Short-range order strengthening in boron-doped high-entropy alloys for cryogenic applications

Abstract Boron doping with an adequate concentration is highly desirable for alloy development because it profoundly improves the material's interface cohesion via interfacial segregation. However, scientific and applicable potentials of soluble boron that resides at the alloy internal grains are generally overlooked. Here we report a strategy for overcoming the typically low strengths of face-centered cubic high-entropy alloys (HEAs) through exploiting soluble boron instead of the interfacial boron. We find that soluble boron increases stress/strain field at the recrystallized HEA grain structure, leading to the generation of short-range order (SRO) in those deformation structure under load at 77 K. The highly increased degree of SRO at planar dislocation slip band that forms during straining, proved by electron microscopy and synchrotron X-ray diffraction, strengthens a typical non-equimolar Fe40Mn40Co10Cr10 (at%) HEA, particularly increasing yield strengths by ∼32%, to ∼1.1 GPa compared to those of boron-free reference materials with similar grain sizes. The advent of deformation-induced SRO domains causes severe lattice distortion (specifically, contraction), leading to the increased cryogenic yield strength of ∼210 MPa, but generating micro-voids at grain boundaries. This study on deformation-induced SRO via boron advances the fundamental understanding of SRO impacts on HEA grains and mechanical properties at cryogenic temperatures, which may pave a general pathway for developing a wide range of ultrastrong alloys for cryogenic applications.