At the atomic scale, generally speaking, solids are either crystalline or amorphous. In crystals, the atoms or molecules fill space in a repeating, orderly arrangement, like stacked cannonballs or cells in a honeycomb. Table salt is a handy example. In amorphous materials, however, the atoms or molecules are packed together randomly, without repeating arrangements. Glass is the common example, and indeed “amorphous” and “glassy” are commonly used as synonyms.
Many solids can exist in either a crystalline or a glassy state. If a molten material is frozen quickly, the atoms or molecules have less time to settle into an orderly arrangement before they’re out of energy, and have to freeze where they are. The resulting solid is more likely to be glassy. If frozen slowly, however, the atoms or molecules can find their “proper” places and settle into a regular structure. The resulting solid is more likely to be crystalline.
The properties of metals are commonly manipulated this way, using various heat treatments. Under microscopic examination, metals are grainy, and each grain is a volume inside which the atoms are arranged in a highly ordered crystal structure. Generally speaking, the faster the metal is cooled, the smaller the crystalline grains, and the question naturally arises: Is it possible to freeze molten metal fast enough that crystal grains do not form at all? The resulting piece of metal would be completely glassy, with an essentially random packing of its atoms.
Turns out this is very hard to do. With pure metals, it is nigh impossible: All the atoms are the same size, and pack together easily, and there is no practical way to move heat out of a molten sample fast enough to keep them from falling into their orderly crystal arrangement, like billiard balls in a triangle. Once you begin mixing metals to make alloys, however, you’re dealing with atoms that aren’t all the same size, and a strategy suggests itself: Choose a mixture of atoms with a distribution of sizes that does not pack neatly in space.
It was not until 1960, however, that glassy metals were actually produced in a lab, from an alloy consisting of three atoms silver for each atom silicon. Elaborate means were required to cool the samples fast enough, and they had to be small and thin—wires, ribbons, or foils less than 100 micrometers thick. But the concept was proven, and the goal for researchers became to lower the “critical cooling rate” far enough that glassy metal objects could be made in larger sizes, without expensive cooling methods.
In the 1990s, researchers at CalTech made it work. The first commercial amorphous metal alloy was brought to market in 2003 by CalTech spin-off Liquidmetal Technologies. Called Vitreloy 1, it’s about 40% zirconium, 20% berylium, and 10% each of titanium, copper, and nickel.
Vitreloy 1 and its successors have a number of exceptional properties. They are strong, hard, and (unlike crystalline metals) they do not shrink appreciably on freezing, and thus can be injection-molded, blow-molded, and otherwise formed using the same economical processes as plastics. Designing tooling for amorphous metals requires paying attention to the critical cooling rate of the material; if some volume of the part cools too slowly, the alloy there will crystallize, shrink, and spoil it.
Today, Liquidmetal’s amorphous alloys are found in sporting gear, military hardware, and consumer electronics. Bits of it have gone into iPhones and other cell phones. If you’re hot to buy a piece, right now, your best bet is probably to snag one of SanDisk’s Cruzer Titanium model USB drives, which sport cases made from the stuff. Some Head tennis rackets and Rawlings baseball bats also contain amorphous alloy parts. Liquidmetal Technologies, unfortunately, does not provide samples, except by negotiated sale, for test and evaluation purposes, under a non-disclosure agreement. They’ve acquired a number of patents, however, that are chockablock with interesting technical details. US7017645 is particularly instructive of the methods for designing molds for amorphous alloys, and the original Vitreloy patent (US5288344), set to expire in 2013, includes eight formulations for alloys that “have been shown to be amorphous when cast in a layer 5mm thick.”
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