|Abstract:||T91 ferritic-martensitic (F/M) steel is one of the leading candidates for high temperature structural materials in advanced nuclear power applications. In situ Wide-angle X-ray Scattering (WAXS) was used to investigate T91 during tensile test under 3 different temperatures: room temperature (RT), 450C, 550C, respectively. By fitting scattering patterns, information of diffraction peaks and their variation trends with respect to macroscopic strain were recorded for further analysis of Fe matrix, M23C6 and MX precipitates. Lattice strain of Fe matrix and precipitates were obtained from peaks shift in WAXS pattern during tensile test. Load partitioning effect, which is known as precipitates can bear higher load than Fe matrix, was found to be less obvious within plastic regime with the increase of temperature from RT to 550C. Peak broadening, represented by full width at half maximum (FWHM), was carefully analyzed using modified Williamson-Hall (W-H) plots to separate strain and crystalline size effects due to their different theta dependence. It was found that dislocation density rho in Fe matrix behaved differently within different tensile regimes: before Ultimate Tensile Strength (UTS), they all increased; after UTS, it plateaued first then increased at end for RT, remained stable for 450C while decreased continuously for 550C. Coherent scattering length L310 calculated by Scherrer Equation using Fe(310) plane behaved as below: before UTS, they all decreased; after UTS, it decreased continuously for RT, remained stable for 450C and rose dramatically for 550C. The variation trends of both dislocation density and coherent scattering length L310 were impacted by combination effect from high temperature and tensile flow stress evolution.
Scanning Electron Microscopy (SEM), Optical Micropy (OM) and Transmission Electron Microscopy (TEM) were also used to study the post-tensile samples. SEM found that at RT tensile fracture happened violently with huge cracks on necking center cross-section, while fracture happened slowly with voids and dimples on cross-section under higher temperatures. SEM Energy Dispersive Spectroscopy (EDS) mapping also found that rupture protrusions on necking center edges had higher C and N concentration than that of metallic elements. OM revealed the martensitic plates and ferrites in T91 after polishing and etching. Lastly, TEM experiment was also carried out using Focused Ion Beam (FIB) prepared specimen to characterize precipitates, dislocations and other microscale features, and to compare with the previous X-rays Diffraction (XRD) results.