Amorphous metals whose atomic arrangements resemble those of a liquid more than a crystal are often exceptionally strong when compared to their crystalline counterparts, but they are typically much less resistant to fatigue, greatly hampering their use. Now scientists find they can dramatically improve amorphous metals by engineering them so cracks move in zigzags instead of straight lines within them, report findings detailed in the Proceedings of the National Academy of Sciences.
Metals are often crystals, with atomic arrangements consisting of orderly rows of atoms. In contrast, in amorphous materials, the atoms are arranged randomly. One example of an amorphous material is the glass found in windows, which is more akin to an extremely viscous, immobilized liquid than a solid. Another is the liquid water that is the most dominant form of water in the universe, clumping from water vapor on cold surfaces such as interstellar dust into comets.
Since the atoms in amorphous materials are arranged in a disorderly manner, the defects that inevitably emerge in crystals are not present. As such, amorphous metals, also known as metallic glasses, often have superior mechanical and temperature properties compared with crystalline metals, leading researchers to suggest they could find use in everything from submarine hulls to skyscraper girders.
However, metallic glasses often possess a poor and inconsistent resistance to fatigue – that is, failure associated with cyclic loading. This severely limits their potential use as reliable engineering materials, since fatigue is the most common form of failure in structures.
The level of resistance a material has to fatigue is linked with how well its microscopic structural features can stop the spread of tiny cracks. The fact that metallic glasses lack these microstructures helps explain their weakness and inconsistency against fatigue.
Now materials scientist and mechanical engineer Robert Ritchie at Lawrence Berkeley National Laboratory in Berkeley, Calif., together with his colleagues at Caltech, have studied a palladium-based glass with unexpectedly high fatigue resistance. The secret lies in how cracks propagating in them move in a staircase-like manner.
The metallic glass in question has a high Poisson’s ratio – that is, when compressed along one axis, it expands along the other two perpendicular axes more than some other materials, much like how a jelly doughnut squirts outward when squeezed. This suggests zones where there is intense shearing strain known as shear bands form and proliferate when the metallic glass is deformed.
Any cracks that emerge in this metallic glass cannot move in straight lines because they get blocked by these networks of shear bands. Instead, cracks have to try and move in a staircase-like manner around these shear bands that blunts their progress. This gives the metallic glass fatigue resistance “comparable to many crystalline metals,” Ritchie says.
Ritchie cautioned it can take structural materials decades to see commercial application after they are first developed. “Also, we can only make this material in very small sizes at this point, so it’s premature to think about applications now,” he says. “Moreover, palladium is ferociously expensive – one would likely want a metallic glass made from a much cheaper material to commercialize.”