Researchers Uncover Metallic Glass Structure

By Anonymous

Researchers at Johns Hopkins University have uncovered the atomic- scale structure of metallic glasses that gives them their distinctive mechanical and magnetic properties.

“How the atoms pack themselves in metallic glass has been a mystery,” said Howard Sheng, an associate research scientist at Johns Hopkins’ Department of Materials Science and Engineering.

In traditional metals, scientists can view the configuration of atoms through transmission-electron microscopes (TEM). With metallic glasses, however, the TEM results in an image of a scattered array of atoms forming no obvious pattern. Researchers at Johns Hopkins launched a two-pronged approach to solve the mystery of how metallic glass atoms are arranged.

“Our goal was to advance the understanding of atomic packing in metallic glasses,” said Evan Ma, a professor of materials science and engineering at Johns Hopkins. “This is a difficult task because of the long-range order in these amorphous structures. Yet it is of fundamental importance because it is the structure that determines properties.”

Researchers made samples of a number of binary metallic glasses, each composed of two elements, and subjected them to tests to gather information about the samples’ three-dimensional atomic configurations (Figure 2).

Metallic glass atoms do not arrange themselves in a completely random way, researchers discovered. Instead, groups of 7-15 atoms tend to arrange around a central atom, forming three-dimensional shapes called Kasperpolyhedra. Similar shapes are found in crystalline metals, but in metallic glasses, these polyhedra are distorted and do not align themselves in long rows. In metallic glass, the polyhedra join together in unique ways as small nanometer- scale clusters. These structural features were described as chemical and topological short-range order and medium-range order.

Researchers also studied how low-density spaces form among these clusters in metallic glass. These cavities affect the way the material forms as a glass and the mechanical properties it will possess.

“Our findings should allow the people who make metallic glass to move closer to intelligent design techniques, developing materials with the precise mechanical characteristics needed for specific products,” said Sheng.

Copyright Minerals, Metals & Materials Society Mar 2006