Study Results Help Design Metallic Materials with Unmatched Damage Tolerance

A new alloy with cold-loving features is reported to be one of the toughest metallic alloys ever, according to a team of Berkeley Lab researchers.

The secrets of a new alloy’s amazing toughness is seen in this transmission electron microscopy movie that shows the formation of nano-sized bridges across a growing crack. These bridges inhibit the crack’s growth, and are one of several mechanisms identified by the scientists that give the alloy incredible toughness and strength. (Credit: Berkeley Lab)

The team has determined multiple mechanisms that make this new material extremely tough.

The alloy is composed of chromium, manganese, iron, cobalt and nickel, and so is referred to as CrMnFeCoNi by the team. At room temperature, this alloy is remarkably strong and tough, exhibiting properties such as incredibly high tensile strength, ductility, and resistance to fracture.

The interesting aspect of this particular alloy is that the colder it gets, the tougher and stronger it becomes, making it a potential material for use in cryogenic applications such as storage tanks for liquefied natural gas.

The Berkeley Lab-led team closely studied this material under strain using transmission electron microscopy to discover its secrets. The images obtained showed many nanoscale mechanisms sequentially activating in the alloy, which together were capable of resisting any damage from spreading.

These mechanisms include include bridges that develop over cracks, thereby hindering their spread. This kind of crack bridging is a well-known toughening mechanism seen in materials such as ceramics and composites, but which has not previously been observed in unreinforced metals.

This breakthrough research will hopefully become a base for future research into developing metallic materials with unparalleled damage tolerance. The findings of this research were published in the Nature Communications journal.

“We analyzed the alloy in earlier work and found spectacular properties: high toughness and strength, which are usually mutually exclusive in a material,” says Robert Ritchie, a scientist with Berkeley Lab’s Materials Sciences Division who led the research with Qian Yu of China’s Zhejiang University and several other scientists.

“So in this research, we used TEM to study the alloy at the nanoscale to see what’s going on,” says Ritchie.

A material’s resistance to deformation is known as its strength, and a material’s resistance to fracture is known as its toughness. It is very unusual for a material to possess both strength and toughness, so CrMnFeCoNi stands out. This type of alloy is known as a high-entropy alloy as it is mainly made up of an uncomplicated solid solution phase and displays a high entropy of mixing.

Back in 2014, Ritchie and his team discovered that when CrMnFeCoNi deforms at very cold temperatures, a process known as “twinning” occurs, where adjoining crystalline areas copy the formation of one another. It is likely that the material's strength and toughness is partly due to that this twinning, but other than in the crack bridges, twinning does not occur in the material at room temperature. However, the alloy’s strength and toughness is still unparalleled.

“If we don’t see twinning at room temperature, then what other mechanisms give the alloy these amazing properties?” asks Ritchie.

A series of straining experiments were conducted at room temperature on the alloy whilst it was observed using transmission electron microscopy. The time-lapse images revealed two occurrences involving shear stress: swift-moving partial dislocations capable of enhancing ductility; and slow-moving ideal dislocations, which add to the strength of the material.

Another occurrence that was noticed was related to partial dislocations called 'three-dimensional stacking fault defects', where alterations occurred in the 3D arrangement of atoms in a region.

These flaws were actually large barriers to dislocation, analogous to stacking bricks against a developing fissure, and this imparted unprecedented toughness to the alloy.

A nanoscale version of chewing a large toffee that sticks together the teeth was also captured in images, where tiny bridges deformed by the twinning phenomenon were produced across a crack, which helped stop the crack from getting any wider.

“These bridges are common in reinforced ceramics and composites,” says Ritchie. “Our research found that all of these nanoscale mechanisms work together to give the alloy its toughness and strength.”

The Department of Energy’s Office of Science partially funded this research.

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