A thermodynamic description of the C-Nb-Ti system over the whole composition and temperature ranges and its application in solidification microstructure analysis

摘要

An understanding of the thermodynamic and phase diagram data for the C-Nb-Ti system is essential for the development of cermets, cemented carbides as well as Nb-based superalloys. Based on a critical evaluation of the literature data, the C-Nb-Ti ternary system has been reviewed and assessed by means of the CALPHAD technique. A self-consistent thermodynamic description for the C-Nb-Ti system has been developed over the whole composition and temperature ranges. Comprehensive comparisons between the calculated and measured phase diagrams available in the literature show that the reliable experimental information can be satisfactorily accounted for by the present thermodynamic description. The as-cast microstructures of several Nb-based superalloys reported in the literature have been analyzed by using Scheil solidification simulations based on the presently obtained thermodynamic database. The improvement mechanism of room-temperature hardness and high-temperature yield stress can be successfully interpreted according to the simulated amount of (Nb,Ti)C carbide, the effective strengthening particles in the as-cast Nb-based superalloys, which indicates a powerful tool of thermodynamic calculations in materials development.