Nanostructure and Micromechanical Behaviors of Heterogeneous Materials
Student thesis: Doctoral Thesis
Related Research Unit(s)
Heterogeneous materials with combined super properties are developed for many engineering applications such as mechanical, aerospace, civil, transportation, marine engineering, medicine, sports, recreational goods and others. Because of inherent inhomogeneity of heterogeneous materials, and the local interaction between different constituents, properties of heterogeneous materials are nonlinear and various methods have been developed to study their properties. Approaches including the experimental and theoretical methods have been developed to characterize the structures and predict the properties of heterogeneous materials for revealing the effect of local inhomogeneity on properties. However, as the domain that have dramatically different mechanical and physical properties can be limited to several micrometer and even nanometers in heterogeneous materials such as the thin film system and the nanoparticle reinforced composite system, both experimental and theoretical approaches cannot provide any insight into the detailed inhomogeneous structures and quantify their effect on the properties. An atomistic simulation can be applied to build a bottom-up model starting from the atomistic structures, displaying atom configuration, predicting the materials properties, and quantifying the structure-properties relationship. In this thesis, the structures of different heterogeneous material systems are developed to characterize the local inhomogeneity in these materials and to quantify the relationship between the inhomogeneous structure and the mechanical properties. As materials with ordered structures and materials with disordered structures represent totally different deformation mechanism, two heterogeneous materials namely layered metallic system where the dissimilar constituents can represent a similar deformation mechanism and metallic glass-metal system where the dissimilar constituents have different deformation mechanisms are investigated. In layered metallic systems, the model of metallic thin film deposited on a metallic substrate has been constructed. The interfacial structure evolution and atomistic origin of intrinsic residual stress are figured out through the in-situ characterization of atom arrangement and rearrangement in layered metallic nanocomposites with different interfacial misfit by using an atomistic approach. With the increment of the interfacial misfit, the interface roughens while the intrinsic residual stress which plays a dominant role in the mechanical, optical, magnetic, and thermal properties of nanocomposites increases and then reduces. The film structure dominates the evolution of interfacial structure and intrinsic residual stress when the interfacial misfit is low, whereas the effect of substrate structure on the interface and the stress is as important as the film structure with the increase of the interfacial misfit. In the metallic glass-metal system, a model of amorphous matrix reinforced by crystalline phase has been constructed. To quantify the inhomogeneous structure effect on the properties, the model undergoes a tensile deformation. The deformation of the amorphous matrix dominates the beginning deformation resulting in a linear stress-strain response. The motion of atoms in the amorphous matrix near the interface is significantly deflected, affecting the local stress field. The neighbor shear transformation zones in the amorphous matrix near and far from the interface are activated along different directions, disturbing the formation of a domain shear band. This is different from that in the amorphous matrix without crystalline phase where the neighbor shear transformation zones are activated along the same direction promoting percolation and annihilation of shear transformation zones to form the domain shear band. As the rotation of the crystalline phase caused by phase transformation is confined at the interface, the stress field in the crystalline phase and at the interface is significantly deflected, promoting phase transformation and the free volume increased in shear transformation zones. Overall, this thesis develops models to clearly represent the inhomogeneous structures in different heterogeneous materials and quantify their effect on the mechanical properties. The findings can inspire us to design heterogeneous materials with attractive properties and longer durability.