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Summary of research projects performed in Meshi's group:

1. Subject: Development of strategies for full structure determination of intermetallides using electron crystallography

For technological progress new or optimized multifunctional and structural intermetallics are needed. These phases often possess new structure types, identification and characterization of which is prerequisite for study of their properties. Mostly complex intermetallic phases have large volume of the unit cell and appear as nano-sized particles dispersed in metallic matrices. Thus, single/powder X-ray diffraction methods can not be used for characterization of their structure due to the lack of single crystals and overlapping and/or broadening of powder diffraction peaks. In such cases, electron diffraction (ED) emerges as the only tool for structure determination. In past, conventional ED (such as Selected Area Electron Diffraction (SAED)) was not commonly used for determination of atom positions due to dynamical nature of electron diffraction intensities. The situation has changed when Precession Electron Diffraction (PED), which produces quasi-kinematic intensities, was invented. There exist two main ED data collection methodologies: 1) collection of different PED patterns at zone axis orientation in SAED or nano-beam mode (zonal data); and 2) collection of "off-axis" ED patterns with small angular steps between them (with and w/o precession) (Electron Diffraction Tomography (EDT/ADT) or Rotation Electron Diffraction (RED) approaches). Collection of "off-axis" patterns provides higher completeness of data, so even the most complicated structures with high unit cell parameters can be solved "ab initio". Our group focuses on usage of ED data for structure solution of intermetallics in general and aluminides in particular. We have already successfully solved structures of 5 aluminides (published data), among which is the Al-Ru-Rh complex approximant with 480 atoms in the unit cell. Next step in our research is to implement dynamical refinement for successful determination of partial occupancies.

1. Subject: High Entropy Alloys – development of next generation structural materials

High Entropy alloys (HEA) are currently in the center of researchers' attention due to their promising mechanical properties, some of which overweigh those of steels. The AlCoCrFeNi alloy, which displays a good combination of yield strength and ductility, is one of the most studied HEAs. Vast majority of the studies was performed on this alloy with a purpose to understand the effect of fabrication parameters on its complex microstructure. Ambiguous results were reported. For example, eutectic reaction and spinodal decomposition were proposed as possible mechanisms but neither was proved. Moreover, our study on this alloy revealed characteristic antiphase boundaries (APBs) with different ranges of order as a function of the region in the as-cast alloy (dendrite or interdendrite) (study was published in Scripta Metal.). Not only that the existence of these boundaries was not reported previously, their presence may shed light on some phenomena which were stated but not explained in the literature. There are evidences that APBs are related to the residual strains and the latter seems to play an essential role in this material, especially on phase transformations occurring upon exposure to high temperatures, and was completely ignored in previous researches. Yet, the causes for the formation of the APBs and ordering of these boundaries are not understood. Furthermore, despite its superior properties, AlCoCrFeNi alloy is not implemented in the industry due to its inhomogeneous microstructure, ambiguity in reported results, lack of understanding of the crystallization path and thermodynamics, kinetics of phase transformations and, thus, unpredictability. In the future we intend to perform in-depth metallurgical and structural research to clarify these subjects. Emphasis of this research is on phase transformations occurring in this alloy as a function of temperature and irradiation. Understanding of the phase transformations occurring in the HE alloys will promote their use as structural materials. Important, as well, roles of each element in this system. Currently we are investigating this subject.