Heterogeneous Strain Distribution and Saturation of Geometrically Necessary Dislocations in a Ferritic–Pearlitic Steel during Lubricated Sliding

Abstract

The microstructural evolution of ferritic-pearlitic steel is studied during unidirectional sliding under boundary lubrication using a ball-on-flat configuration. Dislocation activity as a function of distance to surface is determined using orientation gradients by electron backscatter diffraction in logarithmic intervals up to one million of sliding cycles. The orientation gradient results show the formation of geometrically necessary dislocations and low-angle grain boundaries, followed by a consolidation of those grain boundaries as nanocrystalline grains. The density of geometrically necessary dislocations and low-angle grain boundaries is observed to reach a steady state after around 100 000 cycles. Plastic strain is not homogeneously distributed within both material phases, that is, ferrite and pearlite. Most of the plastic deformation is carried by the ferritic phase. The heterogeneous strain distribution between the ferritic and the pearlitic phase observed in the electron backscatter diffraction measurements is attributed to stress incompatibilities at the grain boundaries.

Interactions between Dislocations and Boundaries during Deformation

“…This process proceeds with a reorientation of pearlite colonies and a reduction of lamellar spacing [ 124 , 125 , 126 , 127 ]. It is supposed that the ferrite–cementite interfaces serve as the dislocation sources and strong barriers against dislocation motions, which is confirmed by TEM observations [ 128 , 129 , 130 ]. Thus, at the first stage of plastic deformation, it is assumed that dislocations in ferrite lamellae nucleate firstly at the interface, and then dislocations in cementite lamellae may nucleate at the interface.…”