AFRL scientists made significant progress in developing bulk metallic glasses to improve the durability and performance of aerospace components. They also successfully created working scientific models that can predict the composition of new metallic glasses, a capability that helps researchers determine in advance whether a particular glass can be manufactured in bulk form. As a direct result of their effort, researchers recently discovered several new bulk metallic glasses. Their work also led to the successful development of a new technique to illustrate the topology of amorphous (noncrystalline) metal alloys.

This illustration shows 4 candidate atomic clusters with efficient atomic packing for solute atoms with respective coordination numbers of 9, 10, 12, and 13. These clusters illustrate the local atomic structure in many metallic glasses.
These advancements will enable the Air Force (AF) to develop metallic glasses with the exceptional functional properties required to meet the demands of tomorrow's crucial technologies. Thus, these research results facilitate the development of tougher, higher-performance aerospace components that will benefit the AF, commercial aviation, and industry in general.

As materials with exceptional functional properties— magnetic and structural qualities, in particular—metallic glasses have tremendous potential. However, most metallic glasses must be cooled very quickly, at rates faster than approximately 1000°C/sec. Achieving these high cooling rates typically requires that one dimension of the produced material measure <0.5 mm. Only a few bulk metallic glasses exhibit critical cooling rates low enough to produce bulk pieces >1 mm.

Since research in metallic glasses began about 40 years ago, today's researchers had no basis for determining—prior to manufacturing—whether a new alloy could be produced in bulk form. There existed no clear, reliable methodology for predicting new bulk metallic glass compositions. AFRL research conducted over the past 2 years has remedied this limitation by providing useful predictive models based on the number and sizes of atoms.

The high density of an amorphous alloy, relative to its crystalline form, suggests that efficient atomic packing is a fundamental consideration in the constitution of metallic glasses. Previous efforts to explain the high relative density of amorphous metals based on dense random packing of atoms of differing sizes have been unsuccessful. AFRL researchers took an alternative approach, exploring the concept of efficient atomic packing based on atomic clusters consisting of a central solute atom surrounded by solvent atoms in the first coordination shell (see figure).

The ratio, R, of the solute atom radius to the solvent atom radius was the only topological parameter that researchers considered. A simple analysis of this model leads to the conclusion that specific atomic radius ratios, R*, provide efficient atomic packing over a length scale defined by these atomic clusters. This conclusive result extends earlier descriptions of topological influence on the formation of metallic glasses by identifying a more specific set of conditions for metallic glass formation.

Researchers calculate the values of R* for solute-centered clusters using an analytical expression for the coordination number, NT, of solvent atoms as a function of the radius ratio. They base this expression on the packing of spheres on the curved surface defined by the solute atom and explicitly account for breaks in surface coordination with variations in R. The resulting expression provides an exact solution for well-known clusters such as tetrahedra (R* = 0.225, NT = 4), octahedra (R* = 0.414, NT = 6), and icosahedra (R* = 0.902, NT = 12). Researchers can predict values of R* for solutecentered clusters having 3-24 solvent atoms in the first coordination shell.

The research team analyzed a large number of binary, ternary, and higher-order metallic glasses to validate the prediction of preferred radius ratios in the constitution of metallic glasses. The results showed a clear preference for the predicted critical radius ratios for all major classes of metallic glasses analyzed. The research team formed the following conclusions: (1) efficient atomic packing in the first coordination shell is a fundamental consideration in the constitution of metallic glasses; (2) researchers can predict specific critical radius ratios to provide efficient atomic packing in the first coordination shell of solute-centered atomic clusters; and (3) analysis of a large number of metallic glasses and a selected number of silicate glasses shows a strong preference for these predicted critical radius ratios.

This predictive model provides detailed guidance for selecting new alloys that may ultimately become the next generation of bulk metallic glasses. By selecting alloys with elements that satisfy predicted radius ratios, US scientists recently devised at least three new bulk metallic glass systems, including an iron-based glass that researchers are evaluating as a corrosion-resistant layer for nuclear waste containment.

Dr. Daniel B. Miracle, Capt Wynn S. Sanders, PhD, Dr. Oleg N. Senkov (Universal Energy Systems Corporation), and Dr. Peter S.Meltzer (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document ML-H-05-05.