J. Valdes, X. Liu, West Virginia University, Morgantown, WV; S. Shang, The Pennsylvania State University, University Park, PA; Z. K. Liu, Pennsylvania State University, University Park, PA
Segregation of strengthening phase formation elements, i.e., Al, Ti, and Nb, in Ni-base superalloys could lead to the formation of solidification defects during the processing of turbine components, such as single crystal blades for aerospace engines and large size rotors for land base turbines. In this research, experimental and theoretical methodologies were used to study the partition behavior of Niobium in the model alloys of the Ni-Cr-Nb and Ni-Fe-Nb systems. DTA experiments at different cooling rates together with the thermodynamic and kinetic simulations using different methods and databases were included. The influence of the simulation methods (equilibrium, Scheil model, and back diffusion) on the partition coefficient of Nb was considerable when the solid fraction was greater than 0.5. For the Ni-15Cr-xNb (in weight percent) system, the measured phase transformation temperatures were higher than the ones predicted using the commercial Ni-database and slightly lower for the Ni-20Cr-xNb system. In particular, for the alloy Ni-15Cr-4.5Nb, the measured phase transformation temperatures were closer to the predictions by Du’s database developed recently in comparison to the commercial Ni-database. The kinetic effect on the solidus and liquidus temperatures in the DTA experiments was greater in the cooling stage and strong for the Ni-Fe-Nb, while negligible for the Ni-Cr-Nb.
Summary: Segregation of strengthening phase formation elements, i.e., Al, Ti, and Nb, in Ni-base superalloys could lead to the formation of solidification defects during the processing of turbine components, such as single crystal blades for aerospace engines and large size rotors for land base turbines. In this research, experimental and theoretical methodologies were used to study the partition behavior of Niobium in the model alloys of the Ni-Cr-Nb and Ni-Fe-Nb systems.
