Time-of-flight neutron-diffraction investigations of elastic and anisotropy strains in fatigued austenitic stainless-steel

A study of the elastic and plastic properties of the austenitic base matrix and martensitic precipitates induced in stainless steel during high cycling tensile-compressive loading was made in the in situ stress-rig experiment on the ENGIN instrument at the ISIS facility. It is observed that the elastic constants of the austenitic and martensitic phases for the axial and transverse directions determined by the Rielveld refinement are almost independent of the level of fatigue, while the bulk value of the Young modulus measured by a gauge extensometer decreases with increasing fatigue. The austenitic elastic response is linear throughout the measured stress range (0-500 MPa), while the martensitic response is linear only up to 300-320 MPa, almost the same stress level at which bulk plasticity is observed. Herewith, the ratio of the martensitic elastic constants in the axial and transverse directions is almost twice that expected based purely on the value of the Poisson ratio. Results of the modified Rietveld refinement accounting for the elastic anisotropy in polycrystalline materials under load are presented.