Strong and flexible, HEA offers the ideal additive manufacturing combination

Scientists from the University of Massachusetts Amherst and the Georgia Institute of Technology have 3D printed a dual-phase, nanostructured, high-entropy alloy that exceeds the strength and ductility of other state-of-the-art additive manufactured materials and could be a game-changer for 3D printing.

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.

What makes this material unique?

Made up of five or more elements in near-equal proportions, high entropy alloys have grown in popularity over the last 15 years because they offer the 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 practically a new substance – something ideally suited for metal 3D printing.

3D printing, or additive manufacturing as it is often referred to, has grown explosively in popularity over the last few years, providing an untapped avenue for material development.

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.

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.

Because the process causes materials to melt and solidify very quickly as 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 known as 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, insofar, have no known positioning.

"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."

Generally, a common issue with additive manufacturing is the fragility and brittleness of the finished product. While a 3D printer can design and extrude material into nearly any computer-designed shape, the finished product often has low integrity due to the structure of the metal powders and the precision of printing – instead of the solid mass that comes with casting.

Thus, after a print has been completed, it is often baked or superheated to finalize the hardening. Despite this, microscopic imperfections can occur, leaving conventional metallurgy the top dog in reliability.

If HEAs become more mainstream, or rather, in the sense their chemistries become more viable, then the match made in materials heaven for metal 3D printers could be born.

"The ability to produce strong and ductile HEAs means that these 3D printed materials are more robust in resisting applied deformation, which is important for lightweight structural design for enhanced mechanical efficiency and energy saving," said Chen's Ph.D. student and first author of the paper, Jie Ren.

Zhu's group at Georgia Tech led the computational modeling for the research by developing a dual-phase crystal plasticity computational model to understand the mechanistic roles played by both the face-centered and body-centered layers and how they work together to give the material added strength and ductility.

"Our simulation results show the surprisingly high strength yet high hardening responses in the BCC nanolamellae, which are pivotal for achieving the outstanding strength-ductility synergy of our alloy," added Zhu. "This mechanistic understanding provides an important basis for guiding the future development of 3D printed HEAs with exceptional mechanical properties."

Because 3D printing offers a powerful tool to make geometrically complex and customized parts, companies around the world have begun attempting to develop unique blends of metals into their own proprietary powders.

However, if the full potential of metal 3D printers is to come to fruition, it appears that HEAs are the answer, a tailored near-infinite material for near-infinite designs.

In the future, harnessing 3D printing technology and the vast alloy design space of HEAs opens ample opportunities for the direct production of end-use components for biomedical and aerospace applications – but more than that, it could become the stuff of science fiction turned science reality.