What are some unique properties of metals
Metallic glasses - robust and extremely versatile
Metals have become an integral part of modern civilization and have been with people for thousands of years. In order to create completely new material properties, scientists specifically intervene in the order of the metal atoms. The resulting metallic glasses have unique properties and are one of the most modern types of material of our time.
High-precision gear made of metallic glass
No, it's not about glass. At least not in the everyday sense. Because if we talk about glass in everyday life, we are mostly talking about drinking vessels, window panes or expensive shards. For a materials scientist, however, glass is a much more general term. In science, a material is called glass-shaped if its atoms are not regularly spaced from one another, but are arranged completely irregularly in the material. Metallic glasses are in precisely this state of disorder and this has far-reaching consequences for their properties.
The hardness of metallic glasses puts conventional steel far in the shade. Nevertheless, they can be injected into shape at less than 300 degrees Celsius, similar to plastic. This unique processing of the metallic glasses makes it possible to manufacture components much more precisely than with conventional metals. In this way, subtleties can be poured with an accuracy of just a millionth of a meter - that's sixty times thinner than a hair. Some shapes are even only possible thanks to metallic glasses. Foams that consist of 99 percent air and are 100 times harder than styrofoam sound incredible. However, such materials made of metallic glass have already become a reality. The reason for these amazing properties lies in the arrangement of the atoms in a metallic glass.
Usually, the atoms in a metal are arranged regularly, much like marbles in a box. This ordered state is called crystalline and is typical of metals. The atoms always occupy the same positions and distances from one another over large areas. Such an ordered area is called a grain, and a piece of metal is made up of a huge number of coherent grains.
In contrast to the crystalline state, the atoms in a metallic glass are completely disordered. There are neither grains nor grain boundaries, just mixed-up atoms that freeze in their random positions when the melt cools. This atomic disorder of the metallic glasses has many advantages.
The arrangement of the atoms determines the material properties.
If conventional metal is poured into a mold as a melt, it contracts when it cools, whereby the precision suffers greatly. The cause of the shrinkage lies in the crystalline order. Because the atoms are closer together in their ordered places in the solidified metal than they were before in the melt. Metallic glass, on the other hand, remains exactly in the given shape, because the atoms simply remain in their disorder when it cools, without moving into a narrower lattice. The phenomenon of tidying up the desk is clearly known: the huge, disorganized mountain of paper is suddenly much smaller when you put it in orderly stacks in a box. If, on the other hand, you throw the sheets of paper back and forth in the box, they take up just as much space as before on the desk. In addition, metallic glasses become malleable even at low temperatures. On the one hand, this reduces processing costs and, on the other hand, increases the precision with which the material can be shaped.
Corrosion also has a hard time with metallic glasses. Conventional metals offer chemical decomposition a sensitive target between the individual crystalline areas. Because at the grain boundaries, aggressive substances can penetrate from the outside and attack the metal. In the case of metallic glasses, on the other hand, there are no grain boundaries without crystalline order - and so in many cases corrosion is simply left out.
Another special property of metallic glasses is their hardness: if you drop a steel ball on a plate made of metallic glass, the ball bounces up and down like a bouncy ball. The explanation can again be found in the arrangement of the atoms: conventional metals swallow part of the force when the ball hits the ball, because the atoms in the crystal grains move relative to one another as a result of the impact. Because the order in the tiny grains creates so-called slip planes, along which the atoms move more easily. That cannot happen to the confused mess of metallic glasses, since there are no slip planes without atomic order. So it's no wonder that in 1998 Liquidmetal presented a golf club made of its metallic glass for advertising purposes. Due to the great hardness, the racket transmitted a greater impulse to the ball and thus ensured further tee-offs.
Metallic glasses are much harder than crystalline metal.
But the absence of crystalline grains also brings with it the archenemy of metallic glasses: brittleness. Metallic glasses are very tough, but they give in first, then completely. While their crystalline counterparts initially defy deformations by hardening, a defect in metallic glass propagates like an avalanche. If a break begins to form, the metal at this point becomes softer due to the development of heat, the break progresses and the game starts all over again. So the break can simply run through the entire material. For critical applications in which a sudden total failure of the component would have fatal consequences, there are therefore limits to the use of metallic glasses. However, intensive research is also being carried out into removing this hurdle, and a possible solution to the problem has already been found. For example, researchers at the California Institute of Technology have added individual crystal grains to a metallic glass, thereby limiting the growth of cracks.
