A dual-phase, nanostructured high-entropy alloy that has been 3D printed by researchers from the University of Massachusetts Amherst and the Georgia Institute of Technology is stronger and more ductile than other cutting-edge additively manufactured materials. This discovery could lead to higher-performance components for use in aerospace, medicine, energy, and transportation.

The research was published online by the journal Nature and was headed by Wen Chen, an assistant professor of mechanical and industrial engineering at UMass, and Ting Zhu, a professor of mechanical engineering at Georgia Tech.

High entropy alloys (HEAs), as they are called, have gained popularity as a new paradigm in materials science over the past 15 years. They allow for the creation of a nearly limitless number of different alloy designs since they include five or more elements in nearly equal amounts. Brass, carbon steel, stainless steel, and bronze are examples of traditional alloys that mix a principal element with one or more trace elements.

A cross-sectional electron backscatter diffraction inverse-pole figure map displaying a randomly oriented nanolamella microstructure and images of 3D printed high-entropy alloy components are displayed in front of Wen Chen, an assistant professor of mechanical and industrial engineering at UMass Amherst (right).

Additive manufacturing, also called 3D printing, has recently emerged as a powerful approach to material development. The laser-based 3D printing can produce 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,” says Zhu. 

A HEA was combined with a cutting-edge 3D printing method called laser powder bed fusion by Chen and his team at the Multiscale Materials and Manufacturing Laboratory to create novel materials with unheard-of qualities. When compared to traditional metallurgy, the procedure causes materials to melt and solidify much more quickly, which results in "a very different microstructure that is far-from-equilibrium" on the components produced, claims Chen.

This microstructure, which resembles a net, is composed of alternating layers of face-centered cubic (FCC) and body-centered cubic (BCC) nanolamellar structures that are encased in small, randomly oriented eutectic colonies. The two phases can cooperatively deform thanks to the hierarchical nanostructured HEA.

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

“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,” says Jie Ren, Chen’s Ph.D. student and first author of the paper.  

The computational modelling for the study was coordinated by Zhu's team at Georgia Tech. To comprehend the mechanical roles played by both the FCC and BCC nanolamellae and how they cooperate to provide the material more strength and ductility, he constructed dual-phase crystal plasticity computer models.

“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. This mechanistic understanding provides an important basis for guiding the future development of 3D printed HEAs with exceptional mechanical properties,” Zhu says.  

Additionally, 3D printing provides a strong tool for producing parts with intricate geometries. Future direct manufacture of end-use components for biomedical and aerospace applications will have plenty of options thanks to the combination of 3D printing technology and the enormous alloy design space of HEAs.

You can view the entire study for yourself in the journal Nature.

"Additive manufacturing produces net-shaped components layer by layer for engineering applications1,2,3,4,5,6,7. The additive manufacture of metal alloys by laser powder bed fusion (L-PBF) involves large temperature gradients and rapid cooling2,6, which enables microstructural refinement at the nanoscale to achieve high strength. However, high-strength nanostructured alloys produced by laser additive manufacturing often have limited ductility3. Here we use L-PBF to print dual-phase nanolamellar high-entropy alloys (HEAs) of AlCoCrFeNi2.1 that exhibit a combination of a high yield strength of about 1.3 gigapascals and a large uniform elongation of about 14 per cent, which surpasses those of other state-of-the-art additively manufactured metal alloys. The high yield strength stems from the strong strengthening effects of the dual-phase structures that consist of alternating face-centred cubic and body-centred cubic nanolamellae; the body-centred cubic nanolamellae exhibit higher strengths and higher hardening rates than the face-centred cubic nanolamellae. The large tensile ductility arises owing to the high work-hardening capability of the as-printed hierarchical microstructures in the form of dual-phase nanolamellae embedded in microscale eutectic colonies, which have nearly random orientations to promote isotropic mechanical properties. The mechanistic insights into the deformation behaviour of additively manufactured HEAs have broad implications for the development of hierarchical, dual- and multi-phase, nanostructured alloys with exceptional mechanical properties."

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