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Robert Ritchie   (Source: Roy Kaltschmidt, Berkeley Lab Public Affairs)
This damage-tolerant metallic glass is made of microalloys of palladium with phosphorous, germanium, silicon and silver

Researchers at the California Institute of Technology and the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory have combined efforts to create a glass that is said to be the strongest out of any known material. 

Robert Ritchie, study leader and a material scientist from the Lawrence Berkeley National Laboratory, along with co-authors Marios Demetriou, Glenn Garrett, Joseph Schramm, Maximilien Launey, Douglas Hofmann, and William Johnson of the California Institute of Technology, have developed a glass that is tougher and stronger than steel.

Glassy materials have a non-crystalline, amorphous structure, which makes them strong yet brittle. This type of structure allows cracks and strains to spread throughout the material because the glass' amorphous structure cannot stop crack propagation. To remedy this, the Lawrence Berkeley/California Tech team designed what they called "DH3," which was a fabricated metallic glass capable of preventing cracks from spreading through the use of a second crystalline phase of the metal. While this method prevented the spread of cracks, these researchers wanted to take this research to the next level and make it stronger. 

The Lawrence Berkeley/California Tech team came together once again to develop this new glass, which is a damage-tolerant metallic glass made of microalloys of palladium, which has a high bulk-to-shear stiffness ratio. The difference between this new glass and DH3 is that their new creation also promotes extensive plasticity "through the formation of multiple shear bands before the bands turn into cracks." 

"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," said Ritchie. "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."

Ritchie added that it is important to make a metallic glass with at least five elements in order to quench the material, which means to cool the material. The first samples of the metallic glass consisted of microalloys of palladium with phosphorous, germanium, silicon and silver. This increases the thickness of the glass rods, but the size is limited to its need to rapidly quench the liquid metals for its final structure. When it comes time to quench the liquid metals, it doesn't know which crystal structure to form, so it automatically forms an amorphous structure. 

"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," said Ritchie. "The addition of the palladium provides our amorphous material with an unusual capacity for extensive plastic shielding ahead of an open 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." 

This study was published in Nature Materials.



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RE: iPhone 5!
By Fenixgoon on 1/12/2011 9:26:46 PM , Rating: 3
To refine your answer a little more - stress basically tests the strength of the atomic bonds.

The reason people measure axial strength is because most structures are only loaded in 1 direction. With stress being a tensor, you can apply up to 6 different stresses (but you can always simplify it to 3). We have a machine at work that can do biaxial loading.

Toughness is the material's resistance to incremental crack growth (where the crack presents an extremely large stress concentration).

You can have steel that is tough but not strong (316), strong but not tough (tool steel), or both strong and tough (aircraft grade steel).

Steels (and metals) in general possess both strength and toughness because of their ability to dissipate energy via plasticity (plastic deformation - permanent shape change). Grain boundaries are crystal discontinuities yes, but by that logic intergranular fracture (which is a brittle mode in most cases) should give very high toughness, but it doesn't. Plasticity is the reason for high toughness in metals (often times discontinuities - such as inclusions - raise the stress locally and act as crack initiation sites)

Plasticity in glasses and ceramics is extremely limited in most cases (some people have observed plasticity when the strain rate is very high). Glasses and ceramics can be very strong - usually in compression, because cracks under compression grow stably (tensile stresses will open a crack whereas compressive ones close it). This is why when you pull a ceramic in tension, you get a very low strength value, but compression will give something much higher (tension opens cracks, compression closes them)

All the "glass" in "metallic glass" means is that the solidification rate was fast enough (~10^6 C/s) that no crystalline structure was able to form. If this glass has a toughness closer to that of a typical metal, then the hypothetical iPhone would still dent - not shatter.

For reference, typical toughness values for ceramics are generally < 20, while steels can range anywhere from ceramic-like (tool steels) to above 200 for low-strength/high-toughness steels


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