A Study On The Mechanical Properties Of Nanoporous Metallic Materials With Structural Correlation

Nanoporous metals（NPMs）present novel and unique properties,such as ultrahigh specific surface area,low density,high specific strength,superior catalytic activity,and excellent tissue applicability,showing numerous promising applications in fields of technological areas,including but not limited to,chemical catalysis,shock absorber,micro-nano devices,sensing excitation,fuel cells,and biotechnology.The functional applications,in turn,put forward the corresponding requirements on the mechanical properties of NPMs,such as strength,hardness,ductility,and stability.A better understanding of the relationship between structure and mechanical properties would be of direct relevance to the structural design and functional optimization in various technical applications.In this dissertation,the mechanical responses of NPMs with several typical microstructures were studied via molecular dynamics（MD）simulations,macroscopic experimental tests and theoretical analysis.The sensitivity of mechanical properties of nanoporous materials to topological microstructure was discussed.Four representative types of nanoporous gold（NPG）,including cube,gyroid,diamond,and stochastic bicontinuous microstructure models,were studied and the MD results were compared.It was found that the main deformation behaviors for different models are similar.The bending deformation of ligaments is the dominated elastic deformation and the axial yielding of ligaments is the main plastic deformation.Based on the Gibson-Ashby model,the quantitative scaling laws between mechanical properties and structural parameters were derived.In addition to the similarity with the morphological structure of the experimental specimens,the predicted mechanical properties of the stochastic bicontinuous structure were the closest to the experimental results and theoretical values.By combining phase field decomposition method and Voronoi method,the atomic model of nanocrystalline nanoporous materials was constructed,and the effect of grain size on the mechanical properties of nanocrystalline NPG were studied.The results of tensile tests revealed a reverse Hall-Petch relation between strength and grain size,which was attributed to the combination deformation mechanism of grain boundary sliding,grain rotation and dislocation movement.The Young’s modulus showed a linear relationship with the reciprocal of grain size,which mainly depended on the volume fraction of atoms in grain boundaries.Compared with the strength of monocrystalline structure,the mechanical properties of nanocrystalline NPG were found to be lower,showing an obvious softening phenomenon.Structurally ordered/disordered nanoporous Pt-Co alloys were designed and the tensile mechanical behaviors were investigated via MD simulations.The results demonstrated that alloying could significantly improve the mechanical properties of NPMs,particularly for the ordered treatment.Due to the unique arrangement of atoms,Pt₃Co alloy showed the most obvious enhancement effect（the Young’s modulus and yield strength were increased by more than 100%）among three structurally ordered alloys.For the structurally disordered alloys,the Young’s modulus and strength were the highest when the Co atomic proportion was about 24%,which was close to the atomic ratio of Co in Pt₃Co.The microstructure characteristics of NPMs were extracted,and the stochastic bicontinuous structure model was constructed.The porous resin samples were constructed based on additive manufacturing（AM）technology and the quasi-static/dynamic compressive responses of irregular foams were experimentally tested.Under the compressive load,the ligaments along the loading direction underwent large bending and tensile deformation,driving the horizontal ligaments to deform and improving the energy absorption effect.The porous structure showed excellent energy absorption characteristics.The relative density was the main characteristic parameter to determine the mechanical properties and energy absorption characteristics.The energy absorption characteristics of the porous foams could be effectively regulated by changing the topologic distribution of ligaments.In addition,the quantitative scaling laws for the mechanical parameters and energy absorption characteristics as functions of structural parameters were derived.