| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程學系 === 103 === Equimolar CoCrFeMnNi High Entropy Alloy is a single phase Face-Centered Cubic alloy. In FCC dendrites, Co, Cr, Fe, Mn and Ni these five atoms are randomly distributed. Since these five atoms have different electronegativities and atomic radii, lattice distortion takes place and hence affects its mechanical properties. This study aims at investigating the variation of lattice strain under different conditions by measuring its mechanical properties at different temperature levels and observing the changes of diffraction profiles under different temperatures and with in-situ Neutron Diffraction. Moreover, by observing the surface microstructure under different temperatures with Transmission Electron Microscope (TEM), the relationship between the microstructure and mechanical properties of the material is investigated.
From the result of tensile testing, it is known that CoCrFeMnNi High Entropy Alloy has both good strength and ductility at low temperature, even at room temperature, which is suitable for engineering material application [1]. From the result of in-situ Neutron Diffraction, it shows that the behavior of CoCrFeMnNi High Entropy Alloy is different from that of conventional alloys. The elasticity of High Entropy Alloys is usually smaller and the alloy is insensitive to temperature. Due to lattice distortion, the atoms are not at their lattice positions and hence the system experiences a distinct reaction from ordinary metals when stress is applied. Since high temperature thermal vibration cancels off the lattice strain, leading to the low sensation of the elastic modulus towards temperature.
From the microscopic result of in-situ Neutron Diffraction, we can see that the elastic modulus of each crystal plane increases with the temperature and has two particular types of trends. Plane (111), (200), (311), (400), etc. are in one group as their elastic modulus decreases linearly when the temperature arises while plane (220), (331), (420) and (222) belong to another group as their elastic modulus first increases when the temperature reaches 200oC and 400oC and then decreases after 600OC. This is due to the lattice distortion of High Entropy Alloys: the higher the temperature elevates, the further the distance between the neutrons will be. As a result, the reduction in lattice distortion causes the elastic modulus to go back to its original position.
Last but not least, this study collaborates with two fitting software applications, which are Elastic Visco-Plastic Self-Consistent model fitting (EVPSC) and Convolutional Multiple Whole Profile fitting (CMWP) respectively. EVPSC is used to identify other factors causing the slip mechanism of inner part in CoCrFeMnNi High Entropy Alloy besides the contribution from conventional alloys. The grain size and dislocation density of CoCrFeMnNi High Entropy Alloy can be determined with Convolutional Multiple Whole Profile fitting (CMWP).
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