Strain-Rate Sensitivity and Deformation Mechanisms in CoCrFeMnNi High-Entropy Alloys at Cryogenic Temperatures

Abstract

Background: High-entropy alloys (HEAs), particularly the equiatomic CoCrFeMnNi Cantor alloy, have attracted significant attention due to their exceptional combination of strength and ductility. While room-temperature mechanical behavior has been extensively studied, the strain-rate dependent deformation mechanisms at cryogenic temperatures remain poorly understood. Understanding these mechanisms is critical for evaluating the suitability of HEAs in structural applications where components are exposed to extreme cold, such as in aerospace, liquefied natural gas infrastructure, and superconducting systems.

Methods: We performed uniaxial compression tests on single-phase, face-centered cubic CoCrFeMnNi specimens across a temperature range of 77 K to 298 K, at strain rates spanning four orders of magnitude from 10⁻⁴ to 10² s⁻¹. Deformation microstructures were characterized using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Strain-rate sensitivity parameters were extracted from stress-strain data using standard jump-test and constant-rate protocols, and activation volume analyses were conducted to identify operative deformation mechanisms.

Results: At 77 K, the alloy exhibited a 40% increase in yield strength compared to room temperature, rising from 350 MPa to approximately 490 MPa. Deformation twinning was activated at cryogenic temperatures for all strain rates above 10⁻² s⁻¹, serving as a supplementary deformation mechanism alongside dislocation glide. The strain-rate sensitivity parameter m increased from 0.008 at 298 K to 0.021 at 77 K, indicating enhanced rate-dependent strengthening at low temperatures. TEM analysis revealed a transition from planar dislocation arrays at room temperature to dense dislocation forests interspersed with nanoscale deformation twins at 77 K.

Conclusions: The simultaneous activation of twinning and enhanced strain-rate sensitivity at cryogenic temperatures provides a dual strengthening mechanism that preserves ductility while markedly increasing flow stress. These findings demonstrate that CoCrFeMnNi HEAs hold considerable promise for cryogenic structural applications, particularly where dynamic loading conditions are expected. The results also offer mechanistic insight for designing next-generation HEAs with tailored low-temperature performance.

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