Processing,microstructure, and mechanical properties of (Nb)/Nb5Si3 two-phase alloys

It has been shown that a fine lamellar structure composed of Nb solid solution, (Nb), and Nb₅Si₃ is formed through eutectoid decomposition in the Nb-Si binary system and its ternary derivatives. Such alloys would exhibit a high strength at over 1400 K, yet showing room-temperature toughness of over 10 to 20 MPa m^1/2 if a proper lamellar spacing is chosen. In the present work, effects of processing on the microstructure evolution and mechanical properties are investigated on the Nb-18 at. pct Si alloys prepared by hot pressing (HP) and spark-plasma sintering (SPS). The powders used in the present work are of pure Nb and Nb₅Si₃ in order for the fabrication to become possible at temperatures higher than the melting point of Si and to reduce the formation of SiO₂. The results show that the SPS yields more uniform two-phase microstructure but the alloy fabricated through HP tends to provide higher elevated temperature strength. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.