Investigation of mechanical features of low cycle fatigue specimens of austenitic steel AISI type 321 under applied load by neutron diffraction stress analysis

The elastoplastic properties of a stainless steel with an austenitic matrix and martensitic inclusions induced during cyclic tensile-compressive fatigue loading were studied using neutron diffraction. Specimens of annealed and quenched AISI type 321 steel were subjected to low cycle fatigue ( strain amplitude of 1% at 0.5 Hz). Subsequent in situ loading tests provided the elastoplastic responses of both austenitic and martensitic phases via Rietveld refinement of neutron diffraction spectra. A clear trend of increasing elastic modulus with increasing fatigue level was noted in the austenite matrix. The results of modified refinements accounting for the elastic anisotropy in polycrystalline materials under load are presented. The residual strains in the austenitic matrix and the deviatoric components of both phases' residual microstresses were determined as a function of fatigue cycling. In the Appendix it is demonstrated that the presence of residual stresses can have an effect on the observed elastic moduli in a two phase system, but that for typical magnitudes the effect is negligible.