High strength and large tensile ductility (indicating high static toughness, i.e., the area under the stress-strain curve) are critical prerequisites for the engineering applications of metallic structural materials, especially for those used in low-temperature environments. The strength-ductility-toughness matching of materials is particularly important to avoid catastrophic accidents caused by low-temperature brittleness. This typically requires alloys not only to possess high yield strength (YS, σy > 1.0 GPa) but also to have a high work hardening rate (WHR, Θ) to achieve a large uniform elongation (UE, ɛu > 15%) and high ultimate tensile strength (UTS, σUTS > 2.0 GPa). Currently, widely used low-temperature alloys, such as 316L stainless steel, fail to meet these requirements due to their low volume fractions of strengthening phases (such as BCC phase, B2 phase, etc.) and low-temperature brittleness, which drastically reduces the plasticity and toughness of the alloy. An effective route to improving the low-temperature performance of alloys, compared to traditional low-temperature steels, is the design of complex concentrated alloys, involving multiple principal elements to form iron-rich FCC complex concentrated alloys. While single-phase FCC complex concentrated alloys exhibit high plasticity/toughness, their strength is generally low. In particular, FCC complex alloys with a transformation-induced plasticity (TRIP) effect suffer from very low yield strength, which makes their toughness inadequate for many applications.
To address these challenges, the team led by Academician Sun Jun at the National Key Laboratory of Metal Materials Strength, Xi'an Jiaotong University, was inspired by the microstructure of iron-based and nickel-based high-temperature alloys. They proposed the use of high-volume fraction, coherently bonded L12 nanophase precipitates to strengthen FCC iron-rich complex concentrated alloy (CCA) matrices. To achieve low-temperature high strength with large ductility/toughness, the design concept of this alloy was to construct an ultra-high density of dual-function, coherently bonded L12 nanophase precipitates within the FCC matrix. On the one hand, the L12 phase acts as a dislocation barrier, significantly enhancing yield stress. On the other hand, under sufficiently high stress, the L12 phase acts as a dislocation source, providing abundant partial dislocations to achieve high work hardening performance, thus enabling large uniform elongation. Particularly, the ultra-high density of L12 nanophases significantly increased the alloy’s flow stress during twinning deformation, making the FCC matrix transform into a BCC phase during low-temperature (77 K) tensile tests. This TRIP effect further enhanced the alloy's work hardening rate (WHR > 4 GPa). Due to the similar mobility of screw and edge dislocations in the BCC phase generated by the complex concentrated alloy phase transformation, low-temperature brittleness induced by the FCC-BCC transformation is avoided. This is distinctly different from the low-temperature brittleness induced by the FCC-BCT phase transformation in conventional high-carbon martensitic alloys. Based on this strategy, the team used machine learning assisted by domain knowledge to design an alloy, resulting in the L12 precipitate-strengthened Fe35Co29Ni24Al10Ta2 complex concentrated alloy. The L12 phase in this alloy has a size of approximately 10 nm and a volume fraction of approximately 65 ± 3 vol.%, achieving unprecedented performance combinations at liquid nitrogen temperatures: YS ≈ 1.4 GPa, UTS ≈ 2.25 GPa, UE ≈ 45%, and WHR > 4 GPa. The static toughness of this complex concentrated alloy is higher than all known low-temperature alloys and is expected to be applied in low-temperature fields. The alloy design strategy proposed by the team also provides new ideas for the design of other high-performance alloys.
Figure 1: Microstructure of Fe35Co29Ni24Al10Ta2 alloy in the initial state, room-temperature deformation, and low-temperature deformation.
Figure 2: Performance comparison between the designed Fe35Co29Ni24Al10Ta2 alloy and currently reported high-performance alloys at room temperature/low temperature.
This research was published online in Advanced Materials under the title A Complex Concentrated Alloy with Record-High Strength-Toughness at 77 K.
Co-first authors: Yasir Sohail and Zhang Chongle, Ph.D. students at Xi'an Jiaotong University.
Co-corresponding authors: Prof. Zhang Jinyu, Prof. Xue Dezhen, Prof. Ma En, and Academician Sun Jun.
Other contributors: Prof. Li Suzhi and Prof. Liu Gang.
Corresponding institution: National Key Laboratory of Metal Materials Strength, Xi'an Jiaotong University.
This work was supported by the National Natural Science Foundation, Shaanxi Province Science and Technology Innovation Team Program, and Fundamental Research Funds for Central Universities.
Paper Link: https://onlinelibrary.wiley.com/doi/10.1002/adma.202410923
