Engineers Made One of Metallurgy's Most Brittle Compounds Bend — at 10 Times the Strength of Structural Steel

There is a class of metals engineers have wanted to use in jet engines for decades and mostly cannot. Intermetallics — compounds of two or more metals locked into a highly ordered crystal structure — are enormously strong, melt at very high temperatures, and resist creep, the slow sag a material undergoes when it sits under heat and load for years. They are also, at room temperature, about as forgiving as glass. Bend one and it cracks.

Engineers at Purdue University report in Science Advances that they got one of these compounds, cobalt aluminum, to do both things at once: reach a yield strength of 6 gigapascals — roughly six to ten times the yield strength of high-strength structural steel — while still deforming substantially instead of fracturing.

The trick was to stop casting the material. Conventional casting cools a melt into a crystal, and the resulting structure has very few dislocations — microscopic irregularities where atoms are knocked out of their perfect rows. The word "defect" is misleading here. Dislocations are what let a metal deform under extreme force rather than snapping, and CoAl at room temperature simply does not have enough of them. Earlier attempts to fix its brittleness by tweaking composition or microstructure never generated dislocations in high enough density to matter.

So the Purdue group grew the material from vapor instead, using sputtering deposition, which builds a solid atom by atom out of an alloy vapor and jams in far more dislocations than casting ever permits. On top of that, they engineered what they call a framework of amorphous interfaces — internal boundaries that start out glassy and disordered rather than sharing the crystal structure around them. Under load, parts of those boundaries crystallize and start pumping out new dislocations into the surrounding CoAl.

"This nonequilibrium fabrication approach enables us to fabricate materials from alloy vapor to a solid, introducing a significant number of dislocations in CoAl," said Xinghang Zhang, a professor in Purdue's School of Materials Engineering and the paper's corresponding author. "We were able to achieve significant strength and plasticity in CoAl, which can't be realized via traditional casting."

The team watched it happen. They ran mechanical tests inside a scanning electron microscope, tracking the material's behavior with micrometer precision as it deformed. Collaborators at the University of Houston — professor Yashashree Kulkarni and PhD student Anand Mathew — ran molecular dynamics simulations that showed the amorphous interfaces crystallizing during deformation and dislocations migrating out of those boundaries into the CoAl layers, which is the mechanism that keeps the material from fracturing early.

The immediate application Zhang points to is turbine blades. "High-strength, plastically deformable CoAl alloys could allow an engine or turbo to spin faster while sustaining higher centrifugal force, improving their performance," he said. A blade material that can be shaped without cracking is also cheaper to manufacture into the complex geometries modern engines require.

The result so far is in layered thin films, not a bulk part. The group's next step, led by Zhang's Nanometal Group, is to scale the concept to bulk CoAl nanocomposites and then test whether the same amorphous-interface approach works on other intermetallics. "We will also be testing the concept using other intermetallics, with the goal of establishing the general applicability of FAIs for improving plasticity in this metal class," said Ke Xu, the postdoctoral researcher who is the paper's first author. The work was funded primarily by the National Science Foundation's Metals and Metallic Nanostructures program.