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Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off

Abstract

Metals have been mankind’s most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength–ductility trade-off1,2. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization3,4,5,6. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection7,8,9,10,11, 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 phase12); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase13). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels14,15 and massive solid-solution strengthening of high-entropy alloys3. 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 materials16,17. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

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Figure 1: XRD patterns and EBSD phase maps of Fe80 − xMnxCo10Cr10 (x = 45 at%, 40 at%, 35 at% and 30 at%) HEAs.
Figure 2: Elemental homogeneity among the two phases of Fe50Mn30Co10Cr10 (at%) HEA.
Figure 3: Mechanical behaviour of the TRIP-DP-HEAs compared to various single-phase HEAs.
Figure 4: Deformation micro-mechanisms in the TRIP-DP-HEA with increasing tensile deformation at room temperature.

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Acknowledgements

This work is financially supported by the European Research Council under the EU’s 7th Framework Programme (FP7/2007-2013)/ERC grant agreement 290998. The contributions of H. Springer, S. Zaefferer, M. Nellessen, M. Adamek and F. Schlüter are also gratefully acknowledged.

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Authors

Contributions

C.C.T. and D.R. designed the research; Z.L. was the lead experimental scientist of the study; K.G.P. and Y.D. performed some of the alloy design experiments; and Z.L. and C.C.T. wrote the paper. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Zhiming Li or Dierk Raabe or Cemal Cem Tasan.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Statistical binomial frequency distribution analysis results for the APT tip.

The statistical analysis shows that the tip has an overall composition of Fe48.6Mn27.6Co11.3Cr12.3 (at%). The binomial curves obtained from the experiments match the curves corresponding to a total random distribution. The quality of the fit was quantified using several parameters, as listed in the key. nd is the number of degrees of freedom for a given ion. The values of the normalized homogenization parameter μ for all four elements are close to 0, confirming the random distribution of elements in the DP-HEA.

Source data

Extended Data Figure 2 Strain distribution within the DP-HEA sample upon room-temperature deformation.

a, Evolution of local strain with increasing the global strain (εglo.), indicating an extended uniform deformation process. The red dotted circles in a indicate the local strain values corresponding to various positions in the fractured tensile sample shown in b; four positions with local strains of 10%, 30%, 45% and 65% were highlighted by percentages in red and the corresponding microstructures are shown in Fig. 4. b, Digital image correlation strain map shows the local strain distribution of the tensile sample following fracture. 0 to 11 in b refers to the distance of the sample position from the fracture surface, corresponding to the distance values shown in a.

Extended Data Table 1 Chemical composition of the studied alloys in atomic per cent according to wet-chemical analysis

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Li, Z., Pradeep, K., Deng, Y. et al. Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off. Nature 534, 227–230 (2016). https://doi.org/10.1038/nature17981

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