On a new Ti-carbooxinitride redistribution driven microcrack healing mechanism in an annealed 14YWT nanostuctured ferritic alloy

Abstract

FCRD NFA-1 is a larger best practice heat of Fe-14Cr nanostructured ferritic alloy (NFA) dispersion strengthened by a large population of nano-scale Y2Ti2O7 nano-oxides. NFA-1 was produced by ball milling FeO with argon atomized Fe-14Cr-3W-0.35Ti-0.25Y (wt.%) powders, followed by hot extrusion at 850 °C, followed by 1 h annealing and multipass cross-rolling at 1000 °C to form an ≈ 10 mm thick plate. Cross-rolling results in a large population of {001}<110> cleavage system microcracks (MCs) running on planes parallel to the plate faces. A series of high temperature vacuum anneals were carried out from 1100 to 1300 °C for 1 and 5 h to explore the possibility of self-healing, not only of the MCs, but also macroscopic fatigue cracks. Most notably, a new crack self-healing mechanism was discovered, involving the redistribution and coarsening of initially fine matrix Ti, C, O and N precipitates (TiCONs) to bridging sites on the crack faces. The relocated TiCONs grow and coarsen, thereby consuming a volume of the adjoining Fe-Cr matrix. The displaced matrix atoms diffuse to further fill the crack. Extensive microstructural, fractographic and mechanical property characterization measurements show that MC self-healing begins at 1100 °C/1 h, increasing with annealing time and temperature, up to completion at 1300 °C. The NFA-1 grains and texture are relatively stable. However, at 1300 °C the nano-oxides and dislocations coarsen or recover, respectively, leading to significant softening.