Effect of spark plasma sintering on alloying, densification and mechanical properties of C103 niobium alloy

There is a revival of interest in niobium (Nb) based alloys for high-temperature structural applications, due to their relatively higher melting point, lower density, as well as more tolerance to interstitials compared to other refractory metals. Although some wrought Nb alloys were made by ingot metallurgy in the past, powder metallurgy processing of Nb alloys has not been well developed. In particular, optimum sintering conditions, alloy homogenization and densification behavior in powder metallurgy processing have not been well understood. In this study, alloying and sintering behavior of C103 niobium alloy (Nb-10Hf-1Ti; composition in wt.%), during spark plasma sintering (SPS), have been investigated. The objective is to study the densification, homogenization, and microstructure evolution during spark plasma sintering at temperatures significantly below the solidus temperature. The alloy densified almost fully with very little residual porosity (∼1.3 vol. %) and exhibited 394 MPa yield strength, 458 MPa tensile strength, 23 % elongation and a plane stress fracture toughness (K_Q) value of ∼25 MPa√m. The strength levels are significantly higher than that of the wrought C103 alloy. The ductility was found to be only slightly reduced (from 26 to 23 %) relative to the wrought version, even though the oxygen levels were relatively higher (>2000 ppm) in the present alloy. The strength levels are also significantly higher than those found in additive-manufactured (EB-PBF and LB-PBF) C103 niobium alloys and without any loss in ductility. The strengthening mechanisms as well as the microstructural factors that lead to the ductility and high fracture toughness levels are discussed.