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Acta Metall Sin  2025, Vol. 61 Issue (5): 665-673    DOI: 10.11900/0412.1961.2024.00422
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Achieving Alloys with Concurrent High Strength and High Ductility
MA En (MA Evan)(), LIU Chang
Center for Alloy Innovation and Design, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
Cite this article: 

MA En (MA Evan), LIU Chang. Achieving Alloys with Concurrent High Strength and High Ductility. Acta Metall Sin, 2025, 61(5): 665-673.

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Abstract  

Increasing the yield strength of metallic materials is observed to almost always substantially reduce their tensile ductility. Here we unravel the origin of this perplexing “strength-ductility trade-off”, and conclude that this dilemma does not necessarily preclude concurrent high strength and high ductility. We discuss several strengthening and work hardening mechanisms that regulate dislocation behavior, including traditional ones that have been pushed to their extreme in recent years, as well as new ones that take advantage of the heightened structural and chemical heterogeneities; all these mechanisms are rendered more powerful by emerging complex concentrated alloys that bring in multiple principal elements. These mechanisms, while offering elevated strength, contribute to sustainable strain hardening under high flow stresses, delaying strain localization to allow prolonged uniform elongation. The current status in the pursuit for concurrent high strength and high ductility is reviewed. The goal we set for high yield strength ~2 GPa (rivaling super steels) together with large uniform elongation ~30% (much like un-strengthened elemental metals) is projected to be soon within reach. These take-home messages shed light on some existing puzzles regarding the strength-ductility synergy, and offer new insight into the innovative design of alloys.

Key words:  metallic material      structural heterogeneity      chemical heterogeneity      strength      ductility      strain hardening     
Received:  16 December 2024     
ZTFLH:  TG142  
Fund: National Natural Sciences Foundation of China(52231001);National Natural Sciences Foundation of China(52371162);National Natural Science Fund for Excellent Young Scientists Fund Program (Overseas)
Corresponding Authors:  MA En (MA Evan), professor, Tel:(029)82664764, E-mail: maen@xjtu.edu.cn

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00422     OR     https://www.ams.org.cn/EN/Y2025/V61/I5/665

Fig.1  Engineering ultimate tensile strength (Eng. UTS) versus engineering uniform elongation (Eng.UE), showing the strength-ductility trade-off in alloys, adapted from Ref.[2], tailoring alloy composition in Cu alloys (a, b) and in steels (c) alleviates the trade-off, shifting the banana-shaped curve to a higher level (as indicated by the arrows)
Fig.2  Several typical structural regulation cases
(a) fcc-hcp transformation induced plasticity[8] (εloc—local strain, TD—tensile direction)
(b) fcc-bcc transformation induced plasticity[9] with transformation-induced plasticity (TRIP) induced rapid strain hardening in engineering stress-strain curves (ΔUTS, ΔUE, and ΔYS represent the increase in the UTS, UE, and yield strength (YS), respectively. FNAT: Fe-32.6Ni-6.1Al-2.9Ti; FNAT-m: FNAT-matrix)
(c) twinning induced plasticity with deformation twins and stacking faults (SFs)[10] (Letters M, T, and S represent the fcc-matrix, twin, and SFs, respectively)
(d) bimodal grain size distribution[13]
Fig.3  Several typical cases of chemical/structural heterogeneities that offer mechanisms for strengthening and strain hardening
(a-c) local chemical order (LCO)[19] (dbcc—spacing of the {001} planes in the bcc lattice, dLCO—spacing of the extra LCO)
(d) nanoscale compositional undulation[26]
(e) high-volume-density of nanoprecipitation[27]
(f) intermetallic precipitate with nanoscale disordered interface[28]
Fig.4  Summary diagrams showing tensile yield strength vs uniform strain (elongation)
(a) the regime for yield strength < 1.2 GPa, for which the available data points thus far have already occupied the entire space, with the upper rim (dashed line) exhibiting the shape of an up-side-down banana[17] (HEAs—high-entropy alloys, HN metals—heterogeneous nanostructured metals)
(b) extended regime up to 2.2 GPa, the strength of the strongest bulk super steels (The detailed compositions for these alloys are listed in Supplementary Information 4. The upper-rim dashed line represents a currently reachable envelope, towards which our team is pushing forward with accruing experimental data (green stars). The arrow points to the future direction to go, approaching the upper-right corner where the combination of concurrent high strength and ductility is optimized)
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