Influence of strain amplitude on the development of dislocation structure during cyclic plastic deformation of 304 LN austenitic stainless steel

Abstract

In the present study, electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) were used to document the evolution of dislocations in a 304 LN austenitic stainless steel subjected to cyclic plastic deformation at room temperature using different strain amplitudes. In particular EBSD was used to quantify the development of geometrically necessary dislocation density (GND) with varying strain amplitudes. Solution annealed multiple specimens were cyclically deformed at varying strain amplitudes ranging from ±0.25 to ±1.2% under completely reversed straining conditions until complete failure. It is found that GND density, which is independent of the character of the dislocations, increases with the increase of strain amplitude. This is consistent with the cyclic hardening characteristic of the investigated steel. It is found that the evolution of GND structure during the course of deformation is heterogeneous in nature with small grains having higher GND density than the coarse grains. The clustering of GNDs is visible near grain boundaries leaving the grain interior free. EBSD can capture the variation in GND storage patterns with strain amplitudes. The heterogeneity of the cyclic deformation induced GND structure decreased with the increase of the strain amplitude. The evolution of GND structure is also found to be sensitive to the size and orientation of the austenite grains. It has been found that the GND density is maximum in {111}<0-11> grains, those with the highest Taylor factor. The fidelity of dislocation storage pattern with strain amplitudes is assessed using ECCI. This is consistent with the GND storage pattern shown by EBSD and explains the cyclic hardening behavior of the steel.