Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys close to liquid helium temperatures (2204.01635v1)

Abstract: Medium- and high-entropy alloys based on the CrCoNi-system have been shown to display outstanding strength, tensile ductility and fracture toughness (damage-tolerance properties), especially at cryogenic temperatures. Here we examine the JIc and (back-calculated) KJIc fracture toughness values of the face-centered cubic, equiatomic CrCoNi and CrMnFeCoNi alloys at 20 K. At flow stress values of ~1.5 GPa, crack-initiation KJIc toughnesses were found to be exceptionally high, respectively 235 and 415 MPa(square-root)m for CrMnFeCoNi and CrCoNi, with the latter displaying a crack-growth toughness Kss exceeding 540 MPa(square-root)m after 2.25 mm of stable cracking, which to our knowledge is the highest such value ever reported. Characterization of the crack-tip regions in CrCoNi by scanning electron and transmission electron microscopy reveal deformation structures at 20 K that are quite distinct from those at higher temperatures and involve heterogeneous nucleation, but restricted growth, of stacking faults and fine nano-twins, together with transformation to the hexagonal closed-packed phase. The coherent interfaces of these features can promote both the arrest and transmission of dislocations to generate respectively strength and ductility which strongly contributes to sustained strain hardening. Indeed, we believe that these nominally single-phase, concentrated solid-solution alloys develop their fracture resistance through a progressive synergy of deformation mechanisms, including dislocation glide, stacking-fault formation, nano-twinning and eventually in situ phase transformation, all of which serve to extend continuous strain hardening which simultaneously elevates strength and ductility (by delaying plastic instability), leading to truly exceptional resistance to fracture.

• The paper reveals that CrCoNi alloys achieve record-high fracture toughness at 20 K, with KIc of 415 MPa√m and Kss of 544 MPa√m.
• The research employs nonlinear-elastic J-based mechanics, neutron diffraction, and electron microscopy to uncover deformation mechanisms such as nano-twinning and TRIP effects.
• The findings indicate that CrCoNi-based HEAs promise enhanced performance for cryogenic and aerospace applications by overcoming traditional strength-ductility trade-offs.

Exceptional Fracture Toughness of CrCoNi-based Medium- and High-Entropy Alloys at Cryogenic Temperatures

High-entropy alloys (HEAs) are a novel class of metallic materials that derive their properties from the incorporation of multiple principal elements, contrasting with traditional alloys that rely on a single dominant constituent. This research paper focuses on the outstanding fracture toughness of CrCoNi-based medium- and high-entropy alloys at temperatures approaching that of liquid helium (around 20 K). The paper provides comprehensive data on the exceptional damage tolerance of these alloys, with particular emphasis on the CrCoNi and CrMnFeCoNi alloy systems.

Highlights of the Study

The results indicate that the CrCoNi and CrMnFeCoNi alloys exhibit outstanding mechanical properties under low-temperature conditions, showcasing fracture toughness values that are among the highest reported in the literature. Specifically, the crack-initiation toughness (KIc) for CrCoNi reached 415 MPa√m with a crack-growth toughness (Kss) of 544 MPa√m at 20 K. CrMnFeCoNi showed commendable values as well, with KIc at 235 MPa√m and Kss at 383 MPa√m. These numerical results underscore the exceptional performance of these materials at cryogenic temperatures.

The investigation employs nonlinear-elastic J-based fracture mechanics to determine the fracture toughness of the alloys. In situ neutron diffraction and extensive electron microscopy analyses reveal that the toughness is attributed to a complex synergy of deformation mechanisms. These include dislocation glide, stacking-fault formation, nano-twinning, and phase transformations—all of which contribute to sustained strain hardening that increases both strength and ductility.

Deformation Mechanisms and Microstructure

Electron backscatter diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM) analyses reveal evolution in the microstructure at the crack-tip regions. At 20 K, these CrCoNi-based alloys exhibit heterogeneous nucleation and restricted growth of stacking faults, fine nano-twins, and transformation to a hexagonal close-packed (hcp) phase. The presence of coherent interfaces from these features serves to both arrest and transmit dislocations. This cooperative behavior fosters strain hardening, thereby elevating the strength and ductility and resisting fracture.

An intriguing aspect of the paper is the role of transformation-induced plasticity (TRIP) effects, promoting additional strain hardening due to the in situ transformation to the hcp phase. Such phase transformations are finely balanced to prevent a brittle-ductile transition, maintaining ductility even under extreme conditions.

Implications and Future Work

From a practical perspective, the remarkably high fracture toughness values indicate that CrCoNi-based HEAs are strong candidates for applications in extreme environments, such as cryogenic engineering and aerospace applications where high damage tolerance is crucial. The findings challenge the conventional understanding of trade-offs between strength and ductility in metallic materials, demonstrating that with intricate control over composition and processing, it is possible to achieve both simultaneously at low temperatures.

Theoretically, this paper broadens the understanding of deformation mechanisms in multi-principal element alloys at cryogenic temperatures. It sets a precedent for exploring other HEA-based systems and could influence future alloy design and processing techniques to maximize mechanical performance.

In conclusion, the exceptional cryogenic fracture toughness of CrCoNi-based medium- and high-entropy alloys demonstrates the potential of these materials to redefine design strategies in fields requiring materials with high performance at extreme conditions. Future research may explore tailored compositional modifications and processing techniques to further enhance the unique properties of these and related alloys.