Effect of vacuum annealing temperature and time on the recrystallisation behavior of cold rolled niobium alloy C-103 sheets

Abstract

Niobium alloy C-103 is the lightest among the refractory alloy family and is widely used for high temperature applications in aerospace propulsion systems. In the present study, sheets of 2 × 1000 × 1000 mm were processed through cold rolling. However, the cold rolled sheets exhibited higher strength with a significant reduction in percentage elongation (ductility), making them unsuitable for further cold forming process, such as the fabrication of divergent sections of rocket engines. In order to eliminate the residual stresses and obtain a strain-free microstructure, the cold rolled sheets have been vacuum annealed at varying temperatures of 1100 °C, 1200 °C and 1300 °C for 1, 2 and 4 h. Detailed microstructural analysis and mechanical properties evaluation has been carried out on vacuum annealed samples for arriving at optimum annealing parameters. It is noted that the samples annealed at 1100 °C had remnant elongated grains, whereas coarsening of grains was noticed for samples annealed at 1300 °C for 4 h. The samples annealed at 1200 °C for 2 and 4 h had an optimum grain size of 33 and 39 μm, respectively. The corresponding 0.2 % yield strength, ultimate tensile strength and % elongation for 2 h and 4 h condition was 276 ± 4.4 MPa, 385 ± 3.5 MPa, 39.6 ± 3.1% and 271 ± 1.8 MPa, 388 ± 1.5 MPa, 37.8 ± 1%, respectively. Further, an attempt has been made to understand the grain growth kinetics with respect to variation in annealing temperatures and time. The activation energy was estimated to be 792 kJ/mol in the temperature range of 1100–1300 °C for this alloy. The 0.2% yield strength of C-103 material was further correlated to the grain size, as per Hall-Petch relation and the values of σ₀ (lattice friction resistance constant) and k₀ (grain boundary barrier constant) were estimated to be 210 MPa and 356 MPa$\sqrt{\mu m}$, respectively.