Title : Strain localization in austenitic stainless steels studied by atom force microscope
Abstract:
Almost all metallic alloys deform through nucleation and propagation of slip bands. Slip localization implies that macroscopic deformation is inappropriate in investigating mechanisms of local plastic slip and strain heterogeneity. In this work we use high resolution Atom Force Microscopy (AFM) and Electron BacksScatter Diffraction (EBSD) techniques to statistically characterise slip band nature, topology and their interactions with Grain Boundaries (GBs) in a 316L austenitic stainless steel.
We first study the evolution of the number (or density) of slip bands and the accommodated plastic shear as a function of the macroscopic strain. In 316L steels, it is shown that the number of slip bands saturates at 1% plastic strain. This implies that activation of new dislocation sources, expected to be responsible for the nucleation of a slip bands, tends to drastically decrease in the first stages of plastic deformation. Beyond 1% of plastic strain, deformation is thus accommodated through elongation and intensification of slip inside the bands. Surprisingly, the slip band width was found constant in all grains. Observations show also that the volume of slip bands is only a small fraction of the sample. The effective deformation in the bands can be up to ten times larger than the macroscopic one.
A details statistical study is then made to identify the topology of intragranular bands and of bands interacting with GBs. The measurement of slip along slip bands allows deep investigation of slip propagation inside the grain and across GBs. The study reveals five principal band-GBs interactions: direct or indirect transmission, microvolume formation, no-transmission and absorption. Every type is defined according to the slip profile along the band on both sides of the GB. Dislocation storage is easily determined thanks to simple geometrical relations involving the orientation of the Burgers vector, the local slip band width and accommodated slip. These interaction types are then statistically linked with the nature of the GB, characterized by EBSD. The dislocation flux across GBs is thus deduced from the same geometrical relations.