How to "Confuse" Atoms
But how do you even prevent the atoms from adopting their usual crystalline arrangement in the metal? In hot, liquid metal, the atoms have enough energy to break free from their fixed positions and move freely within the melt. If the liquid melt is cooled down quickly enough, the disordered atoms do not have time to move to their places before the melt solidifies. The result is the frozen atomic disorder of a liquid. The endeavor is far more difficult than it may sound at first.
Researchers at the California Institute of Technology succeeded for the first time in 1960 in producing a metallic glass from a gold-silicon mixture. In order to freeze the atoms in their disorder, they cooled the material very quickly - at a million degrees Celsius per second! This incredibly high cooling speed could only be achieved in pieces of metal tenths of a millimeter thick. Therefore, until the 1980s, it was only possible to produce thin strips of metallic glass. In order to be able to produce larger pieces, scientists have to dig deep into their bag of tricks.
Pure metals with only one type of atom, such as copper or iron, completely refuse the disordered glass state. Because no matter how fast you cool pure metals, the atoms are practically always flexible enough to arrange themselves in crystalline form. A mixture of different chemical elements is therefore required to produce a metallic glass. The main trick here is to mix elements that are as different in size as possible. Because the resulting crystal lattice is so complicated that the various atoms need a lot of time to move from the liquid disorder to their ordered places. This means that even relatively slow cooling of a few degrees Celsius per second is sufficient to freeze the mixture in its glassy state. This approach is called “atomic confusion” for obvious reasons. How such a mixture of atoms can be confused is therefore a key to understanding metallic glasses.
How to freeze atomic disorder
Our working group examined a mixture of palladium, copper, nickel and phosphorus. This alloy is known to solidify in the glass disorder even when it cools slowly. But why exactly the atomic mixture can be confused so easily has so far been unclear. To clarify this question, we measured for the first time how mobile the individual types of atoms in the alloy are during cooling (the principle of the measurement is explained in the info box). Our measurements revealed a crucial phenomenon that prevents crystallization during cooling: even before the melt solidifies and all the atoms adopt their crystalline arrangement, the palladium atoms already come together to form a relatively rigid framework. This something, which is difficult to move, can only very slowly move into the crystalline order. Therefore, cooling the mixture at just a few degrees per second is sufficient to freeze the liquid mess.
The slower cooling not only reduces the technical manufacturing effort, but also makes it possible to produce larger pieces of metallic glass. The record is currently held by researchers from Tohoku University in Japan with a cylinder made of metallic glass with a diameter of about seven centimeters. But the record-holding alloys have a serious disadvantage: They consist to a large extent of palladium or similar exotic elements that are very expensive.
(Almost) arrived in everyday life
In the big department stores, it is impossible to imagine life without the alarm systems in the exit area, which sometimes correctly indicate the theft of goods, sometimes trigger a false alarm because a label has not been deactivated. However, very few of us think of metallic glasses when we hear such an alarm. But it is precisely these that guard the products in the form of small stripes. The strips consist of metallic glasses in which magnetic substances are also inserted into the metal mixture. To secure the goods, use is made of the fact that this metal mixture expands or contracts a little in a magnetic field. Although this effect is also observed with conventional metals such as iron or cobalt, even very weak magnetic fields are sufficient to change the size of the metallic glasses. Since only thin strips are required for the security labels, the production of the glasses for this application is also uncomplicated.
It becomes much more expensive when massive pieces of metallic glass are used. So it is not surprising that, despite its unique properties, the material has only been used in niche applications so far. The properties of metal glasses are particularly useful in medicine. Low wear, corrosion resistance and good biological compatibility predestine the disordered metals for implants. Research is even being carried out on biodegradable screws. After an operation, these would simply be dissolved by the body over time, making further interventions superfluous.
The fields of application of the modern materials are diverse: From skis to cell phone cases and transformers to medical prostheses - the metallic glasses are finding their way into a multitude of technical applications. In our everyday life, glass will in future be much more than a drinking vessel.
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