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Invent byNumberResearchers predict,then produce superiortitanium alloys

Ingenious people have produced alloyssince ancient times. By trial and error, theymixed metals and other elements until thewhole came out better than any componentalone. In recent years, some scientists havesaid that alloy development is a mature fieldwith little room for improvement.

This week, however, researchers report anew method for making titanium-basedalloys with many qualities far superior tothose in any alloy previously known.

The team originally intended to developmaterials for automobile parts, says TakashiSaito of Toyota Central Research and Devel-opment Laboratories in Nagakute, Japan.Instead, he and his coworkers wound upwith materials that Saito says are too expen-sive for mass production in automobilesbut have unusual properties suitable for anew generation of precision screws, eye-glass frames, medical devices, sportinggoods, and spacecraft parts.

The alloys are strong yet unusually elas-tic, so they can deform more than otheralloys and still return to their original shape.Engineers can also readily mold or bend thematerials at room temperature into variousshapes, a property called superplasticity.

The materials also possess two charac-teristics desirable in machine parts thatexperience wild fluctuations in tempera-ture, such as those in a spacecraft. Whilemost metals expand with any rise in tem-perature, the new alloys expand very littlebetween –200°C and 300°C. Moreover,conventional alloys deform differentamounts at different temperatures, but thenew materials show about the same defor-mation whether it’s –200°C or 300°C.

Saito and his coworkers report in theApril 18 Science that they devised calcula-tions with which they can predict whatcombination of elements would make an

alloy with these properties. The research-ers determined that a desirable titanium-based alloy must meet three criteria, whichthey call “magic numbers,” that are based onquantum mechanical calculations reflect-ing the behavior and arrangement of atomsand electrons.

The scientists add that an alloy meetingthe criteria must be cold worked, ordeformed by compression at room tem-perature, before it shows the extraordinaryproperties.

So far, Saito and his colleagues have madeseveral titanium-based alloys that fit thesecriteria and possessthe desired proper-ties. The materialsall include oxygen,and one, for exam-ple, also containsniobium, tantalum,and zirconium.

The new alloys’properties are“remarkable,” saysGary Shiflet of theUniversity of Vir-ginia in Char-lottesville. In partic-ular, he says, theirs u p e r p l a s t i c i t ycould eliminate theneed for expensivemachining tech-niques when shap-ing titanium alloys into products.

Perhaps more importantly, says Shiflet,the new report demonstrates the power ofa computational method to predict alloysthat never would have been made throughtrial and error.

Now, Shiflet asks whether the magic-numbers approach applies to otherimportant classes of alloys, such as thenickel-based ones. “We will try it soon,”says Saito. —J. GORMAN

Between the SheetsIn reactors and nanotubes,errant atoms get a grip

Reach out and touch each other. That’ssomething that carbon atoms in adjoininglayers within graphite aren’t supposed todo—even in the cores of nuclear reactorswhere graphite blocks take a beating fromneutron radiation.

New atomic-level simulations in Englandchallenge that expectation. The findings, ifconfirmed, may revise not only the way thatspecialists handle spent nuclear-reactorcores but also how technologists build novel,nanometer-scale carbon structures.

Graphite has a crystal structure consist-ing of many one-atom-thick carbon sheetsknown as graphene. The spacing betweensheets is more than twice that betweenatoms within the sheets, so the forcesbetween sheets are extremely weak.

Now, Rob H. Telling of the University ofSussex in Brighton and his colleagues findthat carbon atoms near neutron-punchedholes in graphene make surprising, tran-sient movements into the zone between lay-ers. As they briefly dangle there, manyatoms from adjacent sheets meet and formstrong bonds, the researchers suggest.

These and otherunexpected bondsthat the simulationpredicts modify prop-erties ranging from agraphite crystal’s sizeto its electric andthermal conductivi-ties, Telling says.

In the May NatureMaterials, he and hiscolleagues describe asurprising set of irreg-ularities that showedup in computer simu-lations of a 64-atom,two-sheet portion ofa graphite crystal.

Their report “shedssignificant new lighton an issue which has

been studied for some 50 years but stillremains poorly understood,” comments KaiNorlund of the University of Helsinki. “Ifthe results are eventually verified, this workshould become a seminal paper in the field.”

Scientists have long known that thegraphite used to slow down neutronsshooting from the fuel in old-style nuclearreactors accumulates crystal defects thatstore energy. The potential danger of thatenergy was demonstrated in 1957 by theworld’s third-worst nuclear accident, theWindscale fire in Sellafield, England. Too-sudden release of energy stored in graphitedefects made the Windscale reactor corecatch fire and unleash significant radioac-tivity into the air.

While the nuclear-power industry haschanged the type of reactors in use, it stillmust dispose of some 150,000 tons of irra-diated graphite as defunct reactors are dis-mantled. The nuclear-power companyBritish Nuclear Fuels partly funded the Sus-sex study in a quest for deeper understand-ing of radiation-induced defects in graphite.That information could improve disposalmethods to avoid a sudden release of energy.

The new work has also struck a chord inan up-and-coming industrial sector—theLilliputian world of carbon nanostructures.By revealing new atomic arrangementscaused by irradiation in graphite, the sim-ulations may aid scientists using radiation,

ALLOY THERE A new titanium-based alloycontaining four other elements has amarblelike microstructure after the materialis deformed through compression.

SCIENCENEWSThis Week

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