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Nano-Aluminum: Small Size Leads to Big Properties
02-06-03
In the continued quest for aluminum alloys with higher properties, metallurgists have employed approaches such as heat treatment and work hardening with all sorts of alloys processed through a variety of typical as well as non-conventional methods. While steady progress has been made in this area, recent work promises to produce aluminum alloys with a major step up in properties. Nano-aluminum alloys are sub-classified into nanocrystalline aluminum, nanophase aluminum, or amorphous aluminum, with the common theme that the source of property improvement is to produce highly refined (in some cases noncrystalline) microstructures on the nanometer scale. (Conventional aluminum alloys have features, specifically grain sizes, on the micrometer scale typically.)
The primary driving force for development of nano-aluminum is the potential for large increases in strength. Some alloys that have been developed in Japan are called "GIGAS" because they have tensile strength in the one gigapascal (145 ksi) range. Strengthening results from the very fine size of the grains coupled with dispersion strengthening in nanocrystalline alloys and the hindrance of dislocation motion in amorphous alloys. A recent article indicates that these types of alloys are now seeing use in applications such as rapidly repeating machinery parts requiring high specific strength and high specific modulus, as well as main construction parts in robots in Japan. In addition to increases in strength, there are also now reports of improved fatigue properties and high strain rate superplastic forming ability that could bode well for other applications. Improved strength retention at elevated temperatures has also been measured.
Properties are important, but it has always been a challenge to produce very high strength alloys with commercial processes. A number of processing routes have been explored to obtain amorphous and nanocrystalline aluminum. Both the primary synthesis process by which the initial structure is obtained as well as the secondary processing route to obtain a bulk shape are important. Primary processes have included powder atomization, mechanical alloying, or inert gas condensation. All of these are rapid solidification processes, which are required to obtain the desired structures. Coupled with this is alloy development, which has focused on Al-rare earth and transition metal compositions. Secondary processing routes such as extrusion of the rapidly solidified products have been reported. The challenge in secondary processing is to maintain the amorphous or nanocrystalline structure, because as temperature and deformation are applied, the material tends to want to recrystallize and the structure to coarsen. An alternative processing route that has been demonstrated at the laboratory scale is severe plastic deformation, in which a conventionally cast alloy is deformed through processes such as the equal channel angular extrusion process to produce a highly refined structure. Recently, high pressure die casting of favorable aluminum-based alloys has demonstrated the formation of a bulk amorphous phase on the surface of the casting.
A related family of aluminum materials termed near-nanocrystalline are under development with US Air Force funding. Utilizing more conventional alloys of the Al-Mg family, these are produced by mechanical alloying of alloy powders followed by consolidation by conventional wrought powder metallurgy and downstream metalworking processes into extrusions and forgings. While the grain sizes in these alloys are in the 100-500 nanometer range, strengths as high as 1015 MPa (147 ksi) have been measured. These materials are of interest for replacing titanium in rocket propulsion applications.
While it will still be a number of years before nano-aluminum finds its way into significant commercial use (assuming issues related to process scale-up and, of course, material cost can be solved), this development is certainly one that bears watching.
A useful summary of the current state of the art in nano-aluminum can be found in a recent article by Inoue and Kimura in the inaugural issue of the Journal of Light Metals, v.1, no. 1, 2001, pp. 31-41, which can be accessed online at www.lightmetals.net.
Article provided courtesy of The Aluminum Association (www.aluminum.org)
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