Computational Design and Prototype Evaluation of Aluminide-Strengthened Ferritic Superalloys for Power-Generating Turbine Applications up to 1,033 K

The objective of the proposed research is to utilize modern computational tools, integrated with focused experiments, to design innovative ferritic NiAl-strengthened superalloys for fossil-energy applications at temperatures up to 1,033 K. Specifically, the computational alloy design aims toward (1) a steady-state creep rate of approximately 3 x 10{sup -11} s{sup -1} at a temperature of 1,033 K and a stress level of 35 MPa, (2) a ductility of 10% at room temperature, and (3) good oxidation and corrosion resistance at 1,033 K. The research yielded many outstanding research results, including (1) impurity-diffusion coefficients in {alpha} Fe have been calculated by first principles for a variety of solute species; (2) the precipitates were characterized by the transmission-electron microscopy (TEM) and analytical-electron microscopy (AEM), and the elemental partitioning has been determined; (3) a bending ductility of more than 5% has been achieved in the unrolled materials; and (4) optimal compositions with minimal secondary creep rates at 973 K have been determined. Impurity diffusivities in {alpha} Fe have been calculated within the formalisms of a harmonic transition-state theory and Le Claire nine-frequency model for vacancy-mediated diffusion. Calculated diffusion coefficients for Mo and W impurities are comparable to or larger than that for Fe self-diffusion. Calculated activation energies for Ta and Hf impurities suggest that these solutes should display impurity-diffusion coefficients larger than that for self-diffusion in the body-centered cubic Fe. Preliminary mechanical-property studies identified the alloy Fe-6.5Al-10Ni-10Cr-3.4Mo-0.25Zr-0.005B (FBB-8) in weight percent (wt.%) for detailed investigations. This alloy shows precipitation of NiAl particles with an average diameter of 130 nm. In conjunction with the computational alloy design, selected experiments are performed to investigate the effect of the Al content on the ductility and creep of prototype Fe-Ni-Cr-Al-Mo alloys. Three-point-bending experiments show that alloys containing more than 5 wt.% Al exhibit poor ductility (< 2%) at room temperature, and their fracture mode is predominantly of a cleavage type. Two major factors governing the poor ductility are (1) the volume fraction of NiAl-type precipitates, and (2) the Al content in the {alpha}-Fe matrix. A bend ductility of more than 5% can be achieved by lowering the Al concentration to 3 wt.% in the alloy. The alloy containing about 6.5 wt.% Al is found to have an optimal combination of hardness, ductility, and minimal creep rate at 973 K. A high volume fraction of precipitates is responsible for the good creep resistance by effectively resisting the dislocation motion through Orowan-bowing and dislocation-climb mechanisms. The effects of stress on the creep rate have been studied. With the threshold-stress compensation, the stress exponent is determined to be 4, indicating power-law dislocation creep. The threshold stress is in the range of 40-53 MPa. The addition of W can significantly reduce the secondary creep rates. Compared to other candidates for steam-turbine applications, FBB-8 does not show superior creep resistance at high stresses (> 100 MPa), but exhibit superior creep resistance at low stresses (< 60 MPa)