Investigation of deformation and microstructural evolution in Grade 91 ferritic–martensitic steel by in situ high-energy X-rays

In situ wide- and small-angle X-ray scattering (WAXS/SAXS) were used to characterize microstructural evolution in Grade 91 ferritic–martensitic steel during tensile tests at room temperature (RT) and 650 °C. Elastic lattice strains in the Fe matrix, M23C6 and MX precipitates were measured from peak shifts in WAXS patterns. Load transfer, as evidenced by precipitates developing much higher lattice strain than the Fe matrix in the plastic regime, was found for the RT test but was not obvious at 650 °C. Detailed peak-broadening analysis as a function of strain, using modified Williamson–Hall plots, indicates strong dislocation activity in the Fe matrix during the RT test which can be divided into two stages: (i) dislocation multiplication (from 0 to 4.0% strain) and (ii) formation of dislocation walls and cells (after 4.0% strain). This framework is consistent with the observed precipitate strain response and can describe the macroscopic flow stress behavior. At 650 °C, Fe peaks broadened much less with increasing strain, indicating a low dislocation density and providing an explanation for the low precipitate strains and macroscopic flow stress at this temperature. Changes in SAXS intensity were used to infer nanoscale void formation, which occurred immediately after necking at RT but long after necking at 650 °C. Most voids were concentrated at the necking center, and the RT specimen developed more voids than the 650 °C specimen.