Nanoscale particles have been uniformly dispersed in a magnesium alloy, yielding composites with record-breaking strengths — and raising the prospect of using magnesium as a lightweight metal for structural applications. See Letter p.539
Magnesium has a density two-thirds that of aluminium, one-quarter that of steel and only slightly higher than that of many polymers. It is therefore regarded as a potentially ideal substitute for heavier metals — but magnesium's poorer mechanical behaviour has limited its application. On page 539 of this issue, Chen et al.1 report a method of fabricating magnesium composites that gives the materials the highest specific strength and stiffness of any structural metal. A crucial step in the process is the dispersion of a relatively large volume fraction of ceramic nanoparticles in the molten metal, overcoming a long-standing challenge in materials technology.
Magnesium is the eighth most common element in Earth's crust and can be extracted from seawater. It is also recycled easily compared with polymers, which makes it environmentally friendly. The first notable commercial use of this metal for civil structural applications was in the Volkswagen Beetle during the 1930s — each car contained 20 kilograms (ref. 2). Bugatti also produced prototypes of a car called the Aerolithe, which had a body made from magnesium (Fig. 1). But the use of magnesium in vehicles was limited throughout the twentieth century because of the high cost of extracting the metal from its ore, the complexity of its mechanical behaviour, and concerns about its flammability and its susceptibility to corrosion under operational conditions.
Interest in magnesium surged afresh at the turn of this century, motivated by the pressing need to implement environmentally friendly policies in industrial production. The benefits of using lightweight materials have now been amply quantified. For example, a weight reduction of 100 kg for an average car saves about 25 gigajoules of energy and 1,600 kg of carbon dioxide emissions over the car's 10-year lifetime3. A major driver for research in magnesium has been the need to improve its mechanical behaviour dramatically, so that it becomes competitive with widely used heavier materials, such as steel or aluminium alloys.
However, hardening strategies that have led to major improvements in strength for other metals have been less effective for magnesium alloys. For example, the precipitation of a fine dispersion of solid particles of uniform size in aluminium alloys leads to four- to fivefold increases in strength (see ref. 4, for example), because the particles act as obstacles to moving dislocations (crystallographic defects whose movement leads to permanent deformation of materials). Up to now, the most effective precipitation treatments applied to magnesium alloys have barely doubled the alloys' strength5.
A major obstacle to further improvement lies in the difficulty of making a uniform dispersion of closely spaced, fine precipitates that effectively hinders the movement of basal dislocations and 'twins' — the deformation modes that are activated in response to the smallest applied stresses. Magnesium alloys often contain a mixture of precipitate phases that have different geometries and size distributions. A more uniformly sized and homogeneous particle distribution could be achieved by optimizing both the alloy composition and the heat treatment. But optimizing both together is extremely complicated, because small changes in alloy composition or in the temperature and duration of a heat treatment can lead to large and unpredictable changes in precipitate distribution.
Another approach to strengthening a metal is to add reinforcing particles of various types, shapes and sizes (usually micrometre-scale or larger). Typical additions to magnesium alloys include ceramic or metallic particles, oxides, borides and, less frequently, carbon or carbon nanotubes. But the resulting materials often have unpredictable mechanical properties, because they are unable to achieve a uniform dispersion of particles or good particle–matrix bonding. This limits their applications to niche products.
Powders consisting of nanoscale particles have been proposed to be highly effective reinforcers, and the development of inexpensive methods of producing them in large quantities has attracted a substantial effort to fabricate nanocomposites6. However, exploiting the full potential of such materials requires a uniform dispersion of a relatively large volume fraction of individual nanoparticles in the melt of the matrix material7. Chen et al. have succeeded in fabricating a magnesium–zinc alloy densely populated with individual ceramic nanoparticles (14% volume fraction), and in this way have endowed it with outstanding mechanical behaviour. This is the first time that formation of a nanocomposite has led to such a large increase in strength.
The authors began by dispersing a 1% volume fraction of ceramic nanoparticles in the magnesium alloy in the liquid state, and then increased the concentration of particles by partially evaporating away the metallic alloy in a vacuum furnace. The resulting uniform distribution of nanoparticles (see Fig. 1a and b of ref. 1) is extremely effective in arresting basal slip and twin propagation, leading to an increase in the alloy's yield strength (the stress at which the material starts to deform irreversibly) from around 50 megapascals to around 410 MPa, without impairing plasticity. The nanocomposites also have excellent mechanical stability up to temperatures as high as 400 °C.
Chen and colleagues conferred further, extraordinary, strength on the alloy by reducing the size of the grains (small crystals) that make up the bulk metal. The resulting material has a yield strength of 710 MPa, the highest ever reported for polycrystalline magnesium alloys and their composites.
The authors' preparation method has been validated at the laboratory scale, and seems to be particularly suited to fabricating small components made from metals that have melting points similar to, or lower than, that of magnesium (aluminium or zinc, for example). However, further work would be needed to optimize the processing parameters for other metals.
It remains to be seen whether the method could be feasible and environmentally friendly on an industrial scale. Potential problems in scaling up the process might include: the amount and cost of the energy needed; elimination of toxic residues from the evaporated matrix material; and maintenance of the equipment used. In addition, fabricating large amounts of homogeneous nanocomposite could be extremely difficult, because gradient distributions of particles are likely to develop during processing.
But there is no doubt that Chen and colleagues' work constitutes a milestone in our quest to design lighter, stronger materials, and opens up fresh avenues for the development of metals with unprecedented properties. For example, by choosing appropriate particles and optimizing their spatial distribution, it might be possible to make nanocomposites that have enhanced magnetic and electrical properties compared with existing materials.
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Tensile and compressive deformation behavior of peak-aged cast Mg–11Y–5Gd–2Zn–0.5Zr (wt%) alloy at elevated temperatures
Journal of Materials Science (2016)