Multiscale microstructural degradation and nanoindentation creep response of T91 heat-resistant steel after ultra-long-term creep exposure

The microstructural degradation and local mechanical response of T91 heat-resistant steel after creep rupture at 600 °C (2,616–349,446 h) were investigated by nanoindentation, EBSD, TEM and X-ray CT. Nano-hardness decreases from ~2.6 to ~1.9 GPa and the apparent indentation stress exponent from 7.16 to 2.59, indicating progressive loss of local deformation constraint. EBSD reveals heterogeneous recovery of tempered martensite: the low-angle boundary fraction (2°≤θ<15°) drops from 55.5% to ~28–33% while the 45–63° fraction rises, mean GND density falls from 8.38 to ~4.3–5.0 × 1014 m−2 and then levels off, and the orientation-domain size remains ~2.85–3.11 μm, consistent with continuous-dynamic-recrystallisation-like lath subdivision. The apparent growth of reconstructed prior-austenite grains reflects degradation of K–S/N–W orientation rather than real coarsening. TEM shows precipitate coarsening and cavity nucleation at precipitate–matrix interfaces. CT reveals cavity volume fraction rising from ~0.02% to ~0.31% with local clustering, demonstrating multiscale coupling among matrix softening, boundary evolution and cavity damage.