ATOMISTIC STUDIES OF DEFORMATION MECHANISMS IN NANOSCALE MULTILAYERED METALLIC COMPOSITES

Thesis (Ph.D.), School of Mechanical and Materials Engineering, Washington State UniversityThe goal of this thesis is to understand the interaction between dislocations and various metallic interfaces in nanoscale metallic multilayers (NMM). At lower strain rates, this mean understanding the effect of interfaces to the strain hardening of the NMMs; at higher strain rates, this means the effect of the interfaces on the spallation strengths of the NMMs. NMMs possess ultra-high strength level which is owing to the interactions between single dislocations (i.e. no pile-up) and interfaces. In this thesis, aiming at the goal, using atomistic simulations several nanoscale metallic multilayers subjected to different loading conditions and strain rates are being considered. First, a few simulations of nanoindentation are implemented at a low strain rate on a series of the Cu-Ni-Nb- bases metallic nanolayers. This includes the simulation of Cu-Ni and Cu-Nb bilayers, where the mechanisms of the interactions of dislocations with single interfaces are investigated in a rather isolated environment. The strengthening mechanisms of metallic multilayers are examined. Here, several types of interfaces (coherent FCC-FCC, such as Cu-Ni, incoherent FCC-BCC, such as Cu-Nb) are investigated. Several deformation mechanisms of the bilayers are discovered. This is done Chapters two to four. Second, to address the strengthening effect of the interfaces in a more realistic, multiple interface context, a number of simulations of nanoindentations on a few Cu-Ni-Nb- based multilayers with number of interfaces up to 18 was performed. Here the surface deformation (surface pile-up) as well as the hardening behavior of the NMMs has been considered. A constitutive law addressing strain hardening in NMM under nanoindentation is developed. This is done in Chapter five. Third, to investigate the effect of the interface on the spallation strengths of the NMMs, several simulations of the NMMs under high strain rate uniaxial strains are performed. Under two different strain rates, the spallation strength (σs) of the NMMs and its variation with respect to the change of the individual layer thickness (h) are analyzed and explained. The mechanisms of the spallation in different NMMs are observed. This is done in Chapter six.Department of Mechanical Engineering, Washington State Universit