An analysis of dislocation nucleation near a free surface

Abstract

Molecular dynamics analyses of defect-free aluminum single crystals subject to bending are carried out to investigate dislocation nucleation from free surfaces. A principal aim of the analyses is to provide background for the development of dislocation nucleation criteria for use in discrete dislocation plasticity calculations. The molecular dynamics simulations use an embedded atom potential for aluminum. Bending is imposed on a strip by specifying a linear variation of displacement rate on opposite edges. The overall bending response is determined and the character of the dislocations nucleated is identified. It is found that the stress magnitudes at the instant of dislocation nucleation are nearly an order of magnitude smaller than for homogeneous bulk dislocation nucleation. The characterization of dislocation nucleation in terms of various phenomenological nucleation criteria is explored, in particular: (i) a critical resolved shear stress; (ii) the onset of an elastic instability; and (iii) a critical stress-gradient criterion. It is found that dislocation nucleation is not well-represented by a critical value of the resolved shear stress but is reasonably well-represented by the critical stress-gradient criterion.

Molecular dynamics simulations of buckling-induced plasticity

“…The resolved shear stress of ͑111͓͒121͔ slip system reaches consequently a maximum stress of 1.7 GPa for 0 = −7.8% in the compression areas. This result is in good agreement with the work of Liu et al, 16 which shows that for a cell of a single bent Al crystal defined with the following axes ͑O x ͒ = ͓111͔, ͑O y ͒ = ͓101͔, and ͑O z ͒ = ͓121͔, dislocations can be also nucleated from the free surfaces of the crystal. With such an orientation, the observed partial dislocations nucleated in ͕111͖ slip planes rapidly self-annihilate, with a slip distance of 1.39 nm.…”

“…14 It was recently demonstrated that the presence of tilt boundaries results in the formation of buckling blisters with lower energy than the classical elastic ones. 15 Finally, Liu et al 16 studied by molecular dynamics the behavior of bent Al single crystals, which is quite similar to the mechanical solicitations of a buckled structure. They demonstrated that partial dislocations are nucleated in compressive areas near the free surfaces at a resolved shear stress of 1.78 GPa.…”

Changes in internal stress distributions during yielding of square prismatic gold nano-specimens

“…However, the Schmidt factor for the most favorable slip system in the [1,1,1] nanowire is larger in tension than that in compression but the residual compressive stresses counteract this effect producing an equal value of the yield stress in tension and compression. Even though Diao et al [2] found that the critical resolved shear stress does not change appreciably with the cross-sectional area of the nanowire and that it can be used as a criterion for the nucleation of defects, Liu et al [5] and Miller and Rodney [6] have stated that the slip system with the highest resolved shear stress is not always activated at the yield point.…”

Surface versus bulk nucleation of dislocations during contact

“…At present no surface nucleation criterion for use in a discrete dislocation plasticity analysis has been developed. In fact, there is no basis for presuming that a critical Peach-Koehler force is an appropriate criterion for dislocation nucleation at a surface (see Liu et al, 2007). In the absence of a more appropriate surface nucleation criterion, we use a similar criterion as for bulk sources but with possibly different source strengths t n surf .…”