Study of grain stresses and crystallographic slips in duplex steel using neutron diffraction

In this work, a novel method for determination of the stress tensor for groups of grains having preferred texture orientations and Critical Resolved Shear Stresses (CRSSs) necessary for activation of slip systems was applied to study the elastic-plastic properties of textured duplex steel. The methodology is based on in situ neutron diffraction measurements of lattice strains for groups of grains in the ferritic and austenitic phases during tensile test.

Using the stress tensors determined for selected grains, the evolution of the Resolved Shear Stress (RSS) was analysed. As a result, for the first time CRSS values for slip systems activated in both phases of duplex steels have been determined directly from experimental data. The important advantage of the used novel methodology is that the grain stress tensor and CRSSs were determined for representative volumes of polycrystalline grains, without the use of any elastic-plastic models. It was found that, due to the heat treatment of the material, the ferritic phase is significantly harder than the austenitic phase, leading to high yield stress value for the steel under study. For the first time, the evolution of the stress tensor and RSS for austenitic grains with different orientations was determined experimentally and the different mechanical behaviour of these grains was demonstrated.

Finally, the experimental data were compared with the multi-scale Elastic-Plastic Self-Consistent (EPSC) model, which used experimental CRSSs as input data. The agreement of the predicted grain stress and macroscopic stress-strain relationship with the experimental results obtained from the tensile test positively verified the Eshelby-type grain interaction used in the EPSC model. Determining representative CRSS values from the experiment for two-phase textured material, as done for the first time in this work, reduces the number of input parameters of mechanical multiscale models by increasing their unambiguity and allowing their verification.