Ashby map of the damage tolerance of materials. Arrow indicates the combination of toughness and strength potentially accessible to metallic glasses extends beyond the traditional limiting ranges towards levels previously inaccessible to any material. Filled star: data for new metallic glass. X: data for other metallic glasses (three Fe-based glasses, two Zr-based glasses a Ti-based glass and a Pt-based glass). O: data for ductile-phase-reinforced metallic glasses. Yield-strength data shown for oxide glasses and ceramics represent ideal limits. (Credit: Nature Materials/Robert Ritchie.)
If you do a Google search (admittedly not very scientific) for “world’s strongest material” and “world’s toughest material,” among the results graphene and spider silk tend to rank the highest, respectively. If you expand your efforts to search for materials that are both strong and exhibit fracture toughness, you start finding a variety mentioned, including silicon carbide (and several other engineering ceramics), Ni/Ti alloys (and other engineering metals) and metallic glasses.
Engineering ceramics are hard to beat on the strength scale. They are scratch resistant and difficult to bend. However, they suffer from a tendency to brittleness. Some engineering metals tend to have higher numbers for combination of both strength and fracture toughness than engineering ceramics. However, even these metals’ toughness come with a price: a tendency to malleability.
But now a group of researchers from Caltech, Lawrence Berkeley National Lab and University of California, Berkeley report in a new paper in Nature Materials that they have found a new composition for a highly damage-tolerant glass — a metallic glass — that is tougher and stronger than Ni and Ti alloys – that apparently make it the toughest, strongest material ever made. Furthermore, their insights suggest even stronger, tougher materials aren’t too far down the road.
The search for materials that suppress fractures while maintaining high strength can be difficult because these two properties are, generally speaking, mutually exclusive. Fracture-tough crystalline materials resist crack expansions because of plastic shielding (think of tiny areas of “shear” bands of the material sliding by each other) ahead of the crack. But, because this shielding doesn’t require much energy, its easy for it to occur, thus decreasing its overall strength.
Conversely, noncrystalline (i.e., amorphous or glass) materials tend to strongly resist the development of openings, but lack the mobile microstructures to employ plastic shielding. So, once an opening does develop, a crack can readily expand to the point of failure (brittleness).
As a result, scientists and engineers are faced with the dilemma of the trade-off between strength and toughness. Previously, metallic glass compositions had been investigated and found to have shear band-forming properties, but under strain a single shear band would often form and grow extensively, resulting in major material failure.
That’s what is so important about this group’s work: They achieve higher strength and higher toughness, successfully dodging the trade-off, by using a composition and process that creates a glass capable of multiple microscale shear bands when subjected to stress.
The important component of this new material is palladium. Researchers leverage the high bulk-to-shear stiffness ratio of palladium. ACerS Fellow Rob Ritchie, a professor at the university and materials scientist at LBL, explains, “Because of the high bulk-to-shear modulus ratio of the palladium containing material, the energy required 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 acknowledges that finding this combination is a little counterintuitive. “You know,” he says, “who would have really imagined that we’d find these rare combinations of properties in a glass? It’s neither the strongest material nor the toughest material, but its the combination of toughness and strength, or damage tolerance, that’s key and that goes beyond the benchmark ranges established by the toughest and strongest materials known until now.”
Besides palladium, the glass contains phosphorous, silicon, germanium and silver (Pd79Ag3.5P6Si9.5Ge2). Marios Demetriou, one of the paper’s coauthors, has been able to make rods of the glass with diameters of six millimeters.
The researchers think this is only the beginning. Ritchie tells me that they know “this is a compositional thing, not a structural thing. We are looking at a lot of other compositions. We are fairly certain we didn’t just get lucky with our first composition. We are sure extensive plasticity can be induced in other metallic glasses and that even higher levels of damage resistance are accessible.”
But, don’t look for these materials in applications anytime in the near future. “Right now, these glasses are phenomenally expensive to make, and even so, we can only make them in small quantities. This is going to be an immature field for quite some time, and, as with a lot of potential structural materials, they will probably take decades to perfect and mature.”
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. (Credit: Nature Materials/Maximilien Launey.)
Nature has a video of the shear bands/plastic shielding here.