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Micrograph of deformed notch in palladium-based metallic glass shows extensive plastic shielding of an initially sharp crack. Inset is a magnified view of a shear offset (arrow) developed during plastic sliding before the crack opened. (Image courtesy of Ritchie and Demetriou)
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Glass stronger and tougher than steel? A new type of damage-tolerant
metallic glass, demonstrating a strength and toughness beyond that of any known
material, has been developed and tested by a collaboration of researchers with
the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the
California Institute of Technology. What’s more, even better versions of this
new glass may be on the way.
“These results mark the first use of a new strategy for metallic glass
fabrication and we believe we can use it to make glass that will be even
stronger and more tough,” says Robert Ritchie, a materials scientist who led
the Berkeley
contribution to the research.
The new metallic glass is a microalloy 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,” Ritchie says. “The result is
that glass undergoes extensive plasticity in response to stress, allowing it to
bend rather than crack.”
Ritchie, who holds joint appointments with Berkeley Lab’s Materials Sciences
Division and the Univ. of California (UC) Berkeley’s
Materials Science and Engineering Department, is one of the co-authors of a
paper describing this research published in the journal Nature Materials under
the title “A Damage-Tolerant Glass.”
Co-authoring the Nature Materials paper were Marios Demetriou (who
actually made the new glass), Maximilien Launey, Glenn Garrett, Joseph Schramm,
Douglas Hofmann and William Johnson of Cal Tech, one of the pioneers in the
field of metallic glass fabrication.
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Robert Ritchie holds joint appointments with Berkeley Lab and UC Berkeley (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
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Glassy materials have a non-crystalline, amorphous structure that make them
inherently strong but invariably brittle. Whereas the crystalline structure of
metals can provide microstructural 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 microstructural 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.
“Our game now is to try and extend this approach of inducing extensive
plasticity prior to fracture to other metallic glasses through changes in
composition,” Ritchie says. “The addition of the palladium provides our
amorphous material with an unusual capacity for extensive plastic shielding
ahead of an opening crack. This promotes a 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.”
The initial samples of the new metallic glass were microalloys of palladium
with phosphorous, silicon and germanium that yielded glass rods approximately
one millimeter in diameter. Adding silver to the mix enabled the Cal Tech
researchers to expand the thickness of the glass rods to six millimeters. The
size of the metallic glass is limited by the need to rapidly cool or “quench”
the liquid metals for the final amorphous structure.
“The rule of thumb is that to make a metallic glass we need to have at least
five elements so that when we quench the material, it doesn’t know what crystal
structure to form and defaults to amorphous,” Ritchie says.
The new metallic glass was fabricated by co-author Demetriou at Cal Tech in
the laboratory of co-author Johnson. Characterization and testing was done at
Berkeley Lab by Ritchie’s group.
“Traditionally strength and toughness have been mutually exclusive
properties in materials, which makes these new metallic glasses so
intellectually exciting,” Ritchie says. “We’re bucking the trend here and
pushing the envelope of the damage tolerance that’s accessible to a structural
metal.”
The characterization and testing research at Berkeley Lab was funded by
DOE’s Office of Science. The fabrication work at Cal Tech was funded by the
National Science Foundation.
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