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An alloy of iron and aluminium is as good as titanium, at a tenth of the cost

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    Materials science
    Wings of steel
    An alloy of iron and aluminium is as good as titanium, at a tenth of the cost
    Feb 7th 2015 | From the print edition
    A LOT of tosh is talked about “nanotechnology”, much of it designed to separate unwary investors from their hard-earned cash. This does not mean, though, that controlling the structure of things at the level of nanometres (billionths of a metre) is unimportant. In materials science it is vital, as a paper just published in Nature, by Hansoo Kim and his colleagues at the Pohang University of Science and Technology, in South Korea, demonstrates. By manipulating the structure of steel on a nanometre scale, Dr Kim has produced a material which has the strength and the lightness of titanium alloys but will, when produced at scale, cost a tenth as much.
    Steel is useful because it is strong and cheap. But it is also heavy. It has, therefore, always been useless for applications such as aircraft. In a world that demands the ever-more efficient use of fuel in motor cars and lorries, it is now falling out of favour there, too. According to Dr Kim, the share by weight of steel in an average light vehicle fell from 68.1% in 1995 to 60.1% in 2011.

    The obvious way out of this is to alloy steel with a lighter metal. And the obvious one to choose is aluminium, which is, like iron (steel’s principal component), cheap and abundant. An alloy of iron, aluminium and carbon (steel’s other essential ingredient) is too brittle to be useful. Adding manganese helps a bit, but not enough for aluminium-steel to be used in vehicles.
    Dr Kim and his colleagues have, however, found that a fifth ingredient, nickel, overcomes this problem. To a chemist, an alloy is a mixture of materials rather than a true chemical compound. But metals do sometimes react to form real compounds, and one class of these, known as B2 intermetallic compounds (which have equal numbers of atoms of two different metals within them), lies at the heart of Dr Kim’s invention. The nickel reacts with some of the aluminium to create B2 crystals a few nanometres across. These crystals form both between and within the steel’s grains when it is annealed (a form of heat treatment).
    B2 crystals are resistant to shearing, so when a force is applied to the new material they do not break. This stops tiny cracks propagating through the stuff, which gives it strength. That strength, allied with the lightness brought by the aluminium, is what Dr Kim was after.
    There is, of course, many a slip ’twixt laboratory bench and production line, but POSCO, one of the world’s largest steel companies, is sufficiently interested in Dr Kim’s discovery to be planning, later this year, a trial that will produce it at industrial scale. If that goes well, not only may steel’s retreat from cars be reversed, but a steel aircraft may one day take to the skies.
    From the print edition: Science and technology

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