When engineers want to make a metal stronger, one of the most reliable strategies is to use smaller grains – the microscopic crystal regions within the material. But when deformed at extreme speeds, this rule flips and metals with very small grains actually become softer, new Cornell research reveals.

The surprising discovery, published Jan. 9 in the journal Physical Review Letters, is contrary to the so-called Hall-Petch effect, which for more than 70 years has held that smaller grains mean stronger metals.

“We wanted to test the limits of that rule and see whether grain boundary strengthening still holds when metals are pushed into truly extreme deformation rates,” said co-lead author Mostafa Hassani, assistant professor in the Department of Materials Science and Engineering and in the Sibley School of Mechanical and Aerospace Engineering. “What we thought was going to be a straightforward confirmation experiment turned out to be something completely different with unexpected results.”

To probe how metals behave under extreme, ultra-fast deformation, Hassani’s group used laser-induced microprojectile impact testing, which targets metals with microscopic particles at velocities that exceed the speed of sound.

“It had been difficult to study these ultra-high strain rates until recent technical developments enabled us to carry out these experiments,” said doctoral student Laura Wu, who co-led the research. “These tests are uncovering new understandings of how, exactly, materials can behave.”

Wu prepared copper samples with grain sizes of 1 to 100 micrometers, all within the range where the Hall-Petch effect normally applies. In impact tests, larger-grained samples consistently exhibited shallower indentations and dissipated more kinetic energy, clear signs of greater hardness in the copper. Because the result defied decades of understanding, Wu and Hassani initially questioned the data.

“We double-checked all our data collection,” Wu said. “We added new data points and repeated experiments, but the results held every time.”

The researchers attribute the results to how tiny defects, known as dislocations, move when a metal deforms. At ordinary strain rates, grain boundaries and other crystal defects strengthen a metal by blocking the motion of these dislocations. But at ultra-high strain rates, dislocations accelerate fast enough to start interacting with the material’s vibrating atoms. This interaction, called dislocation–phonon drag, can significantly strengthen the metal.

Although the study focused on copper, Hassani said the behavior is universal. His group has begun testing other metals and alloys, and the same trend appears: At extremely high deformation rates, the strengthening effect from dislocation-phonon drag can be greatly reduced and even eliminated in smaller grains.

“For me, the exciting part is both the fundamental discovery and the potential applications,” Wu said. “Now that we know the grain-size trend reverses at a high-strain rate, we can use this to build and improve things that withstand high impacts.”

Those insights could inform the design of materials for applications like lightweight armor, spacecraft that survive collisions with space debris, and additive manufacturing of metal components. 

The research was supported by the National Science Foundation and the Army Research Office.

Syl Kacapyr is associate director of marketing and communications for Cornell Engineering.