Bulk Metallic Glasses-phyd today.pdf

Glasses Stronger than steels but able to be shaped and molded like plastics, bulk Jan Schroers metallic glasses are the quintessential engineering materials. reating a glass is akin to racing against order to crystallize. As a result, supercooled metals a thermodynamic clock. The clock be- tend to crystallize quickly—so quickly that until gins ticking when a liquid is cooled about 60 years ago, it was thought infeasible to cool Cbelow its melting temperature TM . metals into a glassy state. We now know, however, Typically, as the supercooled liquid’s that for certain metals—particularly alloys—under temperature decreases, so do its enthalpy and vol- certain conditions, the thermodynamic clock ticks ume, as illustrated by the plot in figure 1a. (Water, slowly enough to allow glass formation. It’s advan- which has a negative thermal expansion coefficient tageous to cool the metal in an immaculate environ- below 4 °C, is a notable exception.) At what’s known ment to reduce the number of impurities at which as the glass transition temperature TG , the atoms in crystals might preferentially nucleate. But the chief the liquid become so tightly packed that they can no breakthrough was recognizing that certain alloys longer easily rearrange themselves. In that glassy have a dramatically lower propensity to crystallize