Cryogenic Tensile Strength of 1.6 GPa in a Precipitation-Hardened (NiCoCr)99.25C0.75 Medium-Entropy Alloy Fabricated via Laser Powder Bed Fusion
Abstract
1. Introduction
2. Experimental Methods
Material Preparation
3. Results and Discussion
3.1. Microstructure Observation and Analysis Results
3.2. Cryogenic Tensile Mechanical Property
3.3. Fracture and Deformation Mechanisms
4. Conclusions
- The 0.75C MEA fabricated via LPBF exhibited a high relative density (>99.9%), columnar grain structure aligned along the building direction (BD), and refined substructures, all of which were retained after heat treatment. Post-heat treatment at 700 °C promoted the precipitation of Cr-rich carbides (Cr23C6) along grain and substructure boundaries. These precipitates contributed to microstructural stabilization by suppressing grain growth and dislocation motion via the Zener pinning effect. High dislocation densities were observed both before and after heat treatment, with the average dislocation density decreasing to approximately 6.2 × 1013 m−2 after heat treatment. Additionally, the fraction of stacking faults increased after heat treatment, likely due to a reduction in carbon solubility in the matrix, which decreased the stacking fault energy.
- Heat-treated 0.75C MEA exhibited excellent mechanical properties at cryogenic temperature (77 K), with a yield strength of 1.23 GPa, an ultimate tensile strength of 1.60 GPa, and an elongation of 9.4%. These values position it among the top two to three highest strength levels reported for AM-fabricated MEAs and HEAs to date. Notably, its ultimate tensile strength surpasses those of previously reported PBF-built precipitation-strengthened NiCoCr MEA. Compared to room temperature, the alloy demonstrated higher yield and tensile strengths at 77 K, accompanied by a reduction in elongation.
- Post-deformation microstructural analysis revealed that extensive deformation twinning (DT) occurred under cryogenic conditions, which was attributed to the reduced stacking fault energy (SFE). DTs intersected with high-angle grain boundaries and substructure boundaries, leading to dislocation locking and the activation of a dynamic Hall–Petch effect, resulting in a high work hardening rate (WHR). While dislocation restriction by substructures and stacking faults was the dominant strengthening mechanism at room temperature, at 77 K, the combined interaction among DTs, SFs, and precipitates acted as a synergistic strengthening mechanism, contributing significantly to both uniform deformation and high strength retention. Furthermore, WHR vs. true strain analysis showed that the WHR remained consistently higher—by approximately 1 GPa—across the entire strain range at cryogenic temperature. These findings suggest that multiple temperature-sensitive strengthening mechanisms were sequentially activated in this alloy, enabling the simultaneous retention of high strength and superior work hardening capability under cryogenic conditions.
- In this study, a NiCoCr MEA alloy with excellent cryogenic tensile strength was successfully developed by employing the LPBF process in combination with carbide strengthening. For future work, it will be necessary to evaluate the material’s potential as a next-generation structural alloy operable over a wide temperature range by conducting additional tests such as fatigue, creep, and fracture toughness.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Park, S.-Y.; Kim, Y.-K.; Kim, H.S.; Lee, K.-A. Cryogenic Tensile Strength of 1.6 GPa in a Precipitation-Hardened (NiCoCr)99.25C0.75 Medium-Entropy Alloy Fabricated via Laser Powder Bed Fusion. Materials 2025, 18, 3656. https://doi.org/10.3390/ma18153656
Park S-Y, Kim Y-K, Kim HS, Lee K-A. Cryogenic Tensile Strength of 1.6 GPa in a Precipitation-Hardened (NiCoCr)99.25C0.75 Medium-Entropy Alloy Fabricated via Laser Powder Bed Fusion. Materials. 2025; 18(15):3656. https://doi.org/10.3390/ma18153656
Chicago/Turabian StylePark, So-Yeon, Young-Kyun Kim, Hyoung Seop Kim, and Kee-Ahn Lee. 2025. "Cryogenic Tensile Strength of 1.6 GPa in a Precipitation-Hardened (NiCoCr)99.25C0.75 Medium-Entropy Alloy Fabricated via Laser Powder Bed Fusion" Materials 18, no. 15: 3656. https://doi.org/10.3390/ma18153656
APA StylePark, S.-Y., Kim, Y.-K., Kim, H. S., & Lee, K.-A. (2025). Cryogenic Tensile Strength of 1.6 GPa in a Precipitation-Hardened (NiCoCr)99.25C0.75 Medium-Entropy Alloy Fabricated via Laser Powder Bed Fusion. Materials, 18(15), 3656. https://doi.org/10.3390/ma18153656