High entropy alloys (HEAs) are receiving increasing attention in recent days as they represent a novel approach to engineer a material. Due to their complex near equi-molar elemental constitution, HEAs possess complicated compositions, which is different from conventional alloys. Among the HEAs which are intensely studied by researchers, the mechanical behavior and phase stability of CoCrFeNi-based HEAs attract most attention. Some of these HEAs were found to have excellent ductility at room and cryogenic temperatures. Proper secondary phase precipitation would enhance the mechanical properties of CoCrFeNi-based HEAs. Although a plenty of research work has been done to investigate mechanical behavior of HEAs, most of them were performed at ex-situ condition, at small deformation scales (≤20% true strain) or characterizing small sampling area (in-situ TEM). The emphasize on phase transition behaviors was mostly put on the phase component evolution in previous study. The size distribution of precipitates in HEAs is of significant importance for improving its mechanical properties. Hence, it is essential to characterize bulk deformation behaviors at large strains and the precipitates distribution in HEAs. Neutron diffraction method could characterize relatively large sample volume due to its high penetration. This ability also makes it possible to conduct in-situ measurement to study deformation mechanism of HEAs under load. Ultra-small angle neutron scattering is an ideal tool for characterizing the distribution of precipitate with size range from hundred nm to few μm.

In this work, we studied deformation mechanisms of CoCrFeNi and effect of Mo addition to deformation behavior of CoCrFeNi by in-situ neutron diffraction method, and the precipitation behavior of CoCrFeNiMo0.3 by ultra-small angle neutron scattering. It was found that the deformation mechanism of CoCrFeNi under room temperature tension is dominated by dislocation activity. Minor addition of Mo does not change much on mechanical property of CoCrFeNi, while more Mo addition would cause phase separation and affect deformation behavior of CoCrFeNi.

The thesis contains three parts as follows:
Firstly, the deformation behavior of CoCrFeNi HEA, which is a base alloy in frequently studied HEAs, is studied by in-situ neutron diffraction method under tensile loading at room temperature. Texture evolution is observed during tensile loading process. Lattice strain response shows significant dependence on hkl-orientation. The HEA also shows a three-stage work hardening behavior during deformation. The deformation mechanism is dislocation slip and entanglement for CoCrFeNi. Moreover, possible planar defects formation at large strain is suggested in CoCrFeNi HEA.

Secondly, the effect of Mo addition to deformation mechanism of CoCrFeNi HEA is studied by in-situ neutron diffraction method under the same deformation condition. The addition of Mo increases yield strength of CoCrFeNi matrix alloy. The deformation behavior of CoCrFeNiMo0.1 is similar to its CoCrFeNi base alloy. Potential phase separation in CoCrFeNiMo0.2 results in higher dislocation density than that of CoCrFeNiMo0.1, but the work hardening behavior remains similar. Deformation behavior of CoCrFeNiMo0.3 is significantly different from other two HEAs, which is due to the different hardening mechanism induced by intermetallic phases.

Thirdly, precipitation behavior in CoCrFeNiMo0.3 HEA is studied by ultra-small angle neutron scattering (USANS), supplemented with microscopic characterization. The intermetallic precipitates tend to form inside the Mo-rich inter-dendritic regions, suggesting a heterogeneous nucleation during annealing. Precipitate size increases with annealing temperature due to the increasing inter-diffusion rate. The coarsening of precipitates and decrease of solution hardening of matrix contribute to the decrease of hardness after heat treatment at higher temperatures. The CoCrFeNiMo0.3 HEA is hardened over a wide annealing temperature range, which provides a possibility for its application as structural material.