3D printed, nanostructured alloy is both strong and ductile

A US research team, led by the University of Massachusetts Amherst and the Georgia Institute of Technology, has 3D printed a dual-phase, nanostructured high-entropy alloy that is said to exceed the strength and ductility of other additively manufactured materials, which could lead to higher-performance components for applications in aerospace, medicine, energy and transportation. Their work has been published in the journal Nature.

Over the past 15 years, high-entropy alloys (HEAs) have become increasingly popular as a new paradigm in materials science. Comprising five or more elements in near-equal proportions, they offer the ability to create a near-infinite number of combinations for alloy design. Traditional alloys, such as brass, carbon steel, stainless steel and bronze, contain a primary element combined with one or more trace elements.

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, according to Georgia Tech Professor Ting Zhu, “the potential of harnessing the combined benefits of additive manufacturing and HEAs for achieving novel properties remains largely unexplored”.

Researchers at UMass’s Multiscale Materials and Manufacturing Laboratory, led by Assistant Professor Wen Chen, combined an HEA with a 3D printing technique called laser powder bed fusion to develop new materials with apparently unprecedented properties. Because the process causes materials to melt and solidify very rapidly as compared to traditional metallurgy, “you get a very different microstructure that is far from equilibrium” on the components created, Chen said. This microstructure looks like a net and is made of alternating layers known as face-centred cubic (FCC) and body-centred cubic (BCC) nanolamellar structures embedded in microscale eutectic colonies with random orientations. The hierarchical nanostructured HEA enables cooperative deformation of the two phases.

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

“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,” added Jie Ren, Chen’s PhD student and first author of the paper.

Zhu’s group at Georgia Tech meanwhile led the computational modelling for the research, developing dual-phase crystal plasticity computational models to understand the mechanistic roles played by both the FCC and BCC nanolamellae and how they work together to give the material added strength and ductility.

Zhu said, “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.”

In the future, harnessing 3D printing technology and the vast alloy design space of HEAs should enable the direct production of end-use components for biomedical and aerospace applications.

Image caption: UMass Amherst PhD student Jie Ren holds a miniature heatsink fan, one of the 3D printed high-entropy alloy components made in Wen Chen’s lab.

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