Effect of strain and strain rate on the development of deformation heterogeneity during tensile deformation of a solution annealed 304 LN austenitic stainless steel: An EBSD study

Evolution of the microstructure and geometrically necessary dislocation (GND) structure was studied during tensile deformation of a solution annealed 304 LN austenitic stainless steel. Microstructures of the steel at varying engineering strains and strain rates (i.e., 1x10(-4) s(-1), 1x10(-3) s(-1) and 1x10(-2) s(-1)) were analyzed using electron back scatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) at ambient temperature. EBSD was used to quantify the evolution of the GND structure and the martensite formed during tensile straining to reveal the deformation mechanisms in the presence of microstructural heterogeneities. The average GND density and the amount of deformation induced martensite increased with increasing plastic strain at a faster rate than expected, following a concave up pattern that accelerated GND formation as strain increased. ECCI was used to examine the dislocation storage arrangements with plastic strain. Various strain rates were imposed on the steel specimens and the results show that the average GND density increased only slightly with increasing engineering strain rate. This substantiates the small decrease in strain rate sensitivity of the steel that was observed as the plastic strain increased. Therefore, the steel is tolerant to a change in the strain rate during forming. This study provides an understanding for the way in which the plastic deformation behavior of the steel is influenced by the evolution of GND density in the presence of microstructural heterogeneities, and by deformation induced martensitic transformations.