DOE national lab verifies stronger and more flexible 3D-printed metal.

As additive manufacturing continues to shift from hobbyist to mainstream, research has ramped up to delve deeper into its expansive applications. A significant focus lies in the materials, such as those being called high-entropy alloys, that unlock unprecedented compositions unattainable through conventional methods. This innovation being explored by scientists has now been examined to its very atomic foundation, and the results verify that this could be the next generation of metal fabrication.

Toward the end of 2022, researchers from the University of Massachusetts Amherst and the Georgia Institute of Technology announced that they had 3D printed a dual-phase, nanostructured, high-entropy alloy that exceeded the strength and ductility of other state-of-the-art additively manufactured materials.

Led by assistant professor of mechanical and industrial engineering Wen Chen at UMass and Ting Zhu, professor of mechanical engineering at Georgia Tech, the work published in the journal "Nature" describes an alloy that could lead to higher-performance components for aerospace, medicine, energy, and transportation, to name a few.

Alloy alchemy

Made up of five or more elements in near-equal proportions, high-entropy alloys have grown in popularity over the last 15 years due to their ability to create a near-infinite number of permutations for alloy design.

Traditional alloys, such as brass, stainless steel, carbon steel, and bronze, generally contain a primary element combined with one or more trace elements, but a high entropy alloy is a recipe of metals that bakes into a practically new substance – something ideally suited for metal 3D printing.

To capitalize on the potential of this metal "leaven," Chen and his team at the UMass Multiscale Materials and Manufacturing Laboratory combined a HEA with one of the more popular and widespread metal 3D printing techniques called laser powder bed fusion.

Laser-based 3D printing alone produces large temperature gradients and high cooling rates that are not readily accessible by conventional routes; however, "the potential of harnessing the combined benefits of additive manufacturing and HEAs for achieving novel properties remains largely unexplored," said Zhu.

Because the process causes materials to melt and solidify very quickly compared to conventional metallurgy, "you get a very different microstructure that is far-from-equilibrium" on the components created, said Chen.

This microstructure looks like a net and is made of alternating layers called face-centered cubic and body-centered cubic nanolamellar structures embedded in microscale eutectic colonies with random orientations – basically alternating microscopic layers of a cocktail of elements that originally had random orientations.

"This unusual microstructure's atomic rearrangement gives rise to ultrahigh strength as well as enhanced ductility, which is uncommon, because usually strong materials tend to be brittle," Chen added.

Compared to conventional metal casting, "we got almost triple the strength and not only didn't lose ductility but actually increased it simultaneously," he continued. "For many applications, a combination of strength and ductility is key. Our findings are original and exciting for materials science and engineering alike."

Now, after some advanced atomic microscopy, some of the answers have been laid bare.

The right tools for the job

Neutron data from the Spallation Neutron Source at the Department of Energy's Oak Ridge National Laboratory enabled the researchers to examine the interior mechanical load sharing of the HEA samples while under strain.

Using an atom probe instrument from the Center for Nanophase Materials Sciences at ORNL to capture detailed 3D images of the compositions and microstructures, the researchers could begin mapping out the alternating nanolamellar layers.

The laser-based AM process produced nanometer-thick nanolamellae (multiple thin layers of plates), offering high strength, while the plates' distinct edges permitted a greater degree of slippage or ductility.

Consisting of alternating layers of face-centered cubic (FCC) crystal structures that average approximately 150 nanometers thick and body-centered cubic (BCC) crystal structures that average roughly 65 nanometers thick, the resulting HEAs exhibited high yield strengths of around 1.3 gigapascals – which exceed the strongest titanium alloys.

Further, these samples offered an elongation – a measure of how much bending a material can withstand before breaking – of about 14%, which is higher than other additively manufactured metal alloys given the same yield strength.

The phases of different annealed samples were also studied using X-ray diffraction at the Advanced Photon Source, one more DOE Office of Science user facility, but at Argonne National Laboratory.

With some of the nation's top facilities examining this material, and some of the U.S. military beginning to implement metal 3D printers within its strategies, as well as artificial intelligence flooding material sciences with tens of thousands of new compositional chemistries, it truly seems like it won't be long until metals like aluminum, iron, and titanium will be elevated to the realms of Vibranium.