Deformation process revealed in high-entropy alloys at ultra-low temperatures

30 July 2020

Since 2014, The Croucher Foundation has supported novel research into the structure of materials and their dynamics by Professor Wang Xunli, Chair Professor of Physics at City University of Hong Kong, at The City University of Hong Kong - Institute of High Energy Physics Joint Laboratory on Neutron Scattering in Dongguan, Guangdong Province.

The joint lab, which Wang co-directs, serves as a platform for collaboration on neutron scattering between the Hong Kong scientific community and Chinese Academy of Sciences (CAS). Neutron scattering offers a versatile method for characterising materials and can bring fresh insights to fields including medicine, new nuclear energy, nanotechnology, and physics, enhancing science and technology research in the Greater Bay Area.

Now, an international team of scientists, led by Wang and utilising neutron scattering, has discovered that high-entropy alloys (HEAs) exhibit exceptional mechanical properties at ultra-low temperatures due to the coexistence of multiple deformation mechanisms.

HEAs are a relatively new class of structural materials with advantageous mechanical properties, such as an excellent combination of strength and ductility, high fracture toughness, and resistance against corrosion. Why had previously remained a mystery.

To carry out their study, the Hong Kong, Mainland China, and Japanese research team made use of in-situ neutron diffraction (a type of neutron scattering), one of the few ways of observing what is happening during materials’ deformation, Wang explained.

With neutron diffraction measurement “we can see every step – which mechanism kicks in first and how each of them interacts with the others”, he said, adding this was not feasible with conventional experimental methods such as transmission electron microscopy.

The neutron scattering technique was able to conduct measurements at ultra-low temperatures (that is, near absolute zero). In addition, measurements were representative of the bulk of the sample rather than from the surface or localised area, providing microscopic information, for example, how different grains of the materials interact with each other.

Deformation pathway of CrMnFeCoNi HEA sample at 15 K. Vertical dashed lines are drawn to pinpoint the changes in the deformation behaviour: (1) Start of dislocation slip; (2) start of stacking faults; (3) first sign of serrations; and (4) massive serrations coincided with the saturation of dislocation slip.

Using this technique, the team found that at 15 Kelvin (K), the HEA deformed in four stages. It began with dislocation slip, a common deformation mechanism for face-centred-cubic materials, where planes of crystal lattice slide over each other. As the dislocations continued, stacking faults gradually became active and dominant, where the stacking sequence of crystal lattice planes was changed by the deformation. This was then followed by twinning, where the misorientation of lattice planes occurred, resulting in a mirror image of the parent crystal. Finally, it transited to serrations where the HEA showed large oscillations of deforming stress.

In their experiments, the scientists also found that HEAs showed a higher and more stable strain hardening (where materials become stronger and harder after deformation), and exceedingly large ductility as the temperature decreased. Based on quantitative analysis of their in-situ experimental data, they concluded that the three observed additional deformation mechanisms – stacking faults, twinning, and serrations – as well as the interaction among these mechanisms were the source of those extraordinary mechanical properties.

The team’s discovery and findings have been published in Science Advances and may hold the key to designing new structural materials for applications at low temperatures.

While the study took the team almost three years to complete, there are still areas for exploration. “Complicated deformation mechanisms in HEAs at ultra-low temperatures is a new terrain [where] very few people have ventured before. The findings of this study only show the tip of an iceberg,” Wang said.

For their next step, the team will further investigate when stacking faults appear in other alloys and their deformation mechanisms at different temperatures.

Professor Wang Xunli is Chair Professor and Head of the Department of Physics at City University of Hong Kong. He received his BS from Peking University and PhD from Iowa State University, both in Physics. He is an elected Fellow of the American Physical Society (APS), American Association for the Advancement of Science (AAAS), Neutron Scattering Society of America (NSSA), and his early work on welding residual stresses was awarded an A. F. Davis Silver Medal by the American Welding Society.

For more information about the CAS-Croucher funding scheme, please click here.