Microstructure and property tailoring of castable nanostructured alloys through thermomechanical treatments

Three types of microstructures, i.e., tempered-martensite (TM), ferrite (F), and dual-phase (TM + F), were developed in a castable nanostructured alloy that favors a high density of nanoprecipitates compared with the precipitates in current reduced-activation ferritic-martensitic steels. The effect of the distinct microstructures on tensile properties, Charpy impact toughness, and thermal helium desorption behavior was investigated with the full TM structure as a reference. The results indicated that the F domain in the TM + F structure governed the strength and slightly impaired the impact toughness. The full F structure exhibited the highest strength without compromising ductility, but it noticeably diminished impact toughness. All microstructures had a dominant helium desorption peak at ∼1070 °C. The higher density of nanoprecipitates and complex boundaries and dislocations in the TM + F structure enhanced the secondary helium desorption peak and extended the shoulder peak, in contrast to the full TM structure with an enlarged desorption peak associated with the ferrite-to-austenite transformation at ∼810-850 °C and the full F structure with a dominant desorption peak related to bubble migration at ∼1070 °C. These results suggest that components fabricated from functionally graded microstructures could be engineered to exploit the advantages of different microstructures for demanding application requirements.