Initiation and Propagation of Plastic Yielding in Duplex Stainless Steel

The elastic-plastic behavior of a two-phase stainless steel alloy is explored at the crystal scale for five levels of stress biaxiality. The crystal lattice (elastic) strains were measured with neutron diffraction (ND) using tubular samples subjected to different combinations of axial load and internal pressure to achieve a range of biaxial stress ratios. Finite element simulations were conducted on virtual polycrystals using loading histories that mimicked the experimental protocols. Two-phase microstructures were instantiated based on microscopy images of the grain and phase topologies and on crystallographic orientation distributions from ND. Detailed comparisons were made between the measured and computed lattice strains for several crystal reflections in both phases for scattering vectors in the axial, radial, and hoop directions that confirm the model’s ability to accurately predict the evolving local stress states. The strength-to-stiffness parameter for multiaxial stress states developed for single-phase polycrystals was applied to explain the initiation of yielding across the five levels of stress biaxiality. Finally, building off the multiaxial strength-to-stiffness, the propagation of yielding over the volume of a polycrystal was estimated in terms of an equation that provides the local stress necessary to initiate yielding within crystals in terms of the macroscopic stress.