New insights of the strength asymmetry in FCC single-crystalline nanopillars

Abstract

Nanomaterials or structures usually exhibit characteristic performance under complex stress states. The tensile and compressive behaviors of [001]-oriented single-crystalline nanopillars were studied, by performing molecular dynamic simulations on several typical FCC metals. For all those metallic nanopillars, their yield strengths for nucleating the initial dislocation show strong loading direction dependence, i.e., the strength under tension is higher than that under compression, showing the typical T/C asymmetry. The origins of the T/C asymmetry were investigated from the new aspects of the surface energy difference under tension and compression, the large ultimate elastic deformation, and the non-Schmid stress, in detail. The results indicate that both the Schmid factor and the non-Schmid factors change considerably due to the large elastic deformation under tension or compression, which contribute negatively to the T/C asymmetry. The difference in surface energy reduction due to the large elastic deformation is one but not the only factor that results in the T/C asymmetry. Although the non-Schmid factor contributes negatively to the T/C asymmetry, the non-Schmid stress can increase the difference of unstable stacking fault energies under tension and compression, which has a significant positive influence on the T/C asymmetry by changing the ideal shear strength of the slip plane.