Anisotropic dislocation loop distribution in alloy T91 during irradiation creep

Ferritic-martensitic (FM) steel T91 was subjected to irradiation with 3 MeV protons while under load at stresses of 100 MPa, 180 MPa, and 200 MPa. Transmission electron microscopy of crept T91 samples was conducted to determine the nature of the loops and to quantify the edge-on dislocation loop density as a function of their orientation to the tensile axis. Loops were determined to be interstitial in nature of the type ao〈1 0 0〉. The distribution of ao〈1 0 0〉 loops was found to be strongly anisotropic in which loops preferentially formed with their normal in the tensile direction. An empirical correlation was developed to describe the relationship between the orientation of the loop normal to the tensile axis and the local dislocation loop density. Loop anisotropy was found to increase as a function of the applied stress and their strain contributions relative to total creep strain were calculated at 4-11% of the total creep strain. Results from this study provide a means for quantifying empirically measured anisotropic microstructure and including it in irradiation creep mechanisms for FM steels.