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Max Planck, MIT researchers develop new strategy for high-entropy alloys; overcoming the strength/ductility tradeoff

Researchers at the Max Planck Institute in Dusseldorf, Germany, and MIT have developed a novel strategy to design nanostructured, bulk high-entropy alloys (HEAs) (earlier post) with multiple compositionally equivalent high-entropy phases. The new approach is described in a paper this week in the journal Nature.

The result, says C. Cem Tasan, the Thomas B. King Career Development Professor of Metallurgy in MIT’s Department of Materials Science and Engineering, also challenges the conventional wisdom that improving the strength of a metal alloy is always a tradeoff that results in a loss of ductility.

High-entropy alloys, which represent an emerging effort in materials science and engineering, are multi-principal-element alloys that are promising for high-temperature applications due to their high resistance to softening at elevated temperatures and sluggish diffusion kinetics.

However, HEAs so far have either high strength or high ductility; achieving both has been a challenge. Further, the inferior castability and compositional segregation of HEAs have been obstacles for commercialization.

High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys.

In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility.

This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

—Li et al.

The main focus of previous work on HEAs has been on evaluating the proposed single-phase stabilization concept in different alloy systems, according to Tasan. However, aiming for stable single-phase microstructures differs from the approach that has been taken with the most widely used metal of all—steel, of which 1,500 million tons are produced worldwide every year.

One of the reasons for steel’s ubiquity is that its various alloys can be tuned to have a wide range of different properties, depending on the specific application. Advanced steels often have phases that are stable, but also some that are metastable (having more than one stable configuration).

Under stress, metastable phases can transform to stable configurations, which improves their ability to resist fracture.

The new finding now being reported by Tasan and his colleagues is that in HEAs it is metastability, rather than the usually sought-after stability, that 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,” Tasan says. It also offers exceptionally high values of both strength and ductility.

MIT-New-Alloys_0
These images show the crystal configurations, or phases, within samples of metal alloys: a single-phase stable alloy (left) and a dual-phase alloy (right), which is metastable (has more than one stable configuration). The dual-phase material turns out to have better strength and ductility than the singe-phase material, the researchers found. Source: MIT. Click to enlarge.

More important than the properties of this particular alloy, Tasan says, is the underlying strategy used to produce it, which could open up new avenues for the design of alloys with novel properties.

The originally proposed HEA concept has motivated enormous efforts to design new alloys, but few of the resulting alloys have shown properties that justify the increased alloying content, in contrast to the alloy presented here, which exhibits excellent strength–ductility combinations. We emphasize that this alloy design strategy is opposite in approach to that generally used in HEAs design: rather than focusing on phase stabilization and single-phase formation, we propose that phase metastability, and ductile multi-phase configurations should be important future research goals in this field.

—Li et al.

Calling this work “unique and creative,” Ke Lu, professor and director of the Shenyang National Laboratory for Materials Science in China, who was not involved in this research, says:

The authors utilized different strengthening mechanisms into an alloy system in a very smart way, leading to a simultaneous increase of strength and ductility that are often exclusive. While each individual mechanism is well documented in many different systems, a synergic application of these effects is novel and original. The properties achieved are impressive indeed.

—Ke Lu

In addition to Tasan, the work was carried out by Zhiming Li, Konda Pradeep, Yun Deng, and Dierk Raabe at the Max-Planck Institute for Iron Research in Dusseldorf, Germany. The work was supported by the European Research Council.

Resources

  • Zhiming Li, Konda Gokuldoss Pradeep, Yun Deng, Dierk Raabe & Cemal Cem Tasan (2016) “Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off” Nature doi: 10.1038/nature17981

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