New steel alloy can withstand impact without deforming

A team of engineers from the University of California, San Diego, the University of South California and the California Institute for Technology have developed and tested a new type of steel that is able to withstand significant impact without deforming permanently.

Transmission electron microscopy image showing different levels of crystallinity embedded in the amorphous matrix of the alloy. Jacobs School of Engineering/UC San Diego

This record-breaking material, called SAM2X5-630, is an amorphous steel alloy with a wide range of applications from drill bits, to body armor for soldiers, to meteor-resistant casings for satellites.

The new material is affordable to manufacture, incredibly hard, but at the same time, not brittle. SAM2X5-630 is 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.

SAM2X5-630 has the highest recorded elastic limit for any steel alloy, according to the researchers–essentially the highest threshold at which the material can withstand an impact without deforming permanently. The alloy can withstand pressure and stress of up to 12.5 giga-Pascals or about 125,000 atmospheres without undergoing permanent deformations.

“Because these materials are designed to withstand extreme conditions, you can process them under extreme conditions successfully,” said Olivia Graeve, a professor of mechanical engineering at the Jacobs School of Engineering at UC San Diego, who led the design and fabrication effort. Veronica Eliasson, an assistant professor at USC, led the testing efforts.

The team mixed metal powders in a graphite mold in order to make the solid material of the alloy. The powders were then pressurized at 100 mega-Pascals, or 1000 atmospheres, and exposed to a powerful current of 10,000 Ampers at 1165°F (630°C) during a process called spark plasma sintering.

The process created small crystalline regions that are only a few nanometers in size, with hints of structure, which researchers believe are key to the material’s ability to withstand stress.

Researchers at USC tested how the alloy responds to shock without undergoing permanent deformations by hitting samples of the material with copper plates fired from a gas gun at 500 to 1300 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.

“The fact that the new materials performed so well under shock loading was very encouraging and should lead to plenty of future research opportunities,” said Eliasson.

www.jacobsschool.ucsd.edu