Theoretical Study On Temperature-dependent Yield Strength Of Precipitation Strengthening High Entropy Alloys And Size-dependent Elastic Moduli Of Single-walled Carbon Nanotubes

Due to the unique structural features caused by multi-principal element characteristics,high-entropy alloys have excellent mechanical properties.The experimental studies have shown that the precipitation strengthening high-entropy alloy with face-centered cubic structure has high strength and toughness at room and low temperatures.And they were extensively applied in many high-tech fields,such as aerospace,new energy,high-end equipment manufacturing,and so on.During service in the above fields,the precipitation strengthening high-entropy alloy with the face-centered cubic structure are usually subjected to different temperature service environments.As a key indicator that determines the safety of materials,yield strength shows a strong temperature dependence.Theoretical study on the contribution of strengthening mechanisms to the yield strength of the precipitation strengthening high-entropy alloy with face-centered cubic structure and its evolution with temperature and establishing a temperature-dependent yield strength theoretical model has important theoretical significance and engineering application value.Meanwhile,single-walled carbon nanotubes are widely used in aerospace,nanoelectronic devices and other fields owing to their excellent mechanical properties caused by the C-C covalent bond and closed structure.The mechanical properties of single-walled carbon nanotubes are important parameters which can determine their applications.Current studies have shown that the elastic modulus of single-walled carbon nanotubes exhibits an obvious size dependence.It is very necessary to study the evolution of the elastic modulus of single-walled carbon nanotubes with the diameter and establish the corresponding theoretical characterization model.This dissertation uses theoretical methods to study the temperature-dependent yield strength of precipitation strengthening high-entropy alloy with face-centered cubic structure and the size-dependent elastic modulus of single-walled carbon nanotubes.The specific research content is as follows:(1)For the precipitation strengthening high-entropy alloy with face-centered cubic structure,a temperature-dependent yield strength theoretical characterization was developed by considering the influence of lattice friction resistance,solid solution strengthening,grain boundary strengthening and precipitation strengthening to the yield strength and their evolution temperature.The model predictions are well-validated by the available experimental results at different temperatures.As the relative parameters can be easily obtained from literature or material handbooks,the present study offers a convenient method to predict the yield strength for precipitation strengthening high-entropy alloy with face-centered cubic structure at different temperatures.Moreover,the contribution of each mechanism for the yield strength of precipitation strengthening high-entropy alloy with face-centered cubic structure and their evolution with temperature was performed using the proposed model.The optimal precipitate size at different temperatures to achieve maximum strength of precipitation strengthening high-entropy alloy with face-centered cubic structure was also analyzed.The present study offers a helpful approach to improve the yield strength of precipitation strengthening face-centered cubic high entropy alloys at different temperatures.(2)Based on the Force-Heat Equivalence Energy Density Principle,the theoretical models without introducing wall thickness for characterizing the size-dependent Young's and shear moduli of SWCNTs are established in view of the controversy over the selection of wall thickness of single-walled carbon nanotubes.The model predictions are in good agreement with the available numerical simulation results.The relative parameters used in the model predictions can be easily obtained from the literature.Furthermore,the evolution of the Young's and shear moduli of single-walled carbon nanotubes with diameter is studied using the proposed model.The study provides a new method to quantitatively characterize the size-dependent Young's and shear moduli of SWCNTs.