A study of the effects of Hf and Sn additions on the microstructure of Nbss/Nb5Si3 based in situ composites

The phase stability and microstructures of Nb_ss/Nb₅Si₃ based in situ composites alloyed with Hf and Sn have been investigated. The Nb₅Si₃ silicide in the β and γ forms was the primary phase in the Nb–24Ti–18Si–5Cr–5Al–5Hf–2Mo and Nb–24Ti–18Si–5Cr–5Al–5Hf–5Sn–2Mo alloys studied in this work. In the as cast alloys, the formation of the Nb₃Si phase was suppressed by the addition of Hf. The structure of the Nb₅Si₃ phase was mainly affected by the Hf addition. The Hf-rich regions in the 5–3 silicide probably corresponded to the γNb₅Si₃. This silicide was stable up to 1500 °C in the presence of Hf. The Ti solubility in the Hf-rich Nb₅Si₃ was higher than that in the Nb₅Si₃. The Si concentration in the Nb₅Si₃ phase increased slightly and shifted closer to its stoichiometric composition by the addition of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb₅Si₃ and Nb_ss, respectively. The Sn addition had a significant effect on the niobium solid solution leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in Nb_ss increased considerably whilst the Cr solubility decreased. As a result of the decrease of the Cr solubility in Nb_ss, the Sn addition promoted the formation of Si-rich C14 Cr₂Nb Laves phase and stabilised this phase at 1300 °C. TiN and HfO₂ were formed below the surfaces of the heat treated alloys.