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Tension-Compression Asymmetry in Homogeneous Dislocation Nucleation
Abstract. This letter addresses the dependence of homogeneous dislocation nucleation on the crystallographic orientation of pure copper under uniaxial tension and compression. Molecular dynamics simulation results with an embedded-atom method potential show that the stress required for homogeneous dislocation nucleation is highly dependent on the crystallographic orientation and the uniaxial loading conditions; certain orientations require a higher stress in compression (e.g., <110> and <111>) and other orientations require a higher stress in tension (<100>). Furthermore, the resolved shear stress in the slip direction is unable to completely capture the dependence of homogeneous dislocation nucleation on crystal orientation and uniaxial loading conditions.
This manuscript was recently accepted in Applied Physics Letters. I have converted this to the journal format (3-pages) using LaTex. A few points that I have found interesting:
1. The tension-compression asymmetry observed in the dislocation nucleation stress in single crystal copper may help explain the tension-compression asymmetry that exists in the presence of heterogeneities (i.e., grain boundaries in nanocrystalline materials, free surfaces in nanowires). The tension-compression contour plot shows that most orientations require a larger dislocation nucleation stress in compression than tension. Therefore, a nanocrystalline material with random crystallographic texture should require a larger yield stress in compression than tension based solely on the effect of the crystal orientation; previous work confirms that this is the case.
2. The stress required for dislocation nucleation in single crystals depends on both the resolved shear stress on the slip plane in the direction of slip and the stress normal to the slip plane. While not explicitly shown in this letter, we find that there are certain regions of the stereographic triangle in tension where the dislocation nucleation stress correlates best with the resolved stress normal to the slip plane. In other words, while the resolved shear stress is important for the motion of dislocations (Schmid Law), the resolved stress normal to the slip plane can be important for the nucleation of dislocations in a pure fcc material.
I welcome any comments. Thank you.