Dislocation nucleation from surface steps:: atomistic simulation in aluminium

The possible role of surface steps in the nucleation of dislocations from a free surface has been studied by means of a static atomistic simulation using a many-body potential for aluminium. The fee crystal with a {100} free surface containing a monatomic step lying along a [110] dense direction is submitted to an increasing uniaxial stress along a direction belonging to the {100} plane. For a sufficiently high applied stress, well below the theoretical strength, dislocations are nucleated at the step and glide in the {111} planes emerging at the step. The effect of a stress orientation is examined. The type of dislocation formed, that is Shockley partials of 90 degrees and 30 degrees character or perfect dislocations, is rationalized by considering the resolved shear stress in the {111} planes. The plane where glide will occur is favoured well before nucleation; a shear of increasing amplitude and extension is progressively localized on this plane. The role of the stress field due to the step, in the formation of the localized shear, is discussed.