Development of Nb–1%Zr–0.1%C alloy as structural components for high temperature reactors

The Nb–1Zr–0.1C (wt.%) alloy is being considered for structural components in the proposed Compact High-Temperature-Reactors (HTR). The present work reports on the development of 30–50 kg ingots of the alloy in correct composition as well as technology for forming the material in various shapes. The work deals with the deformation behavior of as-cast material at different temperatures and strain rates, recrystallization behavior at different temperature and time and evolution of microstructures at different processing conditions (as-cast, deformed and recrystallized). The as-cast Nb alloys were deformed up to 35% at different temperatures. The deformation results showed that the flow stress of the as-cast Nb alloy increases with increasing temperature from 800 °C to 1000 °C. Beyond 1200 °C, substantial decrease in the strength of the alloy was noticed. To determine the optimum recrystallization temperature and time for the alloy, several heat treatments were conducted by systematically varying temperature and time. It was found that the deformed Nb alloy could be recrystallized by annealing at 1300 °C for 3 h. The microstructures of the as-cast, deformed and recrystallized samples of Nb–1%Zr–0.1%C alloy were systematically characterized by optical, electron back scattered diffraction (EBSD) and transmission electron microscopy techniques. The Nb–1Zr–0.1C alloy showed significant differences in the microstructure after different thermo-mechanical treatments. Microstructures of the Nb alloy showed two phases: the matrix (bcc) phase and the carbide phase. Electron Microscopy and energy dispersive spectroscopic analyses revealed that the carbide precipitation undergoes various phase transformations. The as-cast structure of Nb alloy had hexagonal Nb2C precipitates in the Nb matrix and after extrusion, the deformed microstructure had two types of carbide precipitates: needle and rectangular morphology precipitates. The needle shape precipitates were of (Nb, Zr)2C with orthorhombic structure and rectangular shape precipitates were of (Nb, Zr)3C2 with hexagonal structure. After annealing, the recrystallized microstructures showed equilibrium structure i.e. (Nb, Zr)C precipitates (fcc structure) in the Nb matrix.