Application of the stress function method on HEXRD data for analyzing phase specific stresses in plastically deformed duplex stainless steel

Diffraction-based residual stress analysis using the well-established sin²ψ approach of plastically deformed polycrystals often poses considerable challenges, particularly with regard to the occurrence of non-linear courses of the measured lattice plane distance d vs. sin²ψ. These non-linearities can be attributed to different causes, which can occur individually or together, i.e. crystallographic texture that can develop in the material during plastic deformation, usually resulting in elastically anisotropic material behaviour and the occurrence of intergranular strains, which arise as a result of deformation incompatibilities between neighbouring crystallites during plastic deformation.

While evaluation methods exist for the consideration of crystallographic texture, hardly any of them are suitable to also account for plastically induced intergranular strains. The harmonic method proposed by Behnken for determining stress functions, on the other hand, allows for the consideration of both texture and plastically induced strains. However, it has rarely been applied so far, as it requires the experimental determination of lattice strains of several {hkl}-lattice planes for numerous measurement directions and is moreover rather complex in its evaluation procedure.

In the present work, the comprehensive measurement data required for such an evaluation was generated using 2D high-energy X-ray diffraction (HEXRD) at the Deutsches Elektronen-Synchrotron (DESY). Duplex stainless steel X2CrNiN23-4 (ferritic-austenitic) samples were deformed through defined load increments up to a plastic strain of about 2% using an in-situ tensile testing setup. By using an area detector in transmission geometry, strain pole figures and intensity pole figures of several lattice planes of both phases were determined.

Based on the experimentally determined data, phase-specific stresses were determined using the harmonic stress function method. Following this approach, the phase-specific load partitioning behaviour of the investigated duplex stainless steel could be determined despite the presence of intergranular strains.