Steel alloy shows record-breaking resistance to shock

Transmission electron microscopy image showing different levels of crystallinity embedded in the amorphous matrix of the alloy
A team of engineers from the University of California, San Diego, the University of Southern California and the California Institute of Technology claims to have developed and tested a type of steel with a record-breaking ability to withstand an impact without deforming permanently. The new steel alloy could be used in applications from drill bits, to body armour for soldiers, to meteor-resistant casings for satellites.

The material is said to be an amorphous steel alloy, a subclass of steel alloys made of arrangements of atoms that deviate from steel's classical crystal-like structure, where iron atoms occupy specific locations.

Researchers are increasingly looking to amorphous steel as a source of new materials that are affordable to manufacture, incredibly hard, but at the same time, not brittle. The researchers believe their work on the steel alloy, named SAM2X5-630, is the first to investigate how amorphous steels respond to shock.

According to the researchers, SAM2X5-630 has the highest recorded elastic limit for any steel alloy: able to withstand pressure and stress of up to 12.5GPa (giga-Pascals) or about 125,000 atmospheres without undergoing permanent deformations.

By comparison, stainless steel has an elastic limit of 0.2GPa, while that of tungsten carbide (a high-strength ceramic used in military armour) is 4.5GPa.

"Because these materials are designed to withstand extreme conditions, you can process them under extreme conditions successfully," said Olivia Graeve, professor of mechanical engineering at UC San Diego, who led the design and fabrication effort.

To make the solid materials that comprise the alloy, Prof Graeve’s team mixed metal powders in a graphite mould. The powders were then pressurized at 100MPa (mega-Pascals), or 1000 atmospheres, and exposed to a current of 10,000Ampers at 630°C during a process called spark plasma sintering.

According to Prof Graeve, this technique allows for enormous time and energy savings. "You can produce materials that normally take hours in an industrial setting in just a few minutes," she said.

The process created small crystalline regions a few nanometers in size, with hints of structure, which researchers believe are key to the material's ability to withstand stress. This finding is promising because it shows that the properties of these types of metallic glasses can be fine-tuned to overcome shortcomings such as brittleness, which have prevented them from becoming commercially applicable on a large scale.

Researchers at USC tested how SAM2X5-630 responds to shock by hitting samples of the material with copper plates fired from a gas gun at 500 to 1300m/s. The primary focus of future research efforts on these alloys is increasing the weight of the materials to make them more resistant to impacts.

Author
Tom Austin-Morgan

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