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Abstract: Anisotropic plastic deformation beneath surface step during nanoindentation process is investigated applying the quasicontinuum (QC) method. Microscopic mechanisms of effects of surface step on FCC Al , (111), and (001) planes under the action of a cylindrical indenter are systematically studied. For steps on plane, dislocations nucleation and emission beneath surface steps is the main plastic deformation characteristic; for steps on (111) plane, the crystal lattice beneath the step undergoes severe distortion and rotation; for steps on (001) plane, deformation twinning and dislocation lock can be observed beneath the surface step. Load–displacement responses of various orientations and roughness are discussed in detail. The concept of geometrically necessary displacement is proposed to describe step flattening process. Strain energy for nanoindentation on stepped and flat surfaces under various crystal orientations is analyzed and compared to further investigate the effect of crystal anisotropic plastic deformation. The description about the atomistic behavior beneath surface step and the discussion about the anisotropic plastic deformation of surface step are meaningful for understanding the influence of surface step on the nanoindentation tests. [Copyright &y& Elsevier]

Abstract: The quasicontinuum (QC) multiscale method is used to investigate anisotropic fracture behaviors of body-centered cubic (BCC) rare metal tantalum (Ta) loaded in Mode I and different fracture mechanisms are discussed from nanoscopic to continuum perspectives to have a deep understanding of brittle and ductile fracture. Initial crack deflection, brittle fracture by cleaving along low surface energy plane, ductile fracture as a result of dislocation emission and fracture accompanied by deformation twinning are all observed near crack tips of different crystal orientations. Particularly, some of these fracture mechanisms are found to be consistent with the latest experimental results. By examining different fracture behaviors, we find the surface energy and the available slip planes play a combined role in determining the fracture mechanisms near a crack tip. Both isotropic and anisotropic critical stress intensity factors are derived and compared for different crack orientations. A straightforward criterion that is proved to be applicable is used to distinguish brittle fracture from ductile fracture. [Copyright &y& Elsevier]

Abstract: Multiscale simulations using the quasicontinuum (QC) method with the embedded-atom method (EAM) potential are performed to examine the mechanical response of Cu–Ag bilayer film during nanoindentation tests. An attempt is made, from the viewpoint of collective interaction among misfit dislocations on the interface, to account for the strengthening and weakening mechanisms of interface on Cu–Ag bilayer film. The details of misfit dislocation nucleation, motion and collective interaction on Cu/Ag and Ag/Cu interfaces are discussed systematically, respectively. The investigation shows that the property and performance of Cu–Ag bilayer film mainly depend on the mechanical property of upper film. Both the strengthening and weakening effects are closely related to the collective interaction among misfit dislocations on the interface. Due to the pinning effect of interface on misfit dislocation, both the local interface migration and the voids can be observed at the core region of misfit dislocations. For nanoindentation on Cu/Ag bilayer film, the plastic deformation is localized chiefly in the lower Ag substrate and the void will disappear with the redistribution of misfit dislocations, which indicate that there are distinct protective and strengthening effects of the upper Cu film on the lower Ag substrate. While, for nanoindentation on Ag/Cu bilayer film, both the upper Ag film and the lower Cu substrate experience plastic deformation and the voids will not disappear, which imply that there are an obvious weakening effect of the upper Ag film on the lower Cu substrate. In addition, the multiscale simulation results are consistent with the experimental results. [Copyright &y& Elsevier]

Abstract: The quasi-continuum multiscale method (QC) is applied to investigate position effect of cylindrical indenter on nanoindentation into Cu thin film. Load–displacement responses reflect that indenter position influences the critical load and critical displacement distinctly. The microscopic deformation mechanism shows that it is the retarding of dislocation nucleation beneath the indenter that actually leads to a relative lower critical load. Once the plastic deformation is retarded, the load–displacement curve will undulate several times until dislocations nucleate at the retarded region. It is found that the critical loads periodically change with indenter positions. The period equals to the distance between two adjacent atomic planes in [111] direction. An improved elastic model is proposed to predict the critical load in consideration of the effect of indenter position. The agreement between QC simulation and the present model has shown the effectiveness of the improved model. [Copyright &y& Elsevier]