Inception of plasticity in the presence of vacancies in FCC single crystals: indenter size effect
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Inception of plasticity in the presence of vacancies in FCC single crystals: indenter size effect I. Salehiniaa, V. Pereza, M. Weberb, and D.F. Bahra a School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164, USA b Center for Materials Research, Washington State University, Pullman, WA, 99164, USA ABSTRACT Atomistic simulations of nanoindentation tests were used to study the indenter radius size effect in the presence of vacancies in a (111) single crystal of nickel. For radii from 2 nm to 8 nm, the maximum shear stresses under the indenter at the onset of plastic deformation in crystals with vacancies were compared to those which cause yield in perfect crystals by placing a single vacancy in a position near the maximum shear stress underneath the indenter tip. The effect of the presence of vacancies is lowered by decreasing the indenter radius. Results obtained for several random distributions of vacancies, in the range 3.3e-4 to 0.0033, show that placing a single vacancy near a specific location produces similar results as using larger numbers of vacancies while simplifying the complexity of the simulation. Finally, visualizations of atomic configurations of a single crystal with vacancy concentration of 3.3e-4 for radii of 4 nm and 6nm show that the heterogeneous nucleation is a size dependent phenomenon. INTRODUCTION Nucleation of dislocations is one mechanism responsible for the transition from elastic to plastic deformation during contact loading in relatively defect free single crystalline materials. In a nanoindentation experiment, this transition is demonstrated by a sudden change or discontinuities in the load-depth curve [1]. The nucleation of dislocations has been attributed to both homogeneous [2] or heterogeneous [3] processes. The first happens when the crystal is defect free and no preferred location of nucleation exists in the material, while the latter is dominated by defects in the crystal. Among different types of defects, point defects, and in particular lattice vacancies, are common in single crystals. While the equilibrium concentration of vacancies in most metals is typically very low at room temperature [4], non-equilibrium processes such as quenching, severe plastic deformation and radiation damage can increase the vacancy concentration to a significant degree [5]. Research on the weakening effect that vacancies have on the onset of plastic deformation include both experimental and modeling studies concluding lower elastic modulus and ultimate tensile strength [6], and also lower yield stress and fracture toughness in room temperature [7,8,9] for the crystals with higher concentrations of vacancies. In addition, Salehina and Bahr [11] reported several mechanisms observed during atomistic simulations of indentations, including the nucleation of dislocations from vacancies, migration of a vacancy prior to nucleation of dislocation and the nucleation of a large number of dislocations from a loop already created under the indented surface. Indenter tips w
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