Cryogenic material have a wide range of applications in our life, such as deep-space exploration, applied superconductors, and gas industry. With the development of space technology and fusion reactor field, producing high-performance materials in the extreme conditions, especially at very low temperatures, has become a more and more impending mission. However, it's still a big challenge to produce metals and alloys with high-strength (σ_UTS>1GPa) and excellent ductility (ε_f>60%) at extremely low temperatures.
As a fire-new material, high-entropy alloys (HEAs) exhibit an extremely-broad philosophy on how to combine elements. The potent mixture strategy makes the opportunity to find something new and exciting very high. In this circumstance, the service performance of high-entropy alloy under extremely condition inevitably becomes something we're curious about.
In this paper, the authors have assessed the mechanical response of the CoCrFeNi high-entropy alloy (see the results in Figure 1), and found that this alloy exhibit a high ultimate tensile strength of 1.26 GPa and elongation to failure of 62% at 4.2 K, which are the best among almost all of metallic materials, as shown in Figure 2. This study witnesses the extensive deformation twinning and phase transformation from a face-centered cubic (FCC) structure to a hexagonal close-packed (HCP) structure are responsible for the superior mechanical performance at such low temperature. Moreover, the serration feature appears in the stress-strain curves of this alloy at liquid-helium temperatures and the authors deduce the high-density twinning and phase transformation contribute to the serration feature, and that the FCC-HCP transition makes the serrated flow unstable.
The results of high-mechanical performance at low temperatures, phase transition, and serration, not only exhibit a significant breakthrough in the fundamental materials science, but also indicate HEAs' tremendous prospects and potential applications in the field of extreme cryogenic engineering.
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https://doi.org/10.1007/s40843-018-9373-y
Science China Materials