Numerical investigation of grain misorientations at and close to the free surface of FCC polycrystalline metals

Abstract

The evolution of grain misorientations (also called orientation gradients) at and close to the free surface of polycrystalline aggregates under tensile straining is analyzed within the crystal plasticity finite element model. The grain misorientation is defined as an angle of crystal orientation change with respect to the average grain orientation. The study is based on austenitic stainless steel (300 series) and may be in the future extended to other face-centered-cubic polycrystalline metals.The crystal plasticity model is calibrated to single crystal 316 steel stress–strain curves measured in three tensile directions [001], [111] and [123]. A procedure for automatic identification of the material parameters is introduced. The calibrated model is then used in polycrystalline simulations using Voronoi topologies for the grains with random crystallographic orientations. Mean and median grain misorientations are calculated on a free surface of the model and compared to electron backscatter diffraction (EBSD) measurements on 316 and 304 stainless steel taken from the literature. A linear correlation between the mean/median grain misorientation and the imposed macroscopic plastic strain is found to be in excellent agreement with the measurements. Furthermore, a sensitivity analysis is performed, showing that free surface response is influenced mostly by in-plane anisotropy of the grains and much less by bulk effects. The contributing thickness is found to be limited to only about three grains below the free surface. Among various uncertainties assessed, the variation in model topology (different Voronoi structures) is shown to have the biggest influence on the calculated grain misorientations. Finally, a sharp 40% decrease of grain misorientations is predicted when half (or more) of the average grain size of the material (few 10μm in stainless steel) is removed from the free surface, highlighting the possibly important influence of sample surface preparation (e.g., polishing) to the interpretation of misorientation measurements obtained from surface sensitive techniques.