Amorphous Steel Alloy Is Incredibly Impact Resistant

A team of engineers from the University of California, San Diego (USCD) and the University of Southern California (USC) has developed and tested a material with the ability to withstand the highest impact, without deforming permanently. The new alloy could be used in a wide range of applications, from drill bits to body armor for soldiers, to meteor-resistant casings for satellites.

The material, SAM2X5-630, is an amorphous steel alloy, a subclass of steel alloys made of arrangements of atoms that deviate from steel’s classical crystal-like structure, in which iron atoms occupy specific locations. The researchers believe their work on the alloy is the first to investigate how amorphous steels respond to shock.

Testing showed the alloy can withstand pressure and stress of up to 12.5 giga-Pascals, or about 125,000 atmospheres, without undergoing permanent deformation—the highest recorded elastic limit recorded for a steel alloy.

To make the solid materials that comprise the alloy, a team led by Olivia Graeve, UCSD professor of mechanical engineering, mixed metal powders in a graphite mold. The powders were then pressurized at 100 mega-Pascals, or 1,000 atmospheres, and exposed to a current of 10,000 Amperes at 630 degrees Celsius, in a process known as "spark plasma sintering."

The spark plasma sintering 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,” Graeve says.

The process created small crystalline regions that are only a few nanometers in size, with hints of structure, which the 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, the researchers say.

To test how the alloy responds to shock without undergoing permanent deformation, a team led by Veronica Eliasson, USC assistant professor of aerospace and mechanical engineering, hit samples of the material with copper plates fired from a gas gun at 500 to 1,300 meters per second. The material did deform on impact, but not permanently.

The Hugoniot Elastic Limit—the maximum shock a material can take without irreversibly deforming—of a 1.5-1.8 mm-thick piece of SAM2X5-630 was measured at 11.76 ± 1.26 giga-Pascals. By comparison, stainless steel has an elastic limit of 0.2 giga-Pascals, while that of tungsten carbide, a high-strength ceramic used in military armor, is 4.5 giga-Pascals.

SAM2X5-630 doesn't have the highest elastic limit of any material known—diamonds top out at 60 giga-Pascals—but diamonds are not practical for many real-world applications.

“The fact that the new materials performed so well under shock loading was very encouraging and should lead to plenty of future research opportunities,” says Eliasson. The researchers say the primary focus of future research on these alloys will be increasing the weight of the materials to make them even more resistant to impact.