The 19th century was the age of steel; the 20th was the age of synthetic polymers and plastics, but the 21st century will be the age of glassy metals. High-grade steel making and the invention of cheap, moldable and durable plastics have fundamentally changed industries. Similarly, miracle metals promise to alter manufacturing for years to come, spawn new industries and create and destroy great fortunes.
With a molecular structure similar to that of glass, amorphous metal alloys exhibit many unusual properties, such as: the highest strengths of any known metallic material; exceptional magnetism; wear/corrosion resistance; and, a large capacity to store elastic energy. These properties are impressive, but the ease of manufacturing makes them revolutionary. Unlike conventional metals that turn to liquid at high temperatures, these metals soften gradually when heated and do not shrink during solidification. These properties allow shaping and molding of the material into intricate designs, in near-net shapes with micro-scale precision at relatively low temperatures. In addition, the surfaces of the parts made with these materials are smooth-no sanding or grinding is necessary. As a result, the production of parts outperforming titanium and steel in durability and strength is as easy and cheap as making plastics.
The metals have a wide range of applications, from aerospace structures to golf clubs and medical devices. Some of the products on the market today include grid-scale, low loss transformers, corrosion resistant coatings, tennis racquets, baseball bats, watches, scalpels, molds for nano devices and casing for cell phones, made by companies such as HEAD NV, Rawlings Sporting Goods Company, Swiss luxury watchmaker TAG Heuer, Vertu Ltd., and Samsung.
The U.S. government recognized the strategic importance of glassy metals early on and has been applying them for decades. For example, the U.S. Navy has used amorphous coatings since 1980's and the Defense Advanced Research Projects Agency (DARPA) instituted a multi-billion dollar program to establish the viability of these materials. More specifically, DARP's program for Structural Amorphous Metals (SAM) focuses on military applications in the U.S., such as: hull material for ships; corrosion resistant coatings; lightweight alloys for aircraft and rocket propulsion; and, wear-resistant machinery components for combat vehicles. The government of Japan has also been funding research on metallic glasses for more than 30 years.
What Are Metallic Glasses?
An amorphous metal, also known as metallic glass or glassy metal has a disordered atomic-structure similar to that of glass. When a conventional metal cools from its molten liquid state, its atoms arrange into well-ordered crystalline structures. Most often, the structure is a polycrystalline matrix consisting of randomly oriented crystallite. The boundaries between the crystal grains, and other imperfections, cause the actual mechanical strength of a metal to be less than that of a perfect single crystal, contributing to wear and corrosion. The absence of grain boundaries in metallic glass makes the material much stronger than conventional metals and leads to much greater resilience.
Bulk Metallic Glasses
Bulk Metallic Glass (BMG) is an amorphous glass with a thickness of more than one millimeter. The alloy forming the BMG determines its properties and must contain at least three components. Many glass-forming alloys today contain zirconium and palladium. Other common materials are iron, titanium, copper, magnesium, nickel, and platinum. Primarily, today's BMG's are malleable at 400º Celsius.
The Achilles' Heel
The major obstacle to widespread use of the glassy metals is their brittleness, making them unsuitable as structural elements. Most metals under a heavy load deform gracefully before breaking. In metallic glasses, however, the fracture progresses rapidly and becomes catastrophic. One strategy to reduce brittleness is to create a metal matrix composite, consisting of a metallic glass matrix and dendrite particles or fibers of a ductile crystalline metal. The reinforcement can be an added material or an internally created crystalline.
A Brief History
The story of metallic glasses begins in 1960 in Caltech, where a group of scientists produced the first metallic glass from an alloy of silver. To avoid crystallization, the scientists cooled the glass-forming alloys at the rate of one million Kelvin per second. In 1976, a new method of manufacturing thin ribbons of amorphous metal on a super-cooled spinning wheel led to the creation of Metglas -- the main component of low-loss power distribution transformers used today.
By the 1990s, the application of new alloys lowered the required cooling rate to that achievable by simple casting of molten alloy into metallic molds. In 1992, scientists at Caltech then developed the first commercial amorphous alloy for aerospace applications, Vitreloy. A spin-off company, Liquidmetal Inc., exploited the commercial potential of the material in a variety of applications including golf clubs and cell phone casings.
In 2004, two groups succeeded in producing bulk amorphous steel. One of the two groups, in Oak Ridge National Laboratory, created "glassy steel" that is significantly stronger than the conventional steel; the non-magnetic material is suitable for ship structures. In 2005, a German-Chinese team produced the first amorphous material that gets harder under stress.
Many new promising fields of applications for amorphous metals include biomaterials and Biocompatible metallic glasses foams.
In an implanted device with amorphous surface, the small grain size of the material encourages adhesion and growth of cells surrounding the implant. Such an implanted device will reduce recuperation time and can last longer in the body. Biocompatible metallic glasses do not contain nickel; this avoids triggering allergies.
Thin pieces of BMG, such as wires or foils, have high inherent ductility during bending. Therefore, low-density BMG foams are ductile despite the brittle nature of the bulk form. Products made with this foam will inherit the natural advantages of foam architectures, such as improved density-compensated mechanical properties, acoustic damping and energy absorption at constant stress.
While some products made with metallic glasses are on the market today, further development is necessary to make the material universally practical. Many BMGs in these products contain a large portion of expensive and/or toxic components. Vast amounts of global R&D are focused on these materials and have produced recent advances, however, there is great promise of their prominence as a main stay of many industries in the near future.
Dr. Bellisario is the founder and principal of TranStrategy, a Boston-based consulting firm which specializes in the commercialization of technology. She is also a mentor with MIT's Venture Mentoring Service as well as a catalyst for the Deshpande Center at MIT. www.transtrategy.com