New Materials

New Materials: Metallic Glasses By: Chris Pohorence

What is Metallic Glass?:

Metallic glass is a very unique metallic material. It is a metallic alloy that is cooled from an extremely high temperature to form a solid in a matter of milliseconds. This happens when a metallic liquid becomes increasingly viscous during the cooling cycle and fails to form a crystalline structure. The newly formed material can be called a glass because it lacks the crystalline microstructure that is found in metals. A metallic glass has an amorphous or a non-uniform, disorganized crystal structure, which is different than common paneglass, which has a crystalline structure. This material is unlike the glass that is used in windows. Metallic glasses are usually opaque not clear like normal glass.

The History of Metallic Glass:

Metallic glass was developed around 1960 at the California Institute of Technology. The two principle researchers, Pol Duwez and Ronald H. Willens, demonstrated that an amorphous alloy of 75 percent gold and 25 percent silicon could be quenched from a liquid into solid without crystallization starting within the material. They created a ribbon of metallic glass that was one-tenth of a centimeter thick and several meters long on a quenching machine. This quenching machine melts a metal alloy by means of radio frequencies in a quartz tube. The alloy is heated rapidly in the tube at an extremely high temperature. The molten liquid is poured from the tube onto a high-speed wheel. The liquid is cooled instantly on the wheel at a quenching rate of 1000 degrees Kelvin per second. Then the material is thrown off the wheel as a solid glass material at 3000 rpm or around 50 MPH. This scientific method of creating metallic glass is called melt-spinning. This method described here is for making metallic glass in the laboratory, there is a different way to make glass with melt-spinning in industry. This was the beginning of the development of new materials in the science of amorphous metallic alloys.

The uniqueness of this material sparked an explosion of commercial and academic research in the 1970's and 1980's. From this research, some commercial applications of this material were created. Research continues today to find new uses and techniques to make metallic glass. Presently, a researcher from John Hopkins University is trying to make metallic glass in bulk for additional use in industry. Scientist has already created bars of metallic glass that will have more applications in industry than the metallic glass ribbons and wires that are presently on the market.

How to Make Metallic Glass?:

There are many ways to make metallic glass. The method that is explained in this paper is called melt-spinning. There are also some other ways to produce metallic glass. One way is through thermal annealing, and the next is laser quenching. My research does not give much information on thermal annealing. Thermal annealing is used in the laboratory to make metallic glasses of scientific research. Laser quenching is used for making metallic glass coatings on materials. Laser quenching can be very inconsistent in thickness and in coverage. Laser quenching is as fast as melt-spinning but limited in its function to produce metallic glass for industry.

How to make metallic glass? The actual manufacturing of the glass is rather simple but the science behind the process is very difficult to explain. This process of making metallic glass is called melt-spinning. Melt-spinning is the most common way to make metallic glass. This process is quite different from the technique that is used in the lab. This quenching process in used in industry to make a certain alloy of metallic glass called Met-Glas. Met-Glas is used as the magnetic core inside electrical transformers. The first step into making metallic glass is to place a mixture of solid elements into a furnace. The mixture is heated above the liquidus temperature inside the furnace. The temperature of the molten liquid is held constant to retard the formation of nucleic seeds. For the material to form correctly many metallic glasses are made in rather clean environment. The next step is the liquid metal is poured onto a high-speed cooling wheel. The wheel is composed of copper and has the speed of 3000 RPM. This process will instantly transform the liquid into a glass because the copper wheel will quickly absorb the heat from the material. The wheel quenches the material at a rate about 1000 degrees Kelvin a millisecond or 1,000,000 degrees Kelvin in one-tenth of a second. The result of the reaction is a glass ribbon that is around 0.05 millimeter thick and 50 millimeters wide and several meters long. After checks in thickness and in consistency, the product is spooled and is ready for industry. I enclosed a figure of the melt-spinning process on the next page.

The Properties of Metallic Glass:

Metallic glass has many unique, sometimes unusual properties. The properties of metallic glass stray from the usual properties of glass but it also have many metallic properties associated with it. There are some advantages and disadvantages for using metallic glass. In overall processing time, metallic glass takes less time to manufacture over steels or other metals. The processing temperatures are lower in metallic glasses than metals. Steps like forging and annealing are needed to make steel stronger and more flexible. These processes take lots of energy and time to develop desired properties in metal. These are eliminated because of all of the desired properties metal are already in metallic glass. Normally shaping of a metal happens in the solid phase of the material, but with metallic glass, the shaping of the material happen in the liquid phase. The overall advantage is metallic glass are very cost effective, extremely efficient way to make a material. This is one of the reasons, scientists continue to research the many future possibilities of this material.

Some disadvantages are if the composition of the material is changed in way the strengths or properties of the metallic glass will change. Another disadvantage is that heat must escape from the material rapidly to create a glass-like material. Glass is limited to powders, wires, ribbons and shells for the time being. Recent research has created bars of metallic glass, with further research; sheets and other shapes of metallic glass will be developed. The most important disadvantage is that metallic glass has limited thermal stability. If the glass is heated several hundred degrees Celsius, the microstructure of the material changes from an amphrous to a crystalline structure hence losing all of its unique properties.

Strength:

One of many properties of metallic glass is its ability to be extremely ductile. An aluminum alloy glass ribbon can be folded in half easily without breaking or shattering. This simple test proves that metallic glass is resistant to deformation. An average tensile strength of a piece of aluminum alloy metallic glass is about 750 megapascals of pressure. This material succeeds the strengths of even the strongest aluminum alloy in commercial use. The strongest aluminum glass alloy is a combination of 90 percent aluminum, 5 percent iron, and 5 percent cerium. This material has a tensile strength of 940 megapascals of pressure, which is about one and half times stronger than the strongest aluminum alloy on the market.

