Overcoming strength-ductility trade-offs

Traditionally, new metallic alloys have had one metal constituent dominant with others making up a small fraction of the recipe. But a new study suggests a novel strategy could open the way for new classes of alloys with previously unseen combinations of properties.

C. Cem Tasan, the Thomas B. King Career Development Professor of Metallurgy in the Massachusetts Institute of Technology (MIT) Department of Materials Science and Engineering, says the approach challenges the conventional wisdom that improving the strength of a metal alloy is always a trade-off resulting in a loss of ductility – the property that allows a metal to deform without fracturing, for example when stamped into panels.

“When you start mixing metals in roughly equal amounts, you do not have good mechanical properties in most cases, due to the presence of brittle intermetallic phases,” Tasan explains. But in the last decade, there has been a renewed interest in exploring metal mixtures known as high-entropy alloys (HEAs). These compounds contain multiple metallic elements in roughly equal amounts, which could yield single-phase microstructures with improved mechanical strength and stability.

Most of the compounds studied have failed to produce significant improvements in their properties and still suffer from the strength-ductility trade-off, Tasan says. The focus of the previous work has been on evaluating the proposed single-phase stabilization concept in different alloy systems.

Aiming for stable single-phase microstructures, however, differs from the approach used in developing advanced steel alloys. Advanced steels often have phases that are stable and some that are metastable – having more than one stable configuration. Under stress, metastable phases can transform to stable configurations, improving their ability to resist fracture.

Combining roughly equal portions of metallic elements can achieve a property called increased solid-solution hardening, Tasan says. “So we thought, why not combine the strength of this concept with the strengths of steels?”

Tasan and his colleagues report that in HEAs, metastability, rather than single-phase stability, produces the most promising new alloys. A new alloy designed with these principles, composed of iron, manganese, cobalt, and chromium, outperforms even the highest-performance, single-phase, high-entropy alloy and offers exceptionally high strength and ductility values.

“It’s like combining the best of two worlds: metastability, known from steels, and the solid-solution strengthening of HEAs,” Tasan says. But more important than the properties of this particular alloy is the underlying strategy used to produce it, which could open up new avenues for the design of alloys with novel properties. “We think this is just one example of the kind of alloys that could be produced.”

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