METGLASSES

That, of course, is an exaggeration. But not a very wild one. Space travel, for example, was theoretically almost ready in the last century - celestial mechanics were well understood, as was the principle of the rocket engine, including multistage rockets; what was missing (mainly) were the alloys to withstand the necessary temperatures without melting. The same point held up (basically, anyway) the realization of the jet engine and the jet plane.

And the gas turbine cars with ceramic engines (AtE Jun 76), the ones that would get 50 miles to the gallon at high performance, why aren't they here yet?

Because the ceramics that have been developed in the last few years do not yet have the required properties. They do have the necessary heat resistance and hardness, but not, for example, the needed plasticity; they are still too brittle. Metal parts machined in mass production never fit together exactly, but as they run in an engine or other machine, they are sufficiently ductile and resilient to beat, bend and chew each other into shape within reasonable limits, of course.

Ceramics and glass, which are harder than metal, can't do that; they are brittle and will break instead of yielding. Needed: a material that exceeds metals in both strength and plasticity.

Now what makes a metal a metal and what makes glass glass is atomic structure. In a metal (and most solids), the atoms are arranged in a regular, crystallic lattice structure; in glass (and that makes it an exception among solids), the atoms are clustered together in an irregular, shapeless mass, which is what gives glass its hardness. The absence of regular, crystallic structure is characteristic of liquids, in fact, glass is really a supercooled liquid, which didn't have time to crystallize when it solidified. A metal also loses its crystallic structure when it melts, but when it cools and solidifies, it crystallizes. To make the metal hard, it must be cooled rapidly, partially impeding crystallization.

Without knowing anything about atomic structure, ancient peoples knew that they could get hard - but brittle metal by quenching it, i.e., rapidly cooling it by dipping red-hot metal in water.

But they couldn't cool it fast enough to prevent crystallization altogether. Of course, the presence or absence of a lattice is not the only difference between glass and metal. But whatever the other differences, is it possible to cool molten metal so rapidly as to prevent crystallization and give it some glass-1ike properties while retaining some of its metallic ones?

Yes. By cooling metal at the rate of a million degreesC per second (actually some 1,000 degreesC in about a millisecond), one can obtain materials that Allied Chemical calls "metglasses." They are now under intensive investigation in industry, and they have amazing properties: Some of them have a tensile strength three times greater than stainless steel, yet they can withstand a local plastic shear strain of more than 50% (Strain = relative deformation, e.g., prolongation to original length.)

By comparison, common glass, even in the form of filaments, cannot be strained by more 10% before it breaks, and neither can steel make anything like 50%. That means that metallic glasses do not just combine the good properties of glass and metals; theyare better than either.

Too good to be true?

Yes and no. Read on.