Study On Microstructure Evolution Of Nanoporous Metal Ligaments Under Basic Deformation

When nanoporous metals are deformed under load,their internal ligaments often undergo severe deformation,which affects their reticular shape and macroscopic mechanical properties.Therefore,studying the ligaments of nanoporous metals is of great significance.In this thesis,molecular dynamics simulation methods were used to investigate the mechanical properties and microstructure evolution of nanoporous metal ligaments under tension,torsion,and bending deformation.Firstly,molecular dynamics simulation method was used to study the tensile behavior of cylindrical nanoporous copper ligaments.The microstructure evolution and fracture energy of ligaments with different radial-to-axial aspect ratios were compared during the tensile process.It was found that the fracture energy of nanoporous copper ligaments exhibited obvious size effects,and phase transition energy accounted for more than 50%of the fracture energy.Based on this,the fracture energy per unit area of nanoporous copper was investigated as a function of porosity,and the fracture toughness of nanoporous copper under different porosities was given.Furthermore,the mechanical properties of nanoporous gold ligaments during tensile testing were analyzed.By maintaining the same radial-to-axial aspect ratio as that of nanoporous copper ligaments,it was found that the fracture energy of nanoporous gold ligaments did not exhibit significant size effects.And the average proportion of phase transition energy in fracture energy reached 64.46%.Phase transformation was the main energy dissipation mechanism during the tensile fracture of nanoscale ligaments.Similarly,the fracture energy per unit area of nanoporous gold was analyzed as a function of porosity,and the fracture toughness of nanoporous gold under different porosities was given.On this basis,the microstructure evolution of nanoporous copper and gold ligaments was compared.It was found that the fracture energy of nanoporous gold ligaments was higher than that of copper ligaments,but the fracture toughness of nanoporous gold was lower than that of nanoporous copper under the same porosity.Finally,molecular dynamics simulations were conducted on three sets of nanoporous copper ligaments with different radial-to-axial aspect ratios under reverse torsion at both ends,fixed torsion at one end and torsion at the other end,and bending.During reverse torsion,a quasi-periodic torque with "peaks" and "valleys" was observed in ligaments.By analyzing the microstructure near the "peaks" and "valleys",non-crystalline atoms and even 3-4 ?non-crystalline layers were found inside the ligaments near the "valleys".And dislocations were identified as the main cause driving the generation and disappearance of non-crystalline atoms.The surface area change during torsion showed significant size effect,and the HCP phase transition was higher in the ligaments twisted at one end than those subjected to reverse torsion at both ends.When the ligaments were subjected to bending,the surface area change was different from that during tensile and torsional deformation.Dislocations and phase transitions deviated from the region near the applied force block after a certain degree of bending,and HCP phase transition atoms were arranged at a 45 angle to the axis.This study indicates that the microstructure evolution of nanoporous metal ligaments under basic deformation is closely related to the material and size,and is expected to provide a theoretical basis for other mechanical properties and structural design of nanoporous metals.