Cryogenic strengthening of Fe27Co24Ni23Cr26 high-entropy alloys via hierarchical nanotwin-driven mechanism

FCC-based high entropy alloys (HEAs) with exceptional cryogenic mechanical properties have the potential for use in strategic applications. In this regard, we have studied via transmission electron microscopy (TEM) the microstructural evolution of hierarchical nanotwins in tensile-strained Fe27Co24Ni23Cr26 HEA alloys as a function of true strain at cryogenic temperature and compared the behavior at room temperature. At cryogenic temperature, the alloy had a high strength of ∼1211 MPa and large elongation of 87.2% compared to the room temperature tensile strength of 746 MPa and elongation of ∼61%. At 25 °C, the deformation initiated with the entanglement of wavy dislocations, which transformed into planar configuration and dislocation walls, and lastly to deformation nanotwins. In striking contrast, at −150 °C, numerous nanotwin bundles were formed through the coalescence of deformation nanotwins, which divided the matrix into microscale closed blocks. At higher strain, dense deformation nanotwins refined annealing twins into nanoscale closed blocks. Subsequently, in the interior of annealing twins, deformation nanotwin variants interpenetrated, producing serrated/curved interfacial structures. These interfaces are envisaged to promote the formation of ultra-fine deformation nanotwins and closed blocks that facilitate accommodation of a higher degree of deformation strain at cryogenic temperatures. The study provides new insights and guidelines in the futuristic design of FCC high entropy alloys for use at cryogenic temperatures by embracing the hierarchical nanotwin-driven mechanism.