Magnesiumbase Alloys

A useful application of the MA technique was in the production of supercorroding magnesium alloys that operate as short-circuited galvanic cells to corrode (react) rapidly and predictably with an electrolyte, such as seawater, to produce heat and hydrogen gas (Ref 20, 21). Such an alloy system is suitable as a heat source for warming deep-sea divers, as a gas generator to provide gas for buoyancy, or as a fuel in hydrogen engines or fuel cells. The corrosion rate of alloys can be maximized by providing (a) a short electrolyte path, (b) a large amount of exposed surface area, and (c) a strong bond (weld) between the cathode and the anode. It is also useful to provide a very low resistance path for external currents to flow through the corroding pairs. All these requirements can be met with MA processing. Consequently, magnesium-base alloys containing Fe, Cu, C, Cr, or Ti have been evaluated for such applications. The Mg-5 to 20at.% Fe alloy is ideal because of its extremely fast reaction rate, high power output, and the high percentage of theoretical completion of the actual reaction. For corrodable release links an alloy with a slower reaction rate, such as Mg-5at.% Ti is useful.

There have also been a number of investigations in recent years to examine use of MA to produce metal hydrides. This is because metal hydrides are materials for safe storage of hydrogen, and they can store hydrogen with a higher volume density than liquid hydrogen. However, these are sensitive to surface oxidation and hence can be a limiting factor in their commercial utilization. Nanocrystalline hydrides have a high density of defects and interfaces that could enhance diffusion; therefore, nanocrystalline intermetallics would not require activation treatments at high temperatures and pressures after exposure to air (Ref 22). In comparison to coarse-grained materials, mechanically alloyed nanocrystalline intermetallics exhibit a narrower absorption plateau and a lower plateau pressure. Their hydrogen storage behavior is typical of amorphous systems. Several magnesium-base and iron-base intermetallics are being evaluated for this application.

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