Local chemical order starting from chemical short-range order in high/medium entropy alloys
Prof. E. Ma
Center for Alloy Innovation and Design (CAID), Xi’an Jiaotong University

Complex concentrated solutions composed of multiple principal elements are being widely regarded as “high (or medium)-entropy alloys” (HEAs or MEAs), presumably because they are characterized by the high configurational entropy of an ideal solution. The HEA community used to believe that the H/MEAs are all chemically disordered. However, enthalpic interactions among constituent elements are also expected at normal temperatures, resulting in various degrees of local chemical order (LCO), as discovered in a number of nominally random H/MEAs. Among the LCOs that can develop to different extents, chemical short-range order (CSRO) is arguably the most difficult to decipher, because CSRO is only the incipient stage of LCO and very hard to resolve: the convincing identification of CSRO demands not only irrefutable diffraction evidence, but more importantly also intricate chemical information on sub-nanometer length scale regarding the different preferences of the constituent species to occupy certain lattice planes/sites in the first and second nearest-neighbor atomic shell(s). Such concrete evidence of CSRO has been sorely missing thus far. Here we discover that under an appropriate zone axis, micro/nano beam diffraction, as well as atomic-resolution imaging and chemical mapping in transmission electron microscope, can observe CSRO in face-centered-cubic VCoNi and CrCoNi concentrated solutions. Our complementary suite of tools unequivocally nails down the CSRO, including its spatial extent, atomic packing configuration and preferential lattice occupancy by the chemical species. Modeling of the CSRO order parameters and correlations reveals that the CSRO originates from the nearest-neighbor preference for unlike (e.g., V-Co and V-Ni) pairs and avoidance for like (e.g., V-V) pairs. Our findings have thus resolved the pressing challenge as to if, and what kind of, CSROs exist in H/MEAs, and how to explicitly identify them in concentrated solution alloys. We also use atomic strain mapping to demonstrate the interactions of the CSROs with dislocations, shedding light on their effects on the evolution of plasticity carriers upon deformation. The main contributors to this work include Dr. Xiaolei Wu at the Institute of Mechanics of CAS, Dr. Zhiying Chen at Tsinghua University, and Dr. Qi Wang (my former postdoc at JHU). For details, see X. Chen et al., Nature 592 (2021) 712-716, and L. Zhou et al., Acta Mater. 224 (2022) 117490.

About the Speaker

马恩,2021年入职西安交通大学。1998至2020年为Johns Hopkins大学教授。在非晶、相变存储、高熵、纳米结构合金力学行为等研究领域发表学术论文390+篇,其中Nature,Science,Nature 子刊43篇,论文总引用51000+次,H因子116;获美国金属学会材料科学研究银奖(2004) 、亚稳态材料国际研讨会高级科学家奖(2011);入选ASM Fellow (2009)、APS Fellow (2010)、MRS Fellow (2015)。 

2022-07-07 10:00 AM
Room: Tencent Meeting
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