Low temperature conditions are needed to support green initiatives both the short term and long term. Therefore, metals and alloys that are suitable for cryogenic applications will prove to be extremely valuable assets in the drive to net zero carbon emissions.
Alloys consisting of four or more different primary components all in equal ratios, are known as high entropy alloys (HEAs). As these alloys are made up from different core metals they have the potential to have a remarkable combination of properties that other materials wouldn't have.
If proven to be suitable these HEAs could be extremely useful for the “green growth" of the economy in the short term, as liquefied natural gas (LNG), is stored at −160°C. Therefore, all infrastructure and handling needs to be done using materials that are going to be able to withstand these low temperatures without compromising their physical integrity. Alongside LNG, these HEAs would be able to be used in cryogenic batteries. These batteries provide a clean way to store energy for long periods without any emissions. This is done by cooling a gas to a liquid using renewable energy, and then heating it back up to its gaseous state, which spins a turbine to generate energy, when energy is needed.
In the middle and long term the applications of these cryogenic HEAs can extend to the handling of liquid hydrogen, which is an option for use as a green energy vector as when it is used it produces no carbon emissions: only water is produced when it produces electricity when used in a hydrogen fuel cell. The liquid hydrogen needs to be kept at extremely low temperatures, so materials capable of remaining strong in these frosty conditions are essential. As well as liquid hydrogen, it is anticipated that these materials have the potential to be used in nuclear fusion reactors.
Dr Biao Cai and his collaborators, from the metallurgy and materials department at the University of Birmingham, came to use the Engin-X beamline at ISIS to carry out neutron scattering experiments at low temperatures as low as 15 K on HEAs and steels during tensile loading [1-4].
The analysis of the neutron diffraction data showed that, under strain in cryogenic conditions, the HEAs undergone a change in structure, forming nano-scale twin boundaries, which led to a remarkable increase in both plasticity and strength of the studied HEAs.
The fact that these materials actually become stronger and tougher under low temperature conditions indicates that they could be suitable for use in green energy applications.
(a) Cryogenic tensile rig; (b) Stress-strain curves of a FeCrNiCo HEA; (c) Diffraction patterns during tensile loading at 15 K (Figure credit: Lei Tang)
Further Information
[1] Y. Wang, B. Liu, K. Yan, M. Wang, S. Kabra, Y.L. Chiu, D. Dye, P.D. Lee, Y. Liu, B. Cai, Probing deformation mechanisms of a FeCoCrNi high-entropy alloy at 293 and 77 K using in situ neutron diffraction, Acta Mater. 154 (2018) 79–89. https://doi.org/10.1016/j.actamat.2018.05.013.
[2] B. Cai, B. Liu, S. Kabra, Y. Wang, K. Yan, P.D. Lee, Y. Liu, Deformation mechanisms of Mo alloyed FeCoCrNi high entropy alloy: In situ neutron diffraction, Acta Mater. 127 (2017) 471–480. https://doi.org/10.1016/j.actamat.2017.01.034.
[3] L. Tang, K. Yan, B. Cai, Y. Wang, B. Liu, S. Kabra, M.M. Attallah, Y. Liu, Deformation mechanisms of FeCoCrNiMo0.2 high entropy alloy at 77 and 15 K, Scr. Mater. 178 (2020) 166–170. https://doi.org/10.1016/j.scriptamat.2019.11.026.
[4] L. Tang, L. Wang, M. Wang, H. Liu, S. Kabra, Y. Chiu, B. Cai, Synergistic deformation pathways in a TWIP steel at cryogenic temperatures: In situ neutron diffraction, Acta Mater. 200 (2020) 943–958. https://doi.org/10.1016/j.actamat.2020.09.075.
