Tech Briefs

Advancements in predicting the composition of metallic glasses will help the Air Force meet the requirements of tomorrow's technologies.

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.