Processing, Microstructures, and Properties of Aluminide-Strengthened Ferritic Steels

NiAl-strengthened ferritic steel is a material of interest for steam turbine applications due to their promising creep resistance, high thermal conductivity, low cost and thermal expansion coefficient. However, there are two critical issues restricting their applications: low room temperature ductility and limited creep resistance at temperatures higher than 600°C. The reasons for these behaviors have not been fully understood, especially the processing-microstructure-property relationship has not been clearly identified. The alloys were designed, fabricated, and processed, according to the required microstructures and properties. The matrix and precipitates can be thoroughly characterized by a combination of techniques, including atom probe tomography, neutron diffraction, and electron microscopy experiments. Bending, tension, and creep tests were conducted to study the ductility and creep behaviors of the as-designed alloys. Relationships were subsequently developed between the characterized microstructures and the mechanical behaviors to provide in-depth understandings of the alloy properties. Phase transformations and the resulting microstructure, structure, coherency, chemistry, and volume fraction of the precipitates were determined. The relationship between the precipitate microstructure and the room temperature ductility was thoroughly studied and established. Hot rolling was found to significantly improve the ductility. Moreover, thermodynamic modeling was combined with focused experiments to study the ductility and creep resistance. Thermodynamic calculations were used to identify the potential alloy compositions that may exhibit the balanced room temperature ductility and high temperature creep resistance, which was subsequently verified by experiements. This integrated experimental and modeling approach was found to be very effective in the alloy development process