ATOMISTIC SIMULATION STUDY OF THE EFFECTS OF POINT DEFECTS ON THE INCEPTION OF PLASTIC DEFORMATION IN METALS
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Atomistic simulations have been used to study the effect of various types of point defects on the mechanical response of FCC single crystals in nanoindentation and uniaxial tests. To study the effect of spatial distribution of defects in nanoindentation testing, various point defects were located in different relative positions to the indenter. When the defect position was close to the regions of high shear stresses the nucleation of dislocations was related to the location of the defect; however homogeneous nucleation of dislocations was also observed for defect-containing crystals. The effect of the point defects was independent of the indenter size, and the applied pressure needed to initiate plasticity, when compared to defect-free crystals, was a reduction of approximately 10%, 20%, 20% and 50% for a single vacancy, di-vacancy, self-interstitial atom and stacking fault tetrahedron (SFT), respectively. The stochastic nature of the pop-in loads was further explored for different orientations using molecular dynamics and complementary nanoindentation experiments on (100), (101) and (111) single crystals of copper and Ni200. The sensitivity of the crystal to the presence of internal structural defects depends strongly on its crystallographic orientation. The simulations suggest that the first event observed experimentally may not correspond to the first plastic deformation event. Anisotropy effects were also studied for various orientations in uniaxial tests in the presence of a centered SFT. Both the normal stresses to the slip plane and the relative values of Schmid factor in compression and tension affect the observed compression/tension yield asymmetry. The reduction in yield stress was found to be larger in compression than in tension for almost all orientations. The simulations suggest that compression is a more reliable experimental tool for studying the effect of structural defects on the mechanical behavior of the FCC crystals, while tension may be more useful to determine size effects in deformation. Finally, simulations at high temperatures showed that internal defects are capable of reducing the temperature sensitivity of yielding in various crystal orientations, especially when the stress field is mainly compressive like those in nanoindentation and compression tests.