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In situ neutron diffraction experiments have been performed to investigate the deformation mechanisms on CoCrFeNi high entropy alloys (HEAs) with various amounts of doped Cu. Lattice strain evolution and diffraction peak analysis were used to derive the stacking fault probability, stacking fault energy, and dislocation densities. Such diffraction analyses indirectly uncovered that a lower degree of Cu doping retained the twinning behavior in undoped CoCrFeNi HEAs, while increasing the Cu content increased the Cu clusterings which suppressed twinning and exhibited prominent dislocation strengthening. These results agree with direct observations by transmission electron microscopy. 

A second-generation Ni-based superalloy has been directionally solidified by using a Bridgman method, and the key processing steps have been investigated with a focus on their effects on microstructure evolution and mechanical properties. The as-grown microstructure is of a typical dendrite structure with microscopic elemental segregation during solidification. Based on the microstructural evidence and the measured phase transformation temperatures, a step-wise solution treatment procedure is designed to effectively eliminate the compositional and microstructural inhomogeneities. Consequently, the homogenized microstructure consisting of γ/γ′ phases (size of γ′ cube is ~400 nm) have been successfully produced after a two-step (solid solution and aging) treatment. The mechanical properties of the resulting alloys with desirable microstructures at room and elevated temperatures are measured by tensile tests. The strength of the alloy is comparable to commercial monocrystalline superalloys, such as DD6 and CMSX-4. The fracture modes of the alloy at various temperatures have also been studied and the corresponding deformation mechanisms are discussed.