are not as strong (tensile strength of 20-50 ksi vs. 40-80 ksi) and have a lower modulus of elasticity (6.5 msi vs. 10-11 msi). However, magnesium is significantly lighter (0.063 vs. 0.100lb/in.3) and therefore its alloys are competitive on a specific strength and modulus basis. Magnesium alloys, with their hexagonal close-packed structure, must usually be formed at elevated temperatures, while aluminum can be readily formed at room temperature. In addition, magnesium alloys are normally more expensive than comparable aluminum alloys. However, the biggest obstacle to the use of magnesium alloys is their extremely poor corrosion resistance. Magnesium occupies the highest anodic position on the galvanic series, and, as such, there is always the strong potential for corrosion as shown in the example of Fig. 3.1. However, some of the newer alloys have much better corrosion resistance than the older alloys. As shown in Fig. 3.2, some of the newer cast alloys approach the corrosion resistance of competing aluminum casting alloys. Magnesium alloys do have very good damping capacity and castings have found application in high vibration environments, such as helicopter gear boxes.1

Beryllium is also a very lightweight metal (0.068lb/in.3) that has good strength properties and a very high tensile modulus (44 msi). These properties, combined with its attractive electrical and thermal properties, have led to its use in high value aerospace electronic and guidance system applications. However, beryllium must be processed using powder metallurgy that makes it costly. Like magnesium, its hexagonal close-packed (HCP) crystalline structure greatly impairs its formability. Finally, beryllium powder and dust are toxic, which

Fig. 3.1. Severely Corroded Magnesium Part

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