In recent years, a new class of alloys called high-entropy alloys（HEAs）have attracted significant attention of a tremendous number of materials scientists in the world due to their excellent combination of high strength and high ductility. Research on HEAs has thus become one of the most active areas in materials science today. There is a renewed interest in atomic structures of solid-solution alloys, as a possibility is proposed to further increase the strength by forming short-range ordering（SRO）only through simple heat-treatment. A review is made on how to detect SRO in the CrCoNi HEA, one of the strongest HEAs, and the effects of SRO on the strength.
Observations of deformation behavior of high entropy alloys using in situ neutron diffraction measurements during deformation at various temperatures are reviewed. Neutrons are used to investigate stresses and crystallographic microstructures inside engineering materials, taking advantage of their large penetrating power and the ability to see the arrangement of atoms by diffraction methods. The important structural details of high entropy alloys such as internal stresses, phase conditions, dislocations, texture etc. are discussed in relation to the deformation conditions. Some highlights are introduced:（a）Cooperative deformation in CrMnFeCoNi alloy at ultralow temperatures,（b）Stacking fault energies in CrFeCoNi and CrCoNi alloys, and（c）Load redistribution in eutectic high entropy alloy AlCoCrFeNi2.1 during high temperature deformation.
In Cr containing high and medium entropy alloys, the short-range order（SRO）is expected to decrease in the magnetic frustration of parallel pairs. The SRO in CrMnFeCoNi and CoCoNi alloys was investigated using first principles-based Monte Carlo simulations. In the initial stage of the SRO, the L12-type ordering occurred in CrMnFeCoNi and CrCoNi alloys. After the initial stage in CrMnFeCoNi, the Cr and Mn-rich layers and Co and Ni-rich layers were formed along one of ＜100＞ directions, which led to the the L10-type ordering. In CrCoNi, the MoPt2 type ordering evolved after the initial stage of the SRO.
The high-entropy alloys/medium-entropy alloys（HEA/MEA）have a great deal of attention, since they have exceptional mechanical properties. In order to clarify the mechanism, we investigated the local structure in the prototype equiatomic alloy CrCoNi by the measurement of the extended x-ray absorption fine structure. The structural disorder is larger in the Cr sites than those in other constituent elements, which is consistent with the larger atomic displacement of the Cr atoms predicted by the first-principles calculations. The Cr atom has a solute-type characteristic in the 3d transition metal atoms that enhances the local disorder. If the sample is annealed in the intermediate temperature range, the difference in the local disorder among the constituent elements decreases.
This report introduced the results of recent neutron scattering and X-ray absorption fine structure（EXAFS）experiments on a family of medium-entropy alloys（TrCoNi, Tr＝Cr, Mn, and Fe）. Neutron experiments revealed that the time-temperature-dependent short- and long-range structural transformation occurs in a medium-entropy alloy MnCoNi. Although a sign of short-range ordering was not clear in the EXAFS results, the mean-square relative displacement（MSRD）may catch the difference in local structures between medium-entropy alloys. The relatively large static MSRD of the MnCoNi sample can be interpreted as a sign of short-range tetragonal structural transformation, whereas the small value of the CrCoNi sample cannot be interpreted straightforwardly. A possible interpretation of the difference in MSRD was provided.