Glass Stronger and Tougher than Steel

A new damage-tolerant metallic glass, demonstrating a strength and toughness beyond that of any known material developed and tested by a collaboration of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the California Institute of Technology. In the world of materials, strength (the amount of force a substance can withstand) and toughness (its capacity to resist fracturing) are not merely different attributes; they're very difficult to achieve together. Now researchers from Caltech and the Department of Energy’s Lawrence Berkeley National Laboratory have created a form of glass that has both qualities. It's stronger and tougher than steel or, indeed, any other known material. The material features palladium, a metal whose possible use in glasses was recognized 45 years ago.

The new metallic glass is a micro alloy featuring palladium, a metal with a high "bulk-to-shear" stiffness ratio that counteracts the intrinsic brittleness of glassy materials. Because of the high bulk-to-shear modulus ratio of palladium-containing material, the energy needed to form shear bands is much lower than the energy required to turn these shear bands into cracks. The result is that glass undergoes extensive plasticity in response to stress, allowing it to bend rather than crack.

Glassy materials have a non-crystalline, amorphous structure that makes them inherently strong but invariably brittle. Whereas the crystalline structure of metals can provide micro structural obstacles (inclusions, grain boundaries, etc.,) that inhibit cracks from propagating, there's nothing in the amorphous structure of a glass to stop crack propagation. The problem is especially acute in metallic glasses, where single shear bands can form and extend throughout the material leading to catastrophic failures at vanishingly small strains.

In earlier work, the Berkeley-Cal Tech collaboration fabricated a metallic glass, dubbed "DH3," in which the propagation of cracks was blocked by the introduction of a second, crystalline phase of the metal. This crystalline phase, which took the form of dendritic patterns permeating the amorphous structure of the glass, erected micro structural barriers to prevent an opened crack from spreading. In this new work, the collaboration has produced a pure glass material whose unique chemical composition acts to promote extensive plasticity through the formation of multiple shear bands before the bands turn into cracks.

The addition of the palladium provides this amorphous material with an unusual capacity for extensive plastic shielding ahead of an opening crack. This promotes fracture toughness comparable to those of the toughest materials known. The rare combination of toughness and strength, or damage tolerance, extends beyond the benchmark ranges established by the toughest and strongest materials known.

Since the alloy is titanium and zirconium instead of steel, it weighs significantly less than steel. It also melts at much lower temperatures than steel, making it easier and faster to produce.

This is a major accomplishment," said William Nix of Stanford University. One big advantage of the new glass is that it can be cast right to shape. Just pour it into a mold, let it freeze, and you have properties that are comparable to high-strength steel right from the get-go.

A glass this strong has endless potential in the way of structural application -- think cars, planes, and bridges. Thing is, though, palladium is super expensive, and researchers involved in the project say the best applications are in products like dental implants, which are currently made of soft, stiff noble metals, more likely to cause complications like bone atrophy. Chances are we won't see super strong glass bridges anytime soon, but the new glass dental implants could be in your mouth as early as 2016.