Developing Elastic Metallic Glass
USC scientists have developed a material that is incredibly hard but also elastic, making it potentially useful for applications ranging from drill bits and body armor to meteor-resistant satellite casints.
Engineers have created a new material with an unusual chemical structure that makes it incredibly hard yet elastic. The material can withstand heavy impacts without deforming; even when pushed beyond its elastic limits, it doesn’t fracture, but instead retains most of its original strength. These properties make it potentially useful in a variety of applications ranging from drill bits and body armor for soldiers to meteor-resistant satellite casings.
In the journal Nature Scientific Reports, researchers from USC, the University of California-San Diego, and the California Institute of Technology announced the creation of the material, which was produced by heating a powdered iron composite up to exactly 630°C (1,166°F) and then rapidly cooling it. “In particular, 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 Veronica Eliasson, lead author of the paper and assistant professor at the USC Viterbi School of Engineering.
Bulk Metallic Glass Explained
Dubbed SAM2X5-630, the new material reportedly has the highest impact resistance of any “bulk metallic glass,” which is a class of artificially generated materials first discovered in the 1960s that possess disproportionate strength, resilience and elasticity due to their unusual chemical structure. Typical metals and metal alloys have an organized, crystalline structure at the atomic level. Bulk metal glasses (BMGs) are formed when metal and metal alloys are subjected to extreme heat and then rapidly cooled—exciting their atoms into disorganized arrangements and then freezing them there. In general, BMGs tend to be strong, resist scratching, tough to fracture and highly elastic. One commercially available zirconium-based BMG is pound-for-pound twice as strong as titanium.
“Thanks to Professor Eliasson’s expertise in shocks and high-velocity testing, we were able to observe the remarkable elastic limit of this material at high velocities, a behavior which was not evident from conventional mechanical tests,” said Andrea Hodge, principle investigator of the project and professor at USC Viterbi.
What makes SAM2X5-630 special is that it’s not entirely a glass—just mostly. For some reason, the exact timing and temperatures used to create it leave just a few hints of structure here and there, which seems to be the key to its unique nature. The exact same iron composite heated and cooled slightly differently yields a completely random atomic arrangement that lacks the impressive elastic properties.
“It has almost no internal structure, like glass, but you see tiny regions of crystallization,” Eliasson said. “We have no idea why a small amount of crystalline regions in these bulk metallic glasses makes such a big difference under shock loading.”
Superheat, But Don’t Liquefy
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 GPa. For reference, stainless steel has an elastic limit of 0.2 GPa, while that of tungsten carbide (a high-strength ceramic used in military armor) is 4.5 GPa.
The material was produced at UC San Diego using a spark-plasma sintering process in which the iron compound is powdered, placed in a dye and then zapped with a current—superheating it to the point of binding without ever liquefying it. Eliasson and Hodge collaborated with former doctoral student Gauri Khanolkar of USC, Michael Rauls of Caltech, and James Kelly and Olivia Graeve of the Jacobs School of Engineering at UC San Diego. The research was supported by the Defense Threat Reduction Agency (grant HDTRA1-11-1-0067).
For additional information, visit www.usc.edu.