APPLIED PHYSICS GROUP
Prof. Xun-Li WANG
Chair Professor, Department of Physics
Executive Director, Hong Kong Institute for Advanced Study
City University of Hong Kong
Deformation in High Entropy Alloys
High Entropy Alloys (HEAs), which consist of multi-principal elements, are an intriguing new class of materials with promising mechanical properties. Despite the complex chemistry, HEAs can form a single-phase solid solution with an incredibly simple lattice, e.g., a face-centered-cubic (fcc) structure. Remarkably, both the strength and ductility of HEAs increase when the test temperature is lowered.
In situ neutron diffraction is a powerful technique to identify deformation mechanisms. We have been using neutron diffraction to investigate in situ the deformation pathway in HEAs at low temperatures. The goal is to elucidate the interaction and competition between different deformation mechanisms, which lead to the extraordinary ductility.
Stress-strain curve obtained at 15 K for CrMnFeCoNi HEA, showing high strength and extraordinarily large ductility. In situ neutron diffraction measurements show that the deformation at 15 K began with dislocation slip, followed by stacking faults and twinning, before transitioning to inhomogeneous deformation by serrations.
(Naeem et al., Science Advances, 2020)
Key publications from our group:
 M. Naeem, H. Y. He, F. Zhang, H. L. Huang, S. Harjo, T. Kawasaki, B. Wang, S. Lan, Z. D. Wu, F. Wang, Y. Wu, Z. P. Lu, Z. W. Zhang, C. T. Liu, and X.-L. Wang*, “Cooperative deformation in high-entropy alloys at ultralow temperatures,” Science Advances, 6, eaax4002 (2020).
[CrossRef] [Google Scholar] [CityU Research Story] [PressRelease from Japan Atomic Energy Agency] (Selected to be Croucher Foundation feature news [Croucher News])
 D. Ma, A. D. Stoica, X.-L. Wang*, Z. P. Lu, B. Clausen, D. W. Brown, “Moduli inheritance and the weakest link in metallic glasses,” Physical Review Letters, 108, 085501 (2012).
[CrossRef] [Google Scholar] [Covered by News and Views, Nature Materials]
 S. Cheng, A.D. Stoica, X.-L. Wang*, Y. Ren, J. Almer, J.A. Horton, C.T. Liu, B. Clausen, D.W. Brown, P.K. Liaw, and L. Zuo, “Deformation cross-over: from nano to meso scales,” Physical Review Letters, 103, 035502 (2009).
[CrossRef] [Google Scholar] (Selected for inclusion in August 3 issue of Virtual Journal of Nanoscale Science & Technology)
 Y.D. Wang, H. Tian, A. D. Stoica, X.-L. Wang*, P. K. Liaw, and J.W. Richardson, “Development of Large Grain-Orientation-Dependent Residual Stresses in a Cyclically-Deformed Alloy,” Nature Materials, 2, 103-106 (2003).
[CrossRef] [Google Scholar] [Covered by Materials Today]