This is not only true for aluminum based glasses, but some steel-based glasses are also extremely strong. The average tensile strength for steel glass ribbons is around 1000 megapascals of pressure. This also succeeds some of the strongest steels made today. Steel always has one limiting factor, if the strength of the steel increases, the ductility of the material decreases. Steel alloy metallic glasses eliminates this factor. If the strength of a steel glass ribbon is maximized, the ductility of the material will be at its maximum. The strongest steel glass alloy is a composition of 60 percent iron, 6 percent chromium, 6 percent molybdenum, and 28 percent boron. This material has a tensile strength of 4500 megapascals, which is two times stronger than the steel belts that are in tires. The ultimate tensile strength of steel is around 68 megapascals of pressure.

One of the reasons of the tremendous strength in metallic glass is the way that the microstructure of the metallic glass is created. Scientists have discovered that a material has to be highly developed and carefully treated to become very strong. When a metallic glass is created the percentages of the raw elements and the specifications of making the material are carefully controlled. The environment which metallic glasses are formed and processed must be very clean. If the environment was not clean, a material like this could not have the chance of being formed. Scientists also discovered while examining the microstructure of various metallic glasses, that many glass alloys have no signs of the usual properties of ductility that is found in metals. They also found that when a force is placed on a piece of material, the force is distributed throughout the microscopic bands within the material. The force acts upon the bands in both the longitudinal and the transverse axis. This is highly usually of a metal to have this unique characteristic. A normal metal will only withstand a force that is placed upon it only in the longitudinal axis. Then if the force succeeds the maximum strength of the material, the microstructure of the material begins to break at the metallic bonds of the material. This movement of atoms in called slipping, and it happens along slip lines in the microstructure. In metallic glasses this does not happen, scientists have discovered that certain metallic glasses have either microscopic slip planes or no slip planes at all. They have certainty that the necking process happens within some sort of microscopic bands in the microstructure. They have no clear explanation for this phenomenon but research still continues in how metallic glass fracture. The process of work softening reduces the strength of certain metallic glasses. The plastic elongation of the material in greatly reduced causing the glass alloy to become very brittle and also lacking in fatigue resistance.

Corrosion Resistance:

A minor property of metallic glass, corrosion resistance, has proven to be a good protective coating in industry. The metallic glass coating produces hydrated chromium oxyhydroxides upon the material being protected. This will protect the piece from the effects of corrosion. Metallic glass coatings are normally made from an iron-chromium alloy. Iron-chromium alloys have excellent resistance to chlorides (saltwater and ferric chloride solution). There are possible marine applications for iron-chromium glass coated materials. Metallic glass coatings are placed on a piece of material by the means of laser glazing. This technique is not very efficient; it is hard to get a uniform coating throughout surface of the material.

Magnetization:

One of the most useful properties of metallic glass is in magnetization of the material. Most magnetized metallic glasses are alloys of iron because of the superior conductivity of the material. The material before becoming magnetized must be annealed, because many strong glasses can not be magnetized as easily as weaker glasses. Annealing metallic glass make the glass softer, more workable, it also reduces the strength and increases the ductility of the glass. The weaker in strength the glass in, the more magnetic capacity is can hold. Some metallic glasses in this nature are able to become superconductors at absolute zero.

The Applications of Metallic Glass:

Metallic glass is easy to manufacture compared to common steels, aluminum and other metals. Metals require many processing steps to create a finished product. Metal usually goes through a number of forging, rolling, annealing, and drawing step to create a product. A metallic glass eliminates all of these steps and creates a material that is equal to or superior to steel or any type of metal.

Some of the most practical applications so far for metallic glasses are in electrical transformers. A steel alloy of metallic glass is used in place of copper as magnetic cores inside of transformers. The magnetic core of the transformer is composed of numerous layers of metallic glass ribbon. Scientific tests have shown that a transformer with a glass core is much more efficient as a core made from copper. The transfer of energy from the supplier to the consumer in a copper core transformer leads to a loss of electric, which is around 50%. With a metallic glass transformer, the net loss of electric from server to consumer is virtually eliminated. The cause of this is that when a metallic glass is magnetized, the material has no easy axis of magnetization. Magnetization could reverse direction twice each cycle for 120 cycles per second (US current standard) without the staggering loss in energy. It is estimated that $500 million dollars (1980) of electricity could save annually in the United States with transformer and other electric equipment made with metallic glass. Metallic glass cuts energy waste in half because glasses could regulate electricity much better than copper. The next step is for scientist to create electric motors with the same results as the metallic glass transformers.

The magnetic properties of metallic glass could be used in generators, amplifiers, switches, recording heads, delay lines, transducers and shielding. The corrosion properties of metallic glass have future practical applications in marine and biomedical fields. Some future products are scalpels and razor blades made from glass for medical use and numerous naval applications. Structural applications for metallic glasses are flywheels, belts for transmissions in machinery and automobiles, and belting material for tires.

Metallic glasses are a very practical material from a standpoint of an engineer. Metallic glass can be made very quickly and have less initial cost over steel. It can be woven into a fabric, braided into tubes, made into cylinders, and formed into bars. Metallic glass has superior strength to metal, are extremely flexible like modern plastics, and saves energy in production and in electronic applications. Metallic glass is the future of materials, because of their numerous application and superior properties. Scientists will examine metallic glass to understand more about this material and develop more uses for this material.

Bibliography

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2.)Metallic Glasses, Gilman, J.J., "Science", Vol. 208, May 23, 1980, pp. 856-861

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