  The elastoplastic 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 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. For this, two-phase microstructures were instantiated
based on microscopy images of the grain and phase topologies and on
crystallographic orientation distributions from neutron diffraction. 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 accurate predict
the evolving local stress states. A strength-to-stiffness parameter for
multiaxial stress states was applied to explain the initiation of yielding
across the polycrystalline samples 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 within crystals in terms
of the macroscopic stress.
