The Hard Facts About Metallic Glass

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The Hard Facts About Metallic Glass

For thousands of years, civilizations have crafted metal and glass, creating not only objects of beauty, but also elements for industry. These distinct materials each possess strengths and weaknesses that determine their applications.

Common glass is strong and resists being deformed, but is brittle and cracks easily. This is due to its non-crystalline, disordered atomic structure, called an amorphous structure. Metals, on the other hand, resist cracking, but are subject to bending, stretching, and flattening. Their crystalline structure provides micro-structural obstacles that keep metal from cracking and allow it to bend.

In 1960, at Caltech, a new class of material was discovered called "metallic glass." When we think of glass, we immediately think of window panes. But metal can exist as a glass as well. Why? Because, by definition, "a glass is any material that goes from a liquid to a solid without crystallizing." Metallic glass holds the promise of combining many of the desirable attributes of glass and metals.

It wasn't until the 1990s that metallic glass was produced in bulk. Although it offered the tantalizing promise of being stronger and tougher than any known material, two major hurdles have kept metallic glass from being widely adopted for useful applications:

  1. The size of metallic glass parts that can be produced
  2. The inherent brittleness that metallic glass
    exhibits

The size limitation for parts is related to the way metallic glass is made. First, the material is heated to its glass-transition phase of between 500 and 600 degrees Centigrade. At this point, the material softens into a liquid state that can be molded and shaped. The challenge is that, in this state, the atoms tend to automatically arrange themselves into crystals — this needs to be avoided to create an amorphous structure, which makes it strong. Common glasses can take hours to crystallize, offering ample time to form and shape the glass. However, metallic glass usually crystallizes almost instantly upon reaching the thick-liquid state.

To avoid this crystallization, the material needs to be heated quickly and uniformly throughout and then injected into a mold where it freezes. The larger the amount of material, the greater the challenge of achieving this quick heating. Consequently, the size of parts that could be produced has been limited.

But now, researchers at Caltech have developed a new technique that heats and processes metallic glass extremely quickly, allowing time for injection and freezing before crystallization.1 The technique, called ohmic heating, uses an intense pulse of electrical current that delivers energy of over 1,000 joules in about 1 millisecond...

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