Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures (1602.01155v1)

Abstract: High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centered cubic (fcc) solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature the alloy shows tensile strengths of almost 1 GPa, failure strains of ~70%, and KJIc fracture-toughness values above 200 MPa.m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa.m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.

• The paper demonstrates that CrCoNi exhibits a tensile strength above 1.3 GPa and failure strains up to 90% at cryogenic temperatures.
• The paper details microstructural analysis revealing an FCC structure with pronounced dislocation activity and deformation-induced nano-twinning.
• The paper compares CrCoNi with five-component HEAs, highlighting its superior yield strength, ductility, and fracture toughness for extreme environments.

Damage-Tolerance of CrCoNi Medium-Entropy Alloy at Cryogenic Temperatures

The paper investigates the mechanical properties of a medium-entropy alloy consisting of chromium, cobalt, and nickel (CrCoNi), which demonstrates exceptional damage tolerance, especially at cryogenic temperatures. This work extends the understanding of medium-entropy alloys (MEAs), positioning them as highly competitive materials compared to high-entropy alloys (HEAs) and conventional metallic systems for specific applications.

Key Findings and Results

1. Mechanical Properties:
   • At room temperature, CrCoNi exhibits a tensile strength near 1 GPa, failure strain of approximately 70%, and a fracture toughness ($K_{JIc}$) exceeding 200 MPa√m.
   • These properties improve significantly at cryogenic temperatures, with tensile strength rising above 1.3 GPa, failure strains up to 90%, and $K_{JIc}$ values reaching 275 MPa√m. Such mechanical performance positions CrCoNi among the toughest metallic materials currently reported.
2. Microstructural Analysis:
   • The alloy maintains a face-centered cubic (FCC) solid solution structure, crucial for its mechanical properties.
   • Microstructural observations reveal pronounced dislocation activities and deformation-induced nano-twinning as dominant mechanisms contributing to the enhanced strength, ductility, and toughness.
3. Comparative Analysis:
   • CrCoNi's fracture toughness properties surpass those of the widely researched five-component CrMnFeCoNi HEA, particularly at cryogenic temperatures.
   • The paper presents that the yield strength, tensile ductility, and work of fracture at room temperature and cryogenic settings are markedly higher for CrCoNi compared to its five-component counterpart.

Implications and Future Directions

The research introduces critical insights for the application of MEAs in conditions involving extreme temperatures, emphasizing CrCoNi's potential in cryogenic environments such as space, aeronautics, and superconducting technologies. The findings here challenge the preconceived reliance on HEAs by highlighting the comparable, if not superior, properties of MEAs under specific conditions.

From a theoretical perspective, the paper reinforces the significance of element selection over mere compositional complexity in achieving desirable mechanical properties. This suggests a more nuanced approach towards alloy design, focusing on optimizing element interactions rather than maximizing entropy alone.

Future exploration could explore:

• Enhancing processing techniques to achieve uniform microstructures in MEAs, potentially improving their properties further.
• Investigating other MEAs with varying elemental combinations to develop a broader understanding of their mechanical capabilities and practical applicability.

Overall, this paper extends the frontier of alloy research by underlining the utility of medium-entropy systems, particularly in applications demanding exceptional mechanical strength and damage tolerance at low temperatures. As the field progresses, the insights from this research could pave the way for more targeted alloy development tailored to specific application requirements rather than relying on high compositional complexity.