Abstract
Effects of Zr addition on the microstructure formation of two intermetallic phases of Fe2Nb Laves (TCP) and Ni3Nb (GCP) in a carbon-free Fe-20Cr-35Ni-2.5Nb (at.%) novel austenitic heat-resistant steel at elevated temperatures were examined from the viewpoints of thermodynamics and kinetics. From a thermodynamic perspective, the maximum solubility of Zr in the matrix phase is about 0.1 at.% under homogenization treated states at around 1473 K. The dissolved Zr in the matrix eventually reduces the solubility limit of Nb, thereby playing an important role in promoting the formation of grain-boundary Fe2Nb phase as well as that of thermodynamically stable Ni3Nb-δ phase within grain interiors after long-term aging at 1073 K. The Zr dissolved in the matrix after the homogenization treatment is found to enrich in δ phase formed during the aging and raises the temperature limit of formation of δ phase by about 50 K. From a kinetic perspective, because of the increase in Nb supersaturation, the dissolved Zr not only enhances the precipitation of Laves phase at grain boundaries but also promotes the homogeneous nucleation of metastable Ni3Nb-γ″ phase within grain interiors from the beginning of aging and retards the phase transformation from γ″ to δ. These microstructure changes by Zr can be interpreted in terms of the phase equilibria and relative phase stability among different phases. The present results reveal that even a small amount of Zr in solution, not segregation to grain boundaries, has tremendous effects on the precipitation behavior and morphology of intermetallic phases.
