text: While repeatedly reversed loading commonly leads to localisation of dislocation glide, creating linear extrusions and intrusions on a free surface, similar features can arise even if there is no load reversal.  These arise from dislocations gliding on a particular slip plane, in a particular slip direction (within a single grain), under an external load.  Steps can be created on the free surface as a consequence of the tendency for dislocations to follow one another along a glide path, of which there may be several in parallel with each other in the grain concerned.  Prior passage of dislocations apparently makes glide easier for subsequent ones, and the effect may also be associated with dislocation sources, such as a Frank-Read source, acting in particular planes.
  The appearance of such bands, which are sometimes termed “persistent slip lines”, is similar to that of those arising from cyclic loading, but the resultant steps are usually more localised and have lower heights.  They also reveal the grain structure. They can often be seen on free surfaces that were polished before the deformation took place.  For example, the figure shows micrographs (taken with different magnifications) of the region around an indent created in a copper sample with a spherical indenter.  The parallel lines within individual grains are each the result of several hundred dislocations of the same type reaching the free surface, creating steps with a height of the order of a few microns.  If a single slip system was operational within a grain, then there is just one set of lines, but it is common for more than one system to be activated within a grain (particularly when the strain is relatively high), leading to two or more sets of parallel lines.  Other features indicative of the details of how the plastic deformation took place, such as a region of cooperative shear caused by deformation twinning, can also sometimes be seen on such surfaces.  In the optical micrograph shown, there is also evidence of grain rotations – for example, at the “rim” of the indent and in the form of depressions at grain boundaries.  Such images can thus be very informative.