Quick Reference Activity Guide: Amorphous Metal
Activity Materials
- Two acrylic tubes
- A stainless steel base and a stainless steel base with an amorphous metal disk glued on with epoxy
- Two stainless steel ball bearings
- The face-centered cubic (fcc) model built with the solid state model kit
- Different sized marbles in a jar (represents an amorphous arrangement of atoms)
Starting Points
Let's do an experiment. One base is steel, the other base has a thin disk of amorphous metal on the surface. Will the ball bounce better off of the steel base or the amorphous metal base? Let’s see how many times the ball bounces on each surface. Designate a "ball dropper" and a "bounce counter". What will happen if we increase the height we drop the ball from?
Do you know the difference between crystalline and amorphous materials? These are two ways that atoms can be arranged in a material. A crystalline material has a repeating pattern of atoms while the atoms in an amorphous material have no regular pattern (they are all jumbled). The properties of a material are highly dependent on the arrangement of atoms in that material. Let’s compare the rebounding properties of a crystalline and an amorphous metal.
Which material would you like to use for the head of your golf club? What are the important properties of a golf club? Let’s compare the two surfaces to figure out which one would make a better golf club.
Demonstration Procedures
Have a volunteer drop one ball bearing down the center of the tube with the stainless steel base and watch how it bounces on the surface. Count the number of times that the ball bearing bounces. Then, take the same ball and repeat the experiment using the base with the amorphous metal. Again, count how many times the ball bearing bounces. How does it compare between the two materials? The ball bearing bounces on the amorphous metal as if it were on a trampoline.
After the ball bearing is released, it has kinetic energy, or energy of motion that is determined by an objects mass and how fast it is moving. Each time the ball bounces its kinetic energy decreases. Older students might be able to list the ways in which the ball is losing energy (air friction, sound, collisions with the acrylic tube, impact of the ball with the surface).
Have the students look closely at the surface of each base. What do they notice? They should notice lots of pits in the stainless steel base. Ask the students how they think the pits got there (sometimes they don’t make the connection that the pits are a result of the ball hitting the surface). And what about the amorphous metal? There are hardly any pits. This gives us a clue as to why the ball is bouncing more on the amorphous metal surface.
At this point explain the different rebounding properties are related to the structures. A crystalline metal has slip planes, planes of atoms that slip easily past one another and are a result of the repeating pattern of atoms. Show the students a slip plane using the fcc model by lifting a lower corner "atom" of the model. When the ball hits the crystalline surface, some energy goes to moving the surface atoms out of the way and forming the pits (i.e., atoms are slipping past one another). In many cases this atomic motion results in permanent deformation of the solid. Another way to think of it is that a crystalline material is like an energy sponge taking energy from the ball and dissipating it in the form of an atomic scale friction.
An amorphous metal has varying sizes of atoms that exist in a random arrangement in the solid, which eliminates the possibility of slip planes (dislocations). Because there are no slip planes in an amorphous material, the material can be thought of as atomic gridlock - the atoms do not move. This means that the kinetic energy of the ball is not transferred as easily to the amorphous metal and the ball bounces much longer. One consequence of this atomic gridlock is that some amorphous metals are very hard; amorphous metal is two times harder than stainless steel. However, besides being a very hard material, this amorphous metal is brittle.
Fact Sheet
The Vitreloy amorphous metal used in the demonstration is composed of
Zr41.2Be22.5Ti13.8Cu12.5Ni10.0
This particular amorphous metal was developed by Prof. W.L. Johnson from the Department of Applied Physics at the California Institute of Technology in 1993.
Most amorphous metals are formed by rapidly cooling the material from a molten state, Vitreloy is particularly impressive because it can be cooled from liquid state at rates as low as 1oC per second and still form an amorphous solid. The slow cooling rate of this amorphous metal allows it to be cast into molds.
Vitreloy is 2-3 times more resistant to permanent deformation than conventional metals, has a density between that of titanium and steel, and is corrosion resistant.
Applications
Golf clubs with amorphous metal heads, created by Liquidmetal®, have been on the market in 1998.
Industrial coatings for equipment and machinery that are exposed to environments of high wear, temperature, and corrosion. Amorphous metal has the lowest coefficient of friction of any metallic coating, and significantly extends part lifetime. Example: the wall of a refinery coker.
Armor-piercing ammunition that enhances the performance and safety levels for users. The military is developing armor-piercing ammunition that uses amorphous metal instead of Depleted Uranium (DU) alloy.
Casings for electronics and telecommunications equipment, such as cellular handsets, that are stronger, smaller, and thinner.
Amorphous metal knives are used in medical fields, such as ophthalmic medicine, because they are sharper than steel, less expensive than diamond, and higher quality than diamond; they are more consistently manufactured than steel or diamond; and they have longer lasting blades.
Solar wind collector tiles on NASA's Genesis spacecraft, the first mission to collect and return samples of the solar wind. The tiles are comparable to a coffee cup lid, and will play a key role in the collection process. The mission is designed to measure the composition of isotopes in solar matter. For information see: http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1783.pdf
Background and Supporting Information
Many scientists and engineers focus their energy on the development and characterization of new materials. These materials often enable new technologies. Many of the advancements of nanotechnology have come about because of the development of new nanoscale materials or the understanding of materials at the nanoscale.
One such important area of study is the fundamental structure of materials. We know from nature that atoms can arrange themselves with regularity (crystalline) or randomly (amorphous).
Amorphous rocks and minerals occur in nature. One example is obsidian (volcanic glass). Other examples are diatomite and radiolarite, rocks high in amorphous silica content, formed from the shells of tiny fresh- and salt-water organisms called diatoms and radiolaria. The mineral opal is a hydrated amorphous silica. Although these materials have very different properties from each other and from the Vitreloy, they share a commonality in their atomic structure — the atoms of these materials do not have a regular arrangement, they are randomly arranged and unorganized.
Crystalline materials are the opposite of amorphous. They have a regular atomic structure where the atoms are neatly arranged with a simple organization that can be described by a unit cell. The face-centered cubic (fcc) model built with the Solid State Model Kit is an example of one such unit cell. Although we often do not think of them that way, most metals are crystalline. They are often polycrystalline, meaning that they contain many individual crystals packed together in what is call a grain structure. This crystallinity is what gives metals much of the familiar behaviors we associate with them, such as malleability and ductility.
Most crystalline solid structures contain missing atoms, called defects, impurity atoms of other elements, and misaligned planes of atoms called dislocations. Impurity atoms, defects, and dislocations all have an important impact on the physical and chemical properties of the solid. For example, copper wire is easy to bend because the structure contains planes of atoms that can slip easily past one another. The planes that dislocations move along are called slip planes. A slip plane can be observed in the face-centered cubic (fcc) model by picking up one of the bottom corners of the model so that half of the model slides up. Amorphous materials do not contain such planes.
Many students have heard of atoms, but few have an understanding of how the arrangement of atoms can affect the properties of materials. In addition, most students do not know what the word amorphous means. Our goal is to introduce the concepts of crystalline and amorphous structures and the link between structure and properties. Students should be encouraged to come up with different experiments after observing and discussing the differences between the two materials.
One difference that can be observed by the students is the pits that have been made in the steel base where the ball has bounced on it. These pits are a direct result of the impact producing many dislocations and causing permanent deformation. If the ball starts out with an initial amount of kinetic energy then some of the energy lost with each bounce goes into the movement of the dislocations, the misaligned planes of atoms. Such pits are not seen in the amorphous metal because the random arrangement of atoms in this material precludes dislocations. Thus energy is not lost in the movement of dislocations in the amorphous material.
There are several ways in which the ball loses kinetic energy: sound, random collisions with the sides of the tubes, and friction with the air. Assuming these factors are the same, on average, for both bases, the difference in the bouncing of the ball must be due mainly to the difference in energy transfer between the ball and each surface. The fact that the ball bounces longer on the amorphous metal surface indicates a much different energy transfer interaction than that of the stainless steel base. The difference in "reboundability" (coefficient of restitution) of the ball bearing with each metal surface is due to differences in how the atoms pack.
The coefficient of restitution represents the degree to which an impact is elastic. Upon collision, one or both of the objects mar deform, and this deformation may be elastic or plastic (permanent). A perfectly elastic impact, in which the kinetic energy loss is zero, would have a coefficient of restitution of one. A perfectly inelastic impact, where all of the energy goes into plastic deformation, will have is no rebound and a coefficient of restitution of zero. The coefficient of restitution can be determined for the two cases in this demonstration by measuring the height from which the ball is dropped (h1) and the height of the first bounce (h2).
Coefficient of Restitution = e = (h1/h2)1/2
Stainless steel is a crystalline metal made primarily from iron with a few percent of additional elements such as carbon, chromium, and sometimes nickel. Steel has an ordered, repeating pattern of atoms, while the Vitreloy amorphous metal is comprised of five elements (41.2% zirconium, 22.5% beryllium, 13.8% titanium, 12.5% copper, and 10.0% nickel). The different atomic radii of atoms in Vitreloy promote its highly disordered arrangement. The different sized marbles in a jar can be used to represent this amorphous arrangement of atoms. With all of the different atomic sizes it is difficult for the atoms to pack neatly.
It is important to note, however, there are many amorphous materials that are not made up of a collection of differently sized atoms. When a material is molten the atoms are farther apart because they have more thermal energy, their arrangement is irregular and random. If a material can be frozen (i.e. quenched) into the solid state fast enough, crystallization can be prevented and the random arrangement can be trapped in a tightly-packed solid form. This occurs because the atoms do not have enough time to arrange themselves in an ordered structure.
Because there are no planes of atoms in an amorphous material, the atoms are gridlocked into the glassy structure, making the movement of groups of atoms very difficult. One consequence of this atomic gridlock, is that some amorphous metals are very hard. Vitreloy is more than two times harder than stainless steel. However, besides being a very hard material, this amorphous alloy has a low elastic (or Young's) modulus.
Demonstrate the relationship between wavelength and frequency by watching the ball bounce on the amorphous metal surface. Explain that the distance the ball travels in one bounce represents the wavelength of a photon of electromagnetic radiation. The time it takes for the ball to bounce once represents the frequency of the photon. As the "wavelength" decreases the frequency of the bouncing ball increases demonstrating the inverse relationship of the formula c = ln.
By the end of the 8th grade, students should know that:
- Energy cannot be created or destroyed, but only changed from one form into another.
- Energy appears in different forms. Heat energy is in the disorderly motion of molecules; chemical energy is in the arrangement of atoms; mechanical energy is in moving bodies or in elastically distorted shapes; gravitational energy is in the separation of mutually attracting masses.
By the end of the 12th grade, students should know that:
- Whenever the amount of energy in one place or form diminishes, the amount in other places or forms increases by the same amount.
Additional Information
References
See http://www.liquidmetaltechnologies.com for further information about general applications.
K. J. Nordell, N. D. Stanton, G. C. Lisensky & A. B. Ellis. The "Atomic Trampoline" Kit: Demonstrations with Amorphous Metal (ICE, Madison, WI, 2000).
Authors: Amy Payne, Wendy Crone, George Lisensky, Cindy Widstrand, Janet Kennedy, Mike Condren, Ken Lux, Karen Nordell, Nick Stanton, and Arthur EllisAmy Payne, Wendy Crone, George Lisensky, Cindy Widstrand, Janet Kennedy, Mike Condren, Ken Lux, Karen Nordell, Nick Stanton, and Arthur Ellis